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What is anatomy?
Anatomy includes those structures that can be seen grossly (without the aid of magnification) and microscopically (with the aid of magnification). Typically, when used by itself, the term anatomy tends to mean gross or macroscopic anatomy—that is, the study of structures that can be seen without using a microscopic. Microscopic anatomy, also called histology, is the study of cells and tissues using a microscope.
Anatomy forms the basis for the practice of medicine. Anatomy leads the physician toward an understanding of a patient’s disease, whether he or she is carrying out a physical examination or using the most advanced imaging techniques. Anatomy is also important for dentists, chiropractors, physical therapists, and all others involved in any aspect of patient treatment that begins with an analysis of clinical signs. The ability to interpret a clinical observation correctly is therefore the endpoint of a sound anatomical understanding.
Observation and visualization are the primary techniques a student should use to learn anatomy. Anatomy is much more than just memorization of lists of names. Although the language of anatomy is important, the network of information needed to visualize the position of physical structures in a patient goes far beyond simple memorization. Knowing the names of the various branches of the external carotid artery is not the same as being able to visualize the course of the lingual artery from its origin in the neck to its termination in the tongue. Similarly, understanding the organization of the soft palate, how it is related to the oral and nasal cavities, and how it moves during swallowing is very different from being able to recite the names of its individual muscles and nerves. An understanding of anatomy requires an understanding of the context in which the terminology can be remembered.
How can gross anatomy be studied?
The term anatomy is derived from the Greek word temnein, meaning “to cut.” Clearly, therefore, the study of anatomy is linked, at its root, to dissection, although dissection of cadavers by students is now augmented, or even in some cases replaced, by viewing prosected (previously dissected) material and plastic models, or using computer teaching modules and other learning aids.
Anatomy can be studied following either a regional or a systemic approach.
With a regional approach, each region of the body is studied separately and all aspects of that region are studied at the same time. For example, if the thorax is to be studied, all of its structures are examined.
This includes the vasculature, the nerves, the bones, the muscles, and all other structures and organs located in the region of the body defined as the thorax. After studying this region, the other regions of the body (i.e., the abdomen, pelvis, lower limb, upper limb, back, head, and neck) are studied in a similar fashion.
In contrast, in a systemic approach, each system of the body is studied and followed throughout the entire body. For example, a study of the cardiovascular system looks at the heart and all of the blood vessels in the body. When this is completed, the nervous system (brain, spinal cord, and all the nerves) might be examined in detail. This approach continues for the whole body until every system, including the nervous, skeletal, muscular, gastrointestinal, respiratory, lymphatic, and reproductive systems, has been studied.
Each of these approaches has benefits and deficiencies. The regional approach works very well if the anatomy course involves cadaver dissection but falls short when it comes to understanding the continuity of an entire system throughout the body. Similarly, the systemic approach fosters an understanding of an entire system throughout the body, but it is very difficult to coordinate this directly with a cadaver dissection or to acquire sufficient detail.
The anatomical position | Gray's Anatomy |
The anatomical position is the standard reference position of the body used to describe the location of structures (Fig. 1.1). The body is in the anatomical position when standing upright with feet together, hands by the side and face looking forward. The mouth is closed and the facial expression is neutral. The rim of bone under the eyes is in the same horizontal plane as the top of the opening to the ear, and the eyes are open and focused on something in the distance. The palms of the hands face forward with the fingers straight and together and with the pad of the thumb turned 90° to the pads of the fingers. The toes point forward.
Three major groups of planes pass through the body in the anatomical position (Fig. 1.1).
Coronal planes are oriented vertically and divide the body into anterior and posterior parts.
Sagittal planes also are oriented vertically but are at right angles to the coronal planes and divide the body into right and left parts. The plane that passes through the center of the body dividing it into equal right and left halves is termed the median sagittal plane.
Transverse, horizontal, or axial planes divide the body into superior and inferior parts.
Terms to describe location
Anterior (ventral) and posterior (dorsal), medial and lateral, superior and inferior
Three major pairs of terms are used to describe the location of structures relative to the body as a whole or to other structures (Fig. 1.1).
Anterior (or ventral) and posterior (or dorsal) describe the position of structures relative to the “front” and “back” of the body. For example, the nose is an anterior (ventral) structure, whereas the vertebral column is a posterior (dorsal) structure. Also, the nose is anterior to the ears and the vertebral column is posterior to the sternum.
Medial and lateral describe the position of structures relative to the median sagittal plane and the sides of the body. For example, the thumb is lateral to the little finger. The nose is in the median sagittal plane and is medial to the eyes, which are in turn medial to the external ears.
Superior and inferior describe structures in reference to the vertical axis of the body. For example, the head is superior to the shoulders and the knee joint is inferior to the hip joint.
Proximal and distal, cranial and caudal,
Other terms used to describe positions include proximal and distal, cranial and caudal, and rostral.
Proximal and distal are used with reference to being closer to or farther from a structure’s origin, particularly in the limbs. For example, the hand is distal to the elbow joint. The glenohumeral joint is proximal to the elbow joint. These terms are also used to describe the relative positions of branches along the course of linear structures, such as airways, vessels, and nerves. For example, distal branches occur farther away toward the ends of the system, whereas proximal branches occur closer to and toward the origin of the system.
Cranial (toward the head) and caudal (toward the tail) are sometimes used instead of superior and inferior, respectively.
Rostral is used, particularly in the head, to describe the position of a structure with reference to the nose. For example, the forebrain is rostral to the hindbrain.
Two other terms used to describe the position of structures in the body are superficial and deep. These terms are used to describe the relative positions of two structures with respect to the surface of the body. For example, the sternum is superficial to the heart, and the stomach is deep to the abdominal wall.
Superficial and deep can also be used in a more absolute fashion to define two major regions of the body. The superficial region of the body is external to the outer layer of deep fascia. Deep structures are enclosed by this layer. Structures in the superficial region of the body include the skin, superficial fascia, and mammary glands. Deep structures include most skeletal muscles and viscera. Superficial wounds are external to the outer layer of deep fascia, whereas deep wounds penetrate through it. | Gray's Anatomy |
In 1895 Wilhelm Roentgen used the X-rays from a cathode ray tube to expose a photographic plate and produce the first radiographic exposure of his wife’s hand. Over the past 35 years there has been a revolution in body imaging, which has been paralleled by developments in computer technology.
X-rays are photons (a type of electromagnetic radiation) and are generated from a complex X-ray tube, which is a type of cathode ray tube (Fig. 1.2). The X-rays are then collimated (i.e., directed through lead-lined shutters to stop them from fanning out) to the appropriate area of the body. As the X-rays pass through the body they are attenuated (reduced in energy) by the tissues. Those X-rays that pass through the tissues interact with the photographic film.
In the body: air attenuates X-rays a little; fat attenuates X-rays more than air but less than bone attenuates X-rays the most.
These differences in attenuation result in differences in the level of exposure of the film. When the photographic film is developed, bone appears white on the film because this region of the film has been exposed to the least amount of X-rays. Air appears dark on the film because these regions were exposed to the greatest number of X-rays.
Modifications to this X-ray technique allow a continuous stream of X-rays to be produced from the X-ray tube and collected on an input screen to allow real-time visualization of moving anatomical structures, barium studies, angiography, and fluoroscopy (Fig. 1.3).
To demonstrate specific structures, such as bowel loops or arteries, it may be necessary to fill these structures with a substance that attenuates X-rays more than bowel loops or arteries do normally. It is, however, extremely important that these substances are nontoxic. Barium sulfate, an insoluble salt, is a nontoxic, relatively high-density agent that is extremely useful in the examination of the gastrointestinal tract. When a barium sulfate suspension is ingested it attenuates X-rays and can therefore be used to demonstrate the bowel lumen (Fig. 1.4). It is common to add air to the barium sulfate suspension, by either ingesting “fizzy” granules or directly instilling air into the body cavity, as in a barium enema. This is known as a double-contrast (air/barium) study.
For some patients it is necessary to inject contrast agents directly into arteries or veins. In this case, iodine-based molecules are suitable contrast agents. Iodine is chosen because it has a relatively high atomic mass and so markedly attenuates X-rays, but also, importantly, it is naturally excreted via the urinary system. Intra-arterial and intravenous contrast agents are extremely safe and are well tolerated by most patients. Rarely, some patients have an anaphylactic reaction to intra-arterial or intravenous injections, so the necessary precautions must be taken. Intra-arterial and intravenous contrast agents not only help in visualizing the arteries and veins but because they are excreted by the urinary system, can also be used to visualize the kidneys, ureter, and bladder in a process known as intravenous urography.
During angiography it is often difficult to appreciate the contrast agent in the vessels through the overlying bony structures. To circumvent this, the technique of subtraction angiography has been developed. Simply, one or two images are obtained before the injection of contrast media. These images are inverted (such that a negative is created from the positive image). After injection of the contrast media into the vessels, a further series of images are obtained, demonstrating the passage of the contrast through the arteries into the veins and around the circulation. By adding the “negative precontrast image” to the positive postcontrast images, the bones and soft tissues are subtracted to produce a solitary image of contrast only. Before the advent of digital imaging this was a challenge, but now the use of computers has made this technique relatively straightforward and instantaneous (Fig. 1.5). | Gray's Anatomy |
Ultrasonography of the body is widely used for all aspects of medicine.
Ultrasound is a very high frequency sound wave (not electromagnetic radiation) generated by piezoelectric materials, such that a series of sound waves is produced. Importantly, the piezoelectric material can also receive the sound waves that bounce back from the internal organs. The sound waves are then interpreted by a powerful computer, and a real-time image is produced on the display panel.
Developments in ultrasound technology, including the size of the probes and the frequency range, mean that a broad range of areas can now be scanned.
Traditionally ultrasound is used for assessing the abdomen (Fig. 1.6) and the fetus in pregnant women. Ultrasound is also widely used to assess the eyes, neck, soft tissues, and peripheral musculoskeletal system. Probes have been placed on endoscopes, and endoluminal ultrasound of the esophagus, stomach, and duodenum is now routine. Endocavity ultrasound is carried out most commonly to assess the genital tract in women using a transvaginal or transrectal route. In men, transrectal ultrasound is the imaging method of choice to assess the prostate in those with suspected prostate hypertrophy or malignancy.
Doppler ultrasound enables determination of flow, its direction, and its velocity within a vessel using simple ultrasound techniques. Sound waves bounce off moving structures and are returned. The degree of frequency shift determines whether the object is moving away from or toward the probe and the speed at which it is traveling. Precise measurements of blood flow and blood velocity can therefore be obtained, which in turn can indicate sites of blockage in blood vessels.
Computed tomography (CT) was invented in the 1970s by Sir Godfrey Hounsfield, who was awarded the Nobel Prize in Medicine in 1979. Since this inspired invention there have been many generations of CT scanners.
A CT scanner obtains a series of images of the body (slices) in the axial plane. The patient lies on a bed, an X-ray tube passes around the body (Fig. 1.7), and a series of images are obtained. A computer carries out a complex mathematical transformation on the multitude of images to produce the final image (Fig. 1.8).
Nuclear magnetic resonance imaging was first described in 1946 and used to determine the structure of complex molecules. The process of magnetic resonance imaging (MRI) is dependent on the free protons in the hydrogen nuclei in molecules of water (H2O). Because water is present in almost all biological tissues, the hydrogen proton is ideal. The protons within a patient’s hydrogen nuclei can be regarded as small bar magnets, which are randomly oriented in space. The patient is placed in a strong magnetic field, which aligns the bar magnets. When a pulse of radio waves is passed through the patient the magnets are deflected, and as they return to their aligned position they emit small radio pulses. The strength and frequency of the emitted pulses and the time it takes for the protons to return to their pre-excited state produce a signal. These signals are analyzed by a powerful computer, and an image is created (Fig. 1.9).
By altering the sequence of pulses to which the protons are subjected, different properties of the protons can be assessed. These properties are referred to as the “weighting” of the scan. By altering the pulse sequence and the scanning parameters, T1-weighted images (Fig. 1.10A) and T2-weighted images (Fig. 1.10B) can be obtained. These two types of imaging sequences provide differences in image contrast, which accentuate and optimize different tissue characteristics.
From the clinical point of view:
Most T1-weighted images show dark fluid and bright fat—for example, within the brain the cerebrospinal fluid (CSF) is dark.
T2-weighted images demonstrate a bright signal from fluid and an intermediate signal from fat—for example, in the brain the CSF appears white.
MRI can also be used to assess flow within vessels and to produce complex angiograms of the peripheral and cerebral circulation. | Gray's Anatomy |
Diffusion-weighted imaging provides information on the degree of Brownian motion of water molecules in various tissues. There is relatively free diffusion in extracellular spaces and more restricted diffusion in intracellular spaces. In tumors and infarcted tissue, there is an increase in intracellular fluid water molecules compared with the extracellular fluid environment resulting in overall increased restricted diffusion, and therefore identification of abnormal from normal tissue.
Nuclear medicine involves imaging using gamma rays, which are another type of electromagnetic radiation.
The important difference between gamma rays and
X-rays is that gamma rays are produced from within the nucleus of an atom when an unstable nucleus decays, whereas X-rays are produced by bombarding an atom with electrons.
For an area to be visualized, the patient must receive a gamma ray emitter, which must have a number of properties to be useful, including: a reasonable half-life (e.g., 6 to 24 hours), an easily measurable gamma ray, and energy deposition in as low a dose as possible in the patient’s tissues.
The most commonly used radionuclide (radioisotope) is technetium-99m. This may be injected as a technetium salt or combined with other complex molecules. For example, by combining technetium-99m with methylene diphosphonate (MDP), a radiopharmaceutical is produced. When injected into the body this radiopharmaceutical specifically binds to bone, allowing assessment of the skeleton. Similarly, combining technetium-99m with other compounds permits assessment of other parts of the body, for example the urinary tract and cerebral blood flow.
Depending on how the radiopharmaceutical is absorbed, distributed, metabolized, and excreted by the body after injection, images are obtained using a gamma camera (Fig. 1.11).
Positron emission tomography (PET) is an imaging modality for detecting positron-emitting radionuclides. A positron is an anti-electron, which is a positively charged particle of antimatter. Positrons are emitted from the decay of proton-rich radionuclides. Most of these radionuclides are made in a cyclotron and have extremely short half-lives.
The most commonly used PET radionuclide is fluorodeoxyglucose (FDG) labeled with fluorine-18 (a positron emitter). Tissues that are actively metabolizing glucose take up this compound, and the resulting localized high concentration of this molecule compared to background emission is detected as a “hot spot.”
PET has become an important imaging modality in the detection of cancer and the assessment of its treatment and recurrence.
Single photon emission computed tomography (SPECT) is an imaging modality for detecting gamma rays emitted from the decay of injected radionuclides such as technetium-99m, iodine-123, or iodine-131. The rays are detected by a 360-degree rotating camera, which allows the construction of 3D images. SPECT can be used to diagnose a wide range of disease conditions such as coronary artery disease and bone fractures.
Imaging is necessary in most clinical specialties to diagnose pathological changes to tissues. It is paramount to appreciate what is normal and what is abnormal. An appreciation of how the image is obtained, what the normal variations are, and what technical considerations are necessary to obtain a radiological diagnosis. Without understanding the anatomy of the region imaged, it is impossible to comment on the abnormal.
Plain radiographs are undoubtedly the most common form of image obtained in a hospital or local practice. Before interpretation, it is important to know about the imaging technique and the views obtained as standard. | Gray's Anatomy |
In most instances (apart from chest radiography) the X-ray tube is 1 m away from the X-ray film. The object in question, for example a hand or a foot, is placed upon the film. When describing subject placement for radiography, the part closest to the X-ray tube is referred to first and that closest to the film is referred to second. For example, when positioning a patient for an anteroposterior (AP) radiograph, the more anterior part of the body is closest to the tube and the posterior part is closest to the film.
When X-rays are viewed on a viewing box, the right side of the patient is placed to the observer’s left; therefore, the observer views the radiograph as though looking at a patient in the anatomical position.
The chest radiograph is one of the most commonly requested plain radiographs. An image is taken with the patient erect and placed posteroanteriorly (PA chest radiograph; that is, with the patient’s back closest to the X-ray tube.).
Occasionally, when patients are too unwell to stand erect, films are obtained on the bed in an anteroposterior (AP) position. These films are less standardized than PA films, and caution should always be taken when interpreting AP radiographs.
The plain chest radiograph should always be checked for quality. Film markers should be placed on the appropriate side. (Occasionally patients have dextrocardia, which may be misinterpreted if the film marker is placed inappropriately.) A good-quality chest radiograph will demonstrate the lungs, cardiomediastinal contour, diaphragm, ribs, and peripheral soft tissues.
Plain abdominal radiographs are obtained in the AP supine position. From time to time an erect plain abdominal radiograph is obtained when small bowel obstruction is suspected.
High-density contrast medium is ingested to opacify the esophagus, stomach, small bowel, and large bowel. As described previously (p. 6), the bowel is insufflated with air (or carbon dioxide) to provide a double-contrast study. In many countries, endoscopy has superseded upper gastrointestinal imaging, but the mainstay of imaging the large bowel is the double-contrast barium enema. Typically the patient needs to undergo bowel preparation, in which powerful cathartics are used to empty the bowel. At the time of the examination a small tube is placed into the rectum and a barium suspension is run into the large bowel. The patient undergoes a series of twists and turns so that the contrast passes through the entire large bowel. The contrast is emptied and air is passed through the same tube to insufflate the large bowel. A thin layer of barium coats the normal mucosa, allowing mucosal detail to be visualized (see Fig. 1.4).
Intravenous urography is the standard investigation for assessing the urinary tract. Intravenous contrast medium is injected, and images are obtained as the medium is excreted through the kidneys. A series of films are obtained during this period from immediately after the injection up to approximately 20 minutes later, when the bladder is full of contrast medium.
This series of radiographs demonstrates the kidneys, ureters, and bladder and enables assessment of the retroperitoneum and other structures that may press on the urinary tract.
Computed tomography is the preferred terminology rather than computerized tomography, though both terms are used interchangeably by physicians.
It is important for the student to understand the presentation of images. Most images are acquired in the axial plane and viewed such that the observer looks from below and upward toward the head (from the foot of the bed). By implication: the right side of the patient is on the left side of the image, and the uppermost border of the image is anterior.
Many patients are given oral and intravenous contrast media to differentiate bowel loops from other abdominal organs and to assess the vascularity of normal anatomical structures. When intravenous contrast is given, the earlier the images are obtained, the greater the likelihood of arterial enhancement. As the time is delayed between injection and image acquisition, a venous phase and an equilibrium phase are also obtained. | Gray's Anatomy |
The great advantage of CT scanning is the ability to extend and compress the gray scale to visualize the bones, soft tissues, and visceral organs. Altering the window settings and window centering provides the physician with specific information about these structures.
There is no doubt that MRI has revolutionized the understanding and interpretation of the brain and its coverings. Furthermore, it has significantly altered the practice of musculoskeletal medicine and surgery. Images can be obtained in any plane and in most sequences. Typically the images are viewed using the same principles as CT. Intravenous contrast agents are also used to further enhance tissue contrast. Typically, MRI contrast agents contain paramagnetic substances (e.g., gadolinium and manganese).
Most nuclear medicine images are functional studies. Images are usually interpreted directly from a computer, and a series of representative films are obtained for clinical use.
Whenever a patient undergoes an X-ray or nuclear medicine investigation, a dose of radiation is given (Table 1.1). As a general principle it is expected that the dose given is as low as reasonably possible for a diagnostic image to be obtained. Numerous laws govern the amount of radiation exposure that a patient can undergo for a variety of procedures, and these are monitored to prevent any excess or additional dosage. Whenever a radiograph is booked, the clinician ordering the procedure must appreciate its necessity and understand the dose given to the patient to ensure that the benefits significantly outweigh the risks.
Imaging modalities such as ultrasound and MRI are ideal because they do not impart significant risk to the patient. Moreover, ultrasound imaging is the modality of choice for assessing the fetus.
Any imaging device is expensive, and consequently the more complex the imaging technique (e.g., MRI) the more expensive the investigation. Investigations must be carried out judiciously, based on a sound clinical history and examination, for which an understanding of anatomy is vital.
The skeleton can be divided into two subgroups, the axial skeleton and the appendicular skeleton. The axial skeleton consists of the bones of the skull (cranium), vertebral column, ribs, and sternum, whereas the appendicular skeleton consists of the bones of the upper and lower limbs (Fig. 1.12).
The skeletal system consists of cartilage and bone.
Cartilage is an avascular form of connective tissue consisting of extracellular fibers embedded in a matrix that contains cells localized in small cavities. The amount and kind of extracellular fibers in the matrix varies depending on the type of cartilage. In heavy weightbearing areas or areas prone to pulling forces, the amount of collagen is greatly increased and the cartilage is almost inextensible. In contrast, in areas where weightbearing demands and stress are less, cartilage containing elastic fibers and fewer collagen fibers is common. The functions of cartilage are to: support soft tissues, provide a smooth, gliding surface for bone articulations at joints, and enable the development and growth of long bones.
There are three types of cartilage: hyaline—most common; matrix contains a moderate amount of collagen fibers (e.g., articular surfaces of bones); elastic—matrix contains collagen fibers along with a large number of elastic fibers (e.g., external ear); fibrocartilage—matrix contains a limited number of cells and ground substance amidst a substantial amount of collagen fibers (e.g., intervertebral discs).
Cartilage is nourished by diffusion and has no blood vessels, lymphatics, or nerves.
Bone is a calcified, living, connective tissue that forms the majority of the skeleton. It consists of an intercellular calcified matrix, which also contains collagen fibers, and several types of cells within the matrix. Bones function as: supportive structures for the body, protectors of vital organs, reservoirs of calcium and phosphorus, levers on which muscles act to produce movement, and containers for blood-producing cells. | Gray's Anatomy |
There are two types of bone, compact and spongy (trabecular or cancellous). Compact bone is dense bone that forms the outer shell of all bones and surrounds spongy bone. Spongy bone consists of spicules of bone enclosing cavities containing blood-forming cells (marrow). Classification of bones is by shape.
Long bones are tubular (e.g., humerus in upper limb; femur in lower limb).
Short bones are cuboidal (e.g., bones of the wrist and ankle).
Flat bones consist of two compact bone plates separated by spongy bone (e.g., skull).
Irregular bones are bones with various shapes (e.g., bones of the face).
Sesamoid bones are round or oval bones that develop in tendons.
Bones are vascular and are innervated. Generally, an adjacent artery gives off a nutrient artery, usually one per bone, that directly enters the internal cavity of the bone and supplies the marrow, spongy bone, and inner layers of compact bone. In addition, all bones are covered externally, except in the area of a joint where articular cartilage is present, by a fibrous connective tissue membrane called the periosteum, which has the unique capability of forming new bone. This membrane receives blood vessels whose branches supply the outer layers of compact bone. A bone stripped of its periosteum will not survive. Nerves accompany the vessels that supply the bone and the periosteum. Most of the nerves passing into the internal cavity with the nutrient artery are vasomotor fibers that regulate blood flow. Bone itself has few sensory nerve fibers. On the other hand, the periosteum is supplied with numerous sensory nerve fibers and is very sensitive to any type of injury.
Developmentally, all bones come from mesenchyme by either intramembranous ossification, in which mesenchymal models of bones undergo ossification, or endochondral ossification, in which cartilaginous models of bones form from mesenchyme and undergo ossification.
The sites where two skeletal elements come together are termed joints. The two general categories of joints (Fig. 1.18) are those in which: the skeletal elements are separated by a cavity (i.e., synovial joints), and there is no cavity and the components are held together by connective tissue (i.e., solid joints).
Blood vessels that cross over a joint and nerves that innervate muscles acting on a joint usually contribute articular branches to that joint.
Synovial joints are connections between skeletal components where the elements involved are separated by a narrow articular cavity (Fig. 1.19). In addition to containing an articular cavity, these joints have a number of characteristic features.
First, a layer of cartilage, usually hyaline cartilage, covers the articulating surfaces of the skeletal elements. In other words, bony surfaces do not normally contact one another directly. As a consequence, when these joints are viewed in normal radiographs, a wide gap seems to separate the adjacent bones because the cartilage that covers the articulating surfaces is more transparent to X-rays than bone.
A second characteristic feature of synovial joints is the presence of a joint capsule consisting of an inner synovial membrane and an outer fibrous membrane.
The synovial membrane attaches to the margins of the joint surfaces at the interface between the cartilage and bone and encloses the articular cavity. The synovial membrane is highly vascular and produces synovial fluid, which percolates into the articular cavity and lubricates the articulating surfaces. Closed sacs of synovial membrane also occur outside joints, where they form synovial bursae or tendon sheaths. Bursae often intervene between structures, such as tendons and bone, tendons and joints, or skin and bone, and reduce the friction of one structure moving over the other. Tendon sheaths surround tendons and also reduce friction. | Gray's Anatomy |
The fibrous membrane is formed by dense connective tissue and surrounds and stabilizes the joint. Parts of the fibrous membrane may thicken to form ligaments, which further stabilize the joint. Ligaments outside the capsule usually provide additional reinforcement.
Another common but not universal feature of synovial joints is the presence of additional structures within the area enclosed by the capsule or synovial membrane, such as articular discs (usually composed of fibrocartilage), fat pads, and tendons. Articular discs absorb compression forces, adjust to changes in the contours of joint surfaces during movements, and increase the range of movements that can occur at joints. Fat pads usually occur between the synovial membrane and the capsule and move into and out of regions as joint contours change during movement. Redundant regions of the synovial membrane and fibrous membrane allow for large movements at joints.
Descriptions of synovial joints based on shape and movement
Synovial joints are described based on shape and movement: based on the shape of their articular surfaces, synovial joints are described as plane (flat), hinge, pivot, bicondylar (two sets of contact points), condylar (ellipsoid), saddle, and ball and socket; based on movement, synovial joints are described as uniaxial (movement in one plane), biaxial (movement in two planes), and multiaxial (movement in three planes).
Hinge joints are uniaxial, whereas ball and socket joints are multiaxial.
Specific types of synovial joints (Fig. 1.20)
Plane joints—allow sliding or gliding movements when one bone moves across the surface of another (e.g., acromioclavicular joint)
Hinge joints—allow movement around one axis that passes transversely through the joint; permit flexion and extension (e.g., elbow [humero-ulnar] joint)
Pivot joints—allow movement around one axis that passes longitudinally along the shaft of the bone; permit rotation (e.g., atlanto-axial joint)
Bicondylar joints—allow movement mostly in one axis with limited rotation around a second axis; formed by two convex condyles that articulate with concave or flat surfaces (e.g., knee joint)
Condylar (ellipsoid) joints—allow movement around two axes that are at right angles to each other; permit flexion, extension, abduction, adduction, and circumduction (limited) (e.g., wrist joint)
Saddle joints—allow movement around two axes that are at right angles to each other; the articular surfaces are saddle shaped; permit flexion, extension, abduction, adduction, and circumduction (e.g., carpometacarpal joint of the thumb)
Ball and socket joints—allow movement around multiple axes; permit flexion, extension, abduction, adduction, circumduction, and rotation (e.g., hip
Solid joints are connections between skeletal elements where the adjacent surfaces are linked together either by fibrous connective tissue or by cartilage, usually fibrocartilage (Fig. 1.21). Movements at these joints are more restricted than at synovial joints.
Fibrous joints include sutures, gomphoses, and syndesmoses.
Sutures occur only in the skull where adjacent bones are linked by a thin layer of connective tissue termed a sutural ligament.
Gomphoses occur only between the teeth and adjacent bone. In these joints, short collagen tissue fibers in the periodontal ligament run between the root of the tooth and the bony socket.
Syndesmoses are joints in which two adjacent bones are linked by a ligament. Examples are the ligamentum flavum, which connects adjacent vertebral laminae, and an interosseous membrane, which links, for example, the radius and ulna in the forearm.
Cartilaginous joints include synchondroses and symphyses. | Gray's Anatomy |
Synchondroses occur where two ossification centers in a developing bone remain separated by a layer of cartilage, for example, the growth plate that occurs between the head and shaft of developing long bones. These joints allow bone growth and eventually become completely ossified.
Symphyses occur where two separate bones are interconnected by cartilage. Most of these types of joints occur in the midline and include the pubic symphysis between the two pelvic bones, and intervertebral discs between adjacent vertebrae.
The skin is the largest organ of the body. It consists of the epidermis and the dermis. The epidermis is the outer cellular layer of stratified squamous epithelium, which is avascular and varies in thickness. The dermis is a dense bed of vascular connective tissue.
The skin functions as a mechanical and permeability barrier, and as a sensory and thermoregulatory organ. It also can initiate primary immune responses.
Fascia is connective tissue containing varying amounts of fat that separate, support, and interconnect organs and structures, enable movement of one structure relative to another, and allow the transit of vessels and nerves from one area to another. There are two general categories of fascia: superficial and deep.
Superficial (subcutaneous) fascia lies just deep to and is attached to the dermis of the skin. It is made up of loose connective tissue usually containing a large amount of fat. The thickness of the superficial fascia (subcutaneous tissue) varies considerably, both from one area of the body to another and from one individual to another. The superficial fascia allows movement of the skin over deeper areas of the body, acts as a conduit for vessels and nerves coursing to and from the skin, and serves as an energy (fat) reservoir.
Deep fascia usually consists of dense, organized connective tissue. The outer layer of deep fascia is attached to the deep surface of the superficial fascia and forms a thin fibrous covering over most of the deeper region of the body. Inward extensions of this fascial layer form intermuscular septa that compartmentalize groups of muscles with similar functions and innervations. Other extensions surround individual muscles and groups of vessels and nerves, forming an investing fascia. Near some joints the deep fascia thickens, forming retinacula. These fascial retinacula hold tendons in place and prevent them from bowing during movements at the joints. Finally, there is a layer of deep fascia separating the membrane lining the abdominal cavity (the parietal peritoneum) from the fascia covering the deep surface of the muscles of the abdominal wall (the transversalis fascia). This layer is referred to as extraperitoneal fascia. A similar layer of fascia in the thorax is termed the endothoracic fascia.
The muscular system is generally regarded as consisting of one type of muscle found in the body—skeletal muscle. However, there are two other types of muscle tissue found in the body, smooth muscle and cardiac muscle, that are important components of other systems. These three types of muscle can be characterized by whether they are controlled voluntarily or involuntarily, whether they appear striated (striped) or smooth, and whether they are associated with the body wall (somatic) or with organs and blood vessels (visceral).
Skeletal muscle forms the majority of the muscle tissue in the body. It consists of parallel bundles of long, multinucleated fibers with transverse stripes, is capable of powerful contractions, and is innervated by somatic and branchial motor nerves. This muscle is used to move bones and other structures, and provides support and gives form to the body. Individual skeletal muscles are often named on the basis of shape (e.g., rhomboid major muscle), attachments (e.g., sternohyoid muscle), function (e.g., flexor pollicis longus muscle), position (e.g., palmar interosseous muscle), or fiber orientation (e.g., external oblique muscle). | Gray's Anatomy |
Cardiac muscle is striated muscle found only in the walls of the heart (myocardium) and in some of the large vessels close to where they join the heart. It consists of a branching network of individual cells linked electrically and mechanically to work as a unit. Its contractions are less powerful than those of skeletal muscle and it is resistant to fatigue. Cardiac muscle is innervated by visceral motor nerves.
Smooth muscle (absence of stripes) consists of elongated or spindle-shaped fibers capable of slow and sustained contractions. It is found in the walls of blood vessels (tunica media), associated with hair follicles in the skin, located in the eyeball, and found in the walls of various structures associated with the gastrointestinal, respiratory, genitourinary, and urogenital systems. Smooth muscle is innervated by visceral motor nerves.
The cardiovascular system consists of the heart, which pumps blood throughout the body, and the blood vessels, which are a closed network of tubes that transport the blood. There are three types of blood vessels: arteries, which transport blood away from the heart; veins, which transport blood toward the heart; capillaries, which connect the arteries and veins, are the smallest of the blood vessels and are where oxygen, nutrients, and wastes are exchanged within the tissues.
The walls of the blood vessels of the cardiovascular system usually consist of three layers or tunics: tunica externa (adventitia)—the outer connective tissue layer, tunica media—the middle smooth muscle layer (may also contain varying amounts of elastic fibers in medium and large arteries), and tunica intima—the inner endothelial lining of the blood vessels.
Arteries are usually further subdivided into three classes, according to the variable amounts of smooth muscle and elastic fibers contributing to the thickness of the tunica media, the overall size of the vessel, and its function.
Large elastic arteries contain substantial amounts of elastic fibers in the tunica media, allowing expansion and recoil during the normal cardiac cycle. This helps maintain a constant flow of blood during diastole. Examples of large elastic arteries are the aorta, the brachiocephalic trunk, the left common carotid artery, the left subclavian artery, and the pulmonary trunk.
Medium muscular arteries are composed of a tunica media that contains mostly smooth muscle fibers. This characteristic allows these vessels to regulate their diameter and control the flow of blood to different parts of the body. Examples of medium muscular arteries are most of the named arteries, including the femoral, axillary, and radial arteries.
Small arteries and arterioles control the filling of the capillaries and directly contribute to the arterial pressure in the vascular system.
Veins also are subdivided into three classes.
Large veins contain some smooth muscle in the tunica media, but the thickest layer is the tunica externa. Examples of large veins are the superior vena cava, the inferior vena cava, and the portal vein.
Small and medium veins contain small amounts of smooth muscle, and the thickest layer is the tunica externa. Examples of small and medium veins are superficial veins in the upper and lower limbs and deeper veins of the leg and forearm.
Venules are the smallest veins and drain the capillaries.
Although veins are similar in general structure to arteries, they have a number of distinguishing features.
The walls of veins, specifically the tunica media, are thin.
The luminal diameters of veins are large.
There often are multiple veins (venae comitantes) closely associated with arteries in peripheral regions.
Valves often are present in veins, particularly in peripheral vessels inferior to the level of the heart. These are usually paired cusps that facilitate blood flow toward the heart.
More specific information about the cardiovascular system and how it relates to the circulation of blood throughout the body will be discussed, where appropriate, in each of the succeeding chapters of the text. | Gray's Anatomy |
Lymphatic vessels form an extensive and complex interconnected network of channels, which begin as “porous” blind-ended lymphatic capillaries in tissues of the body and converge to form a number of larger vessels, which ultimately connect with large veins in the root of the neck.
Lymphatic vessels mainly collect fluid lost from vascular capillary beds during nutrient exchange processes and deliver it back to the venous side of the vascular system (Fig. 1.28). Also included in this interstitial fluid that drains into the lymphatic capillaries are pathogens, cells of the lymphocytic system, cell products (such as hormones), and cell debris.
In the small intestine, certain fats absorbed and processed by the intestinal epithelium are packaged into protein-coated lipid droplets (chylomicrons), which are released from the epithelial cells and enter the interstitial compartment. Together with other components of the interstitial fluid, the chylomicrons drain into lymphatic capillaries (known as lacteals in the small intestine) and are ultimately delivered to the venous system in the neck. The lymphatic system is therefore also a major route of transport for fat absorbed by the gut.
The fluid in most lymphatic vessels is clear and colorless and is known as lymph. That carried by lymphatic vessels from the small intestine is opaque and milky because of the presence of chylomicrons and is termed chyle.
There are lymphatic vessels in most areas of the body, including those associated with the central nervous system (Louveau A et al., Nature 2015; 523:337-41; Aspelund A et al., J Exp Med 2015; 212:991-9). Exceptions include bone marrow and avascular tissues such as epithelia and cartilage.
The movement of lymph through the lymphatic vessels is generated mainly by the indirect action of adjacent structures, particularly by contraction of skeletal muscles and pulses in arteries. Unidirectional flow is maintained by the presence of valves in the vessels.
Lymph nodes are small (0.1–2.5 cm long) encapsulated structures that interrupt the course of lymphatic vessels and contain elements of the body’s defense system, such as clusters of lymphocytes and macrophages. They act as elaborate filters that trap and phagocytose particulate matter in the lymph that percolates through them. In addition, they detect and defend against foreign antigens that are also carried in the lymph (Fig. 1.28).
Because lymph nodes are efficient filters and flow through them is slow, cells that metastasize from (migrate away from) primary tumors and enter lymphatic vessels often lodge and grow as secondary tumors in lymph nodes. Lymph nodes that drain regions that are infected or contain other forms of disease can enlarge or undergo certain physical changes, such as becoming “hard” or “tender.” These changes can be used by clinicians to detect pathologic changes or to track spread of disease.
A number of regions in the body are associated with clusters or a particular abundance of lymph nodes (Fig. 1.29). Not surprisingly, nodes in many of these regions drain the body’s surface, the digestive system, or the respiratory system. All three of these areas are high-risk sites for the entry of foreign pathogens.
Lymph nodes are abundant and accessible to palpation in the axilla, the groin and femoral region, and the neck. Deep sites that are not palpable include those associated with the trachea and bronchi in the thorax, and with the aorta and its branches in the abdomen.
All lymphatic vessels coalesce to form larger trunks or ducts, which drain into the venous system at sites in the neck where the internal jugular veins join the subclavian veins to form the brachiocephalic veins (Fig. 1.30): | Gray's Anatomy |
Lymph from the right side of the head and neck, the right upper limb, and the right side of the thorax is carried by lymphatic vessels that connect with veins on the right side of the neck.
Lymph from all other regions of the body is carried by lymphatic vessels that drain into veins on the left side of the neck.
Specific information about the organization of the lymphatic system in each region of the body is discussed in the appropriate chapter.
The nervous system can be separated into parts based on structure and on function: structurally, it can be divided into the central nervous system (CNS) and the peripheral nervous system (PNS) (Fig. 1.32); functionally, it can be divided into somatic and visceral parts.
The CNS is composed of the brain and spinal cord, both of which develop from the neural tube in the embryo.
The PNS is composed of all nervous structures outside the CNS that connect the CNS to the body. Elements of this system develop from neural crest cells and as outgrowths of the CNS. The PNS consists of the spinal and cranial nerves, visceral nerves and plexuses, and the enteric system. The detailed anatomy of a typical spinal nerve is described in Chapter 2, as is the way spinal nerves are numbered. Cranial nerves are described in Chapter 8.
The details of nerve plexuses are described in chapters dealing with the specific regions in which the plexuses are located.
The parts of the brain are the cerebral hemispheres, the cerebellum, and the brainstem. The cerebral hemispheres consist of an outer portion, or the gray matter, containing cell bodies; an inner portion, or the white matter, made up of axons forming tracts or pathways; and the ventricles, which are spaces filled with CSF.
The cerebellum has two lateral lobes and a midline portion. The components of the brainstem are classically defined as the diencephalon, midbrain, pons, and medulla. However, in common usage today, the term “brainstem” usually refers to the midbrain, pons, and medulla.
A further discussion of the brain can be found in
Chapter 8.
The spinal cord is the part of the CNS in the superior two thirds of the vertebral canal. It is roughly cylindrical in shape, and is circular to oval in cross section with a central canal. A further discussion of the spinal cord can be found in Chapter 2.
The meninges (Fig. 1.33) are three connective tissue coverings that surround, protect, and suspend the brain and spinal cord within the cranial cavity and vertebral canal, respectively:
The dura mater is the thickest and most external of the coverings.
The arachnoid mater is against the internal surface of the dura mater.
The pia mater is adherent to the brain and spinal cord.
Between the arachnoid and pia mater is the subarachnoid space, which contains CSF.
A further discussion of the cranial meninges can be found in Chapter 8 and of the spinal meninges in Chapter 2.
Functional subdivisions of the CNS
Functionally, the nervous system can be divided into somatic and visceral parts.
The somatic part (soma, from the Greek for “body”) innervates structures (skin and most skeletal muscle) derived from somites in the embryo, and is mainly involved with receiving and responding to information from the external environment.
The visceral part (viscera, from the Greek for “guts”) innervates organ systems in the body and other visceral elements, such as smooth muscle and glands, in peripheral regions of the body. It is concerned mainly with detecting and responding to information from the internal environment.
Somatic part of the nervous system
The somatic part of the nervous system consists of: nerves that carry conscious sensations from peripheral regions back to the CNS, and nerves that innervate voluntary muscles. | Gray's Anatomy |
Somatic nerves arise segmentally along the developing CNS in association with somites, which are themselves arranged segmentally along each side of the neural tube (Fig. 1.34). Part of each somite (the dermatomyotome) gives rise to skeletal muscle and the dermis of the skin. As cells of the dermatomyotome differentiate, they migrate into posterior (dorsal) and anterior (ventral) areas of the developing body:
Cells that migrate anteriorly give rise to muscles of the limbs and trunk (hypaxial muscles) and to the associated dermis.
Cells that migrate posteriorly give rise to the intrinsic muscles of the back (epaxial muscles) and the associated dermis.
Developing nerve cells within anterior regions of the neural tube extend processes peripherally into posterior and anterior regions of the differentiating dermatomyotome of each somite.
Simultaneously, derivatives of neural crest cells (cells derived from neural folds during formation of the neural tube) differentiate into neurons on each side of the neural tube and extend processes both medially and laterally (Fig. 1.35):
Medial processes pass into the posterior aspect of the neural tube.
Lateral processes pass into the differentiating regions of the adjacent dermatomyotome.
Neurons that develop from cells within the spinal cord are motor neurons and those that develop from neural crest cells are sensory neurons.
Somatic sensory and somatic motor fibers that are organized segmentally along the neural tube become parts of all spinal nerves and some cranial nerves.
The clusters of sensory nerve cell bodies derived from neural crest cells and located outside the CNS form sensory ganglia.
Generally, all sensory information passes into the posterior aspect of the spinal cord, and all motor fibers leave anteriorly.
Somatic sensory neurons carry information from the periphery into the CNS and are also called somatic sensory afferents or general somatic afferents (GSAs). The modalities carried by these nerves include temperature, pain, touch, and proprioception. Proprioception is the sense of determining the position and movement of the musculoskeletal system detected by special receptors in muscles and tendons.
Somatic motor fibers carry information away from the CNS to skeletal muscles and are also called somatic motor efferents or general somatic efferents (GSEs). Like somatic sensory fibers that come from the periphery, somatic motor fibers can be very long. They extend from cell bodies in the spinal cord to the muscle cells they innervate.
Because cells from a specific somite develop into the dermis of the skin in a precise location, somatic sensory fibers originally associated with that somite enter the posterior region of the spinal cord at a specific level and become part of one specific spinal nerve (Fig. 1.36). Each spinal nerve therefore carries somatic sensory information from a specific area of skin on the surface of the body. A dermatome is that area of skin supplied by a single spinal cord level, or on one side, by a single spinal nerve.
There is overlap in the distribution of dermatomes, but usually a specific region within each dermatome can be identified as an area supplied by a single spinal cord level. Testing touch in these autonomous zones in a conscious patient can be used to localize lesions to a specific spinal nerve or to a specific level in the spinal cord.
Somatic motor nerves that were originally associated with a specific somite emerge from the anterior region of the spinal cord and, together with sensory nerves from the same level, become part of one spinal nerve. Therefore each spinal nerve carries somatic motor fibers to muscles that originally developed from the related somite. A myotome is that portion of a skeletal muscle innervated by a single spinal cord level or, on one side, by a single spinal nerve.
Myotomes are generally more difficult to test than dermatomes because each skeletal muscle in the body often develops from more than one somite and is therefore innervated by nerves derived from more than one spinal cord level (Fig. 1.37).
Testing movements at successive joints can help in localizing lesions to specific nerves or to a specific spinal cord level. For example: | Gray's Anatomy |
Muscles that move the shoulder joint are innervated mainly by spinal nerves from spinal cord levels C5 and C6.
Muscles that move the elbow are innervated mainly by spinal nerves from spinal cord levels C6 and C7.
Muscles in the hand are innervated mainly by spinal nerves from spinal cord levels C8 and T1.
Visceral part of the nervous system
The visceral part of the nervous system, as in the somatic part, consists of motor and sensory components:
Sensory nerves monitor changes in the viscera.
Motor nerves mainly innervate smooth muscle, cardiac muscle, and glands.
The visceral motor component is commonly referred to as the autonomic division of the PNS and is subdivided into sympathetic and parasympathetic parts.
Like the somatic part of the nervous system, the visceral part is segmentally arranged and develops in a parallel fashion (Fig. 1.39).
Visceral sensory neurons that arise from neural crest cells send processes medially into the adjacent neural tube and laterally into regions associated with the developing body. These sensory neurons and their processes, referred to as general visceral afferent fibers (GVAs), are associated primarily with chemoreception, mechanoreception, and stretch reception.
Visceral motor neurons that arise from cells in lateral regions of the neural tube send processes out of the anterior aspect of the tube. Unlike in the somatic part, these processes, containing general visceral efferent fibers (GVEs), synapse with other cells, usually other visceral motor neurons, that develop outside the CNS from neural crest cells that migrate away from their original positions close to the developing neural tube.
The visceral motor neurons located in the spinal cord are referred to as preganglionic motor neurons and their axons are called preganglionic fibers; the visceral motor neurons located outside the CNS are referred to as postganglionic motor neurons and their axons are called postganglionic fibers.
The cell bodies of the visceral motor neurons outside the CNS often associate with each other in a discrete mass called a ganglion.
Visceral sensory and motor fibers enter and leave the CNS with their somatic equivalents (Fig. 1.40). Visceral sensory fibers enter the spinal cord together with somatic sensory fibers through posterior roots of spinal nerves. Preganglionic fibers of visceral motor neurons exit the spinal cord in the anterior roots of spinal nerves, along with fibers from somatic motor neurons.
Postganglionic fibers traveling to visceral elements in the periphery are found in the posterior and anterior rami (branches) of spinal nerves.
Visceral motor and sensory fibers that travel to and from viscera form named visceral branches that are separate from the somatic branches. These nerves generally form plexuses from which arise branches to the viscera.
Visceral motor and sensory fibers do not enter and leave the CNS at all levels (Fig. 1.41):
In the cranial region, visceral components are associated with four of the twelve cranial nerves (CN III, VII, IX, and X).
In the spinal cord, visceral components are associated mainly with spinal cord levels T1 to L2 and S2 to S4.
Visceral motor components associated with spinal levels T1 to L2 are termed sympathetic. Those visceral motor components in cranial and sacral regions, on either side of the sympathetic region, are termed parasympathetic:
The sympathetic system innervates structures in peripheral regions of the body and viscera.
The parasympathetic system is more restricted to innervation of the viscera only. | Gray's Anatomy |
Spinal sympathetic and spinal parasympathetic neurons share certain developmental and phenotypic features that are different from those of cranial parasympathetic neurons. Based on this, some researchers have suggested reclassifying all spinal visceral motor neurons as sympathetic (Espinosa-Medina I et al. Science 2016;354:893-897). Others are against reclassification, arguing that the results only indicate that the neurons are spinal in origin (Neuhuber W et al. Anat Rec 2017;300:1369-1370). In addition, sacral nerves do not enter the sympathetic trunk, nor do they have postganglionic fibers that travel to the periphery on spinal nerves, as do T1-L2 visceral motor fibers. We have chosen to retain the classification of S2,3,4 visceral motor neurons as parasympathetic. “Parasympathetic” simply means on either side of the “sympathetic,” which correctly describes their anatomy.
The sympathetic part of the autonomic division of the PNS leaves thoracolumbar regions of the spinal cord with the somatic components of spinal nerves T1 to L2 (Fig. 1.42). On each side, a paravertebral sympathetic trunk extends from the base of the skull to the inferior end of the vertebral column where the two trunks converge anteriorly to the coccyx at the ganglion impar. Each trunk is attached to the anterior rami of spinal nerves and becomes the route by which sympathetics are distributed to the periphery and all viscera.
Visceral motor preganglionic fibers leave the T1 to L2 part of the spinal cord in anterior roots. The fibers then enter the spinal nerves, pass through the anterior rami and into the sympathetic trunks. One trunk is located on each side of the vertebral column (paravertebral) and positioned anterior to the anterior rami. Along the trunk is a series of segmentally arranged ganglia formed from collections of postganglionic neuronal cell bodies where the preganglionic neurons synapse with postganglionic neurons. Anterior rami of T1 to L2 are connected to the sympathetic trunk or to a ganglion by a white ramus communicans, which carries preganglionic sympathetic fibers and appears white because the fibers it contains are myelinated.
Preganglionic sympathetic fibers that enter a paravertebral ganglion or the sympathetic trunk through a white ramus communicans may take the following four pathways to target tissues: 1. Peripheral sympathetic innervation at the level of origin of the preganglionic fiber
Preganglionic sympathetic fibers may synapse with postganglionic motor neurons in ganglia associated with the sympathetic trunk, after which postganglionic fibers enter the same anterior ramus and are distributed with peripheral branches of the posterior and anterior rami of that spinal nerve (Fig. 1.43). The fibers innervate structures at the periphery of the body in regions supplied by the spinal nerve. The gray ramus communicans connects the sympathetic trunk or a ganglion to the anterior ramus and contains the postganglionic sympathetic fibers. It appears gray because postganglionic fibers are nonmyelinated. The gray ramus communicans is positioned medial to the white ramus communicans.
2. Peripheral sympathetic innervation above or below the level of origin of the preganglionic fiber
Preganglionic sympathetic fibers may ascend or descend to other vertebral levels where they synapse in ganglia associated with spinal nerves that may or may not have visceral motor input directly from the spinal cord (i.e., those nerves other than T1 to L2) (Fig. 1.44).
The postganglionic fibers leave the distant ganglia via gray rami communicantes and are distributed along the posterior and anterior rami of the spinal nerves. | Gray's Anatomy |
The ascending and descending fibers, together with all the ganglia, form the paravertebral sympathetic trunk, which extends the entire length of the vertebral column. The formation of this trunk, on each side, enables visceral motor fibers of the sympathetic part of the autonomic division of the PNS, which ultimately emerge from only a small region of the spinal cord (T1 to L2), to be distributed to peripheral regions innervated by all spinal nerves.
White rami communicantes only occur in association with spinal nerves T1 to L2, whereas gray rami communicantes are associated with all spinal nerves.
Fibers from spinal cord levels T1 to T5 pass predominantly superiorly, whereas fibers from T5 to L2 pass inferiorly. All sympathetics passing into the head have preganglionic fibers that emerge from spinal cord level
T1 and ascend in the sympathetic trunks to the highest ganglion in the neck (the superior cervical ganglion), where they synapse. Postganglionic fibers then travel along blood vessels to target tissues in the head, including blood vessels, sweat glands, small smooth muscles associated with the upper eyelids, and the dilator of the pupil.
3. Sympathetic innervation of thoracic
Preganglionic sympathetic fibers may synapse with postganglionic motor neurons in ganglia and then leave the ganglia medially to innervate thoracic or cervical viscera (Fig. 1.45). They may ascend in the trunk before synapsing, and after synapsing the postganglionic fibers may combine with those from other levels to form named visceral nerves, such as cardiac nerves. Often, these nerves join branches from the parasympathetic system to form plexuses on or near the surface of the target organ, for example, the cardiac and pulmonary plexuses. Branches of the plexus innervate the organ. Spinal cord levels T1 to T5 mainly innervate cranial, cervical, and thoracic viscera.
4. Sympathetic innervation of the abdomen and pelvic regions and the adrenals
Preganglionic sympathetic fibers may pass through the sympathetic trunk and paravertebral ganglia without synapsing and, together with similar fibers from other levels, form splanchnic nerves (greater, lesser, least, lumbar, and sacral), which pass into the abdomen and pelvic regions (Fig. 1.46). The preganglionic fibers in these nerves are derived from spinal cord levels T5 to L2.
The splanchnic nerves generally connect with sympathetic ganglia around the roots of major arteries that branch from the abdominal aorta. These ganglia are part of a large prevertebral plexus that also has input from the parasympathetic part of the autonomic division of the PNS. Postganglionic sympathetic fibers are distributed in extensions of this plexus, predominantly along arteries, to viscera in the abdomen and pelvis.
Some of the preganglionic fibers in the prevertebral plexus do not synapse in the sympathetic ganglia of the plexus but pass through the system to the adrenal gland, where they synapse directly with cells of the adrenal medulla. These cells are homologues of sympathetic postganglionic neurons and secrete adrenaline and noradrenaline into the vascular system.
The parasympathetic part of the autonomic division of the PNS (Fig. 1.47) leaves cranial and sacral regions of the CNS in association with: cranial nerves III, VII, IX, and X: III, VII, and IX carry parasympathetic fibers to structures within the head and neck only, whereas X (the vagus spinal nerves S2 to S4: sacral parasympathetic fibers innervate inferior abdominal viscera, pelvic viscera, and the arteries associated with erectile tissues of the perineum. | Gray's Anatomy |
Like the visceral motor nerves of the sympathetic part, the visceral motor nerves of the parasympathetic part generally have two neurons in the pathway. The preganglionic neurons are in the CNS, and fibers leave in the cranial nerves.
In the sacral region, the preganglionic parasympathetic fibers form special visceral nerves (the pelvic splanchnic nerves), which originate from the anterior rami of S2 to S4 and enter pelvic extensions of the large prevertebral plexus formed around the abdominal aorta. These fibers are distributed to pelvic and abdominal viscera mainly along blood vessels. The postganglionic motor neurons are in the walls of the viscera. In organs of the gastrointestinal system, preganglionic fibers do not have a postganglionic parasympathetic motor neuron in the pathway; instead, preganglionic fibers synapse directly on neurons in the ganglia of the enteric system.
The preganglionic parasympathetic motor fibers in CN
III, VII, and IX separate from the nerves and connect with one of four distinct ganglia, which house postganglionic motor neurons. These four ganglia are near major branches of CN V. Postganglionic fibers leave the ganglia, join the branches of CN V, and are carried to target tissues (salivary, mucous, and lacrimal glands; constrictor muscle of the pupil; and ciliary muscle in the eye) with these branches.
The vagus nerve [X] gives rise to visceral branches along its course. These branches contribute to plexuses associated with thoracic viscera or to the large prevertebral plexus in the abdomen and pelvis. Many of these plexuses also contain sympathetic fibers.
When present, postganglionic parasympathetic neurons are in the walls of the target viscera.
motor fibers.
Visceral sensory fibers follow the course of sympathetic fibers entering the spinal cord at similar spinal cord levels. However, visceral sensory fibers may also enter the spinal cord at levels other than those associated with motor output. For example, visceral sensory fibers from the heart may enter at levels higher than spinal cord level T1. Visceral sensory fibers that accompany sympathetic fibers are mainly concerned with detecting pain.
Visceral sensory fibers accompanying parasympathetic fibers are carried mainly in IX and X and in spinal nerves S2 to S4.
Visceral sensory fibers in IX carry information from chemoreceptors and baroreceptors associated with the walls of major arteries in the neck, and from receptors in the pharynx.
Visceral sensory fibers in X include those from cervical viscera, and major vessels and viscera in the thorax and abdomen.
Visceral sensory fibers from pelvic viscera and the distal parts of the colon are carried in S2 to S4.
Visceral sensory fibers associated with parasympathetic fibers primarily relay information to the CNS about the status of normal physiological processes and reflex activities.
The enteric system
The enteric nervous system consists of motor and sensory neurons and their support cells, which form two interconnected plexuses, the myenteric and submucous nerve plexuses, within the walls of the gastrointestinal tract (Fig. 1.48). Each of these plexuses is formed by: ganglia, which house the nerve cell bodies and associated cells, and bundles of nerve fibers, which pass between ganglia and from the ganglia into surrounding tissues.
Neurons in the enteric system are derived from neural crest cells originally associated with occipitocervical and sacral regions. Interestingly, more neurons are reported to be in the enteric system than in the spinal cord itself.
Sensory and motor neurons within the enteric system control reflex activity within and between parts of the gastrointestinal system. These reflexes regulate peristalsis, secretomotor activity, and vascular tone. These activities can occur independently of the brain and spinal cord, but can also be modified by input from preganglionic parasympathetic and postganglionic sympathetic fibers. | Gray's Anatomy |
Sensory information from the enteric system is carried back to the CNS by visceral sensory fibers.
Nerve plexuses are either somatic or visceral and combine fibers from different sources or levels to form new nerves with specific targets or destinations (Fig. 1.49). Plexuses of the enteric system also generate reflex activity independent of the CNS.
Major somatic plexuses formed from the anterior rami of spinal nerves are the cervical (C1 to C4), brachial (C5 to T1), lumbar (L1 to L4), sacral (L4 to S4), and coccygeal (S5 to Co) plexuses. Except for spinal nerve T1, the anterior rami of thoracic spinal nerves remain independent and do not participate in plexuses.
Visceral nerve plexuses are formed in association with viscera and generally contain efferent (sympathetic and parasympathetic) and afferent components (Fig. 1.49). These plexuses include cardiac and pulmonary plexuses in the thorax and a large prevertebral plexus in the abdomen anterior to the aorta, which extends inferiorly onto the lateral walls of the pelvis. The massive prevertebral plexus supplies input to and receives output from all abdominal and pelvic viscera.
Specific information about the organization and components of the respiratory, gastrointestinal, and urogenital systems will be discussed in each of the succeeding chapters of this text.
Fig. 1.1 The anatomical position, planes, and terms of location and orientation.
Feet togethertoes forwardHands by sidespalms forwardFace looking forwardInferior margin of orbit level withtop of external auditory meatusSagittal planeCoronal planeSuperiorAnteriorPosteriorMedialLateralInferiorTransverse, horizontal,or axial plane
Fig. 1.2 Cathode ray tube for the production of X-rays.
Fig. 1.3 Fluoroscopy unit.
Fig. 1.4 Barium sulfate follow-through.
Fig. 1.5 Digital subtraction angiogram.
Fig. 1.6 Ultrasound examination of the abdomen.
Fig. 1.7 Computed tomography scanner.
Fig. 1.8 Computed tomography scan of the abdomen at vertebral level L2.
Fig. 1.9 A T2-weighted MR image in the sagittal plane of the pelvic viscera in a woman.
Fig. 1.10 T1-weighted (A) and T2-weighted (B) MR images of the brain in the coronal plane.
Fig. 1.11 A gamma camera.
Fig. 1.12 The axial skeleton and the appendicular skeleton.
Fig. 1.13 Accessory and sesamoid bones. A. Radiograph of the ankle region showing an accessory bone (os trigonum).
B. Radiograph of the feet showing numerous sesamoid bones and an accessory bone (os naviculare).
Fig. 1.14 A developmental series of radiographs showing the progressive ossification of carpal (wrist) bones from 3 (A) to 10 (D) years of age.
Fig. 1.15 T1-weighted image in the coronal plane, demonstrating the relatively high signal intensity returned from the femoral heads and proximal femoral necks, consistent with yellow marrow. In this young patient, the vertebral bodies return an intermediate darker signal that represents red marrow. There is relatively little fat in these vertebrae; hence the lower signal return.
Fig. 1.16 Radiograph, lateral view, showing fracture of the ulna at the elbow joint (A) and repair of this fracture (B) using internal fixation with a plate and multiple screws. | Gray's Anatomy |
Fig. 1.17 Image of the hip joints demonstrating loss of height of the right femoral head with juxta-articular bony sclerosis and subchondral cyst formation secondary to avascular necrosis. There is also significant wasting of the muscles supporting the hip, which is secondary to disuse and pain.
Normal left hipBladderAvascular necrosisWasting of gluteal muscle
Fig. 1.18 Joints. A. Synovial joint. B. Solid joint.
Fig. 1.19 Synovial joints. A. Major features of a synovial joint. B. Accessory structures associated with synovial joints.
Fig. 1.20 Various types of synovial joints. A. Condylar (wrist). B. Gliding (radio-ulnar). C. Hinge (elbow). D. Ball and socket (hip). E. Saddle (carpometacarpal of thumb). F. Pivot (atlanto-axial).
Fig. 1.21 Solid joints.
Fig. 1.22 This operative photograph demonstrates the focal areas of cartilage loss in the patella and femoral condyles throughout the knee joint.
Fig. 1.23 This radiograph demonstrates the loss of joint space in the medial compartment and presence of small spiky osteophytic regions at the medial lateral aspect of the joint.
OsteophytesLoss of joint space
Fig. 1.24 After knee replacement. This radiograph shows the position of the prosthesis.
Fig. 1.25 This is a radiograph, anteroposterior view, of the pelvis after a right total hip replacement. There are additional significant degenerative changes in the left hip joint, which will also need to be replaced.
Fig. 1.26 Axial inversion recovery MR imaging series, which suppresses fat and soft tissue and leaves high signal intensity where fluid is seen. A muscle tear in the right adductor longus with edema in and around the muscle is shown.
Fig. 1.27 Photograph demonstrating varicose veins.
Fig. 1.28 Lymphatic vessels mainly collect fluid lost from vascular capillary beds during nutrient exchange processes and deliver it back to the venous side of the vascular system.
Fig. 1.29 Regions associated with clusters or a particular abundance of lymph nodes.
Cervical nodes(along courseof internaljugular vein)Axillary nodes(in axilla)Deep nodes(related to aortaand celiac trunkand superior andinferior mesentericarteries)Pericranial ring(base of head)Tracheal nodes(nodes related totrachea and bronchi)Inguinal nodes(along course ofinguinal ligament)Femoral nodes(along femoral vein)
Fig. 1.30 Major lymphatic vessels that drain into large veins in the neck.
Fig. 1.31 A. This computed tomogram with contrast, in the axial plane, demonstrates the normal common carotid arteries and internal jugular veins with numerous other nonenhancing nodules that represent lymph nodes in a patient with lymphoma. B. This computed tomogram with contrast, in the axial plane, demonstrates a large anterior soft tissue mediastinal mass that represents a lymphoma.
Fig. 1.32 CNS and PNS.
Fig. 1.33 Arrangement of meninges in the cranial cavity.
Fig. 1.34 Differentiation of somites in a “tubular” embryo.
Fig. 1.35 Somatic sensory and motor neurons. Blue lines indicate motor nerves and red lines indicate sensory nerves.
Somatic sensory neurondeveloping from neural crest cellsEpaxial (back) musclesHypaxial musclesAxon of motor neuronprojects to muscle developingfrom dermatomyotomeSomatic motor neuroncell body in anterior regionof neural tube
Fig. 1.36 Dermatomes. | Gray's Anatomy |
C6 segment of spinal cordSpinal ganglionDermatomyotomeAutonomous region(where overlap ofdermatomes isleast likely)of C6 dermatome(pad of thumb)Skin on the lateral side of the forearm and on thethumb is innervated by C6 spinal level (spinal nerve).The dermis of the skin in this region develops from the somiteinitially associated with the C6 level of the developing spinal cordCaudalCranialSomite
Fig. 1.37 Myotomes.
C6 segment of spinal cordMuscles that abduct the arm are innervated by C5 and C6 spinal levels (spinal nerves) and develop from somites initially associated with C5 and C6 regions of developing spinal cordC5 segment of spinal cordDermatomyotomeSomite
Fig. 1.38 Dermatomes. A. Anterior view. B. Posterior view.
Fig. 1.39 Development of the visceral part of the nervous system.
Motor nerve endingassociated withblood vessels,sweat glands,arrector pili musclesat peripheryPart of neural crest developinginto spinal gangliaVisceral motor ganglionMotor nerve ending associated with visceraDeveloping gastrointestinal tractSensory nerve endingBody cavity(coelom)Visceral sensory neuron developsfrom neural crest and becomespart of spinal ganglionVisceral motorpreganglionicneuron in lateralregion of CNS(spinal cord)Postganglionic motor neuron is outside CNS.An aggregation of postganglionic neuronal cellbodies forms a peripheral visceral motor ganglion.
Fig. 1.40 Basic anatomy of a thoracic spinal nerve.
Fig. 1.41 Parts of the CNS associated with visceral motor components.
SympatheticT1 to L2spinal segmentsBrainstemcranial nervesIII, VII, IX, XS2 to S4spinal segmentsParasympathetic
Fig. 1.42 Sympathetic part of the autonomic division of the PNS.
Abdominal visceraHeartOrgansPeripheralSympathetic nerves followsomatic nerves to periphery(glands, smooth muscle)Pelvic visceraGanglion imparEsophageal plexusPrevertebral plexus
Fig. 1.43 Course of sympathetic fibers that travel to the periphery in the same spinal nerves in which they travel out of the spinal cord.
Gray ramus communicansT10 spinal nervePosteriorramusAnteriorramusPeripheral distribution of sympatheticscarried peripherally by terminal cutaneousbranches of spinal nerve T1 to L2Motor nerve to sweat glands,smooth muscle of bloodvessels, and arrector pilimuscles in the part of T10dermatome supplied by theanterior ramusT10 spinal segmentWhite ramus communicans
Fig. 1.44 Course of sympathetic nerves that travel to the periphery in spinal nerves that are not the ones through which they left the spinal cord.
Sympathetic paravertebral trunksPeripheral distribution ofascending sympatheticsPeripheral distribution ofdescending sympathetics(C1) C2 to C8T1 to L2L3 to CoWhite ramus communicansGray ramus communicansPosterior rootGray ramus communicansGray ramus communicansAnterior root
Fig. 1.45 Course of sympathetic nerves traveling to the heart.
Sympathetic cardiac nervesSympathetic cardiac nervesSympathetic trunkCardiac plexusT1 to T4CervicalWhite ramuscommunicansGray ramuscommunicans
Fig. 1.46 Course of sympathetic nerves traveling to abdominal and pelvic viscera. | Gray's Anatomy |
White ramus communicansGray ramus communicansSacral splanchnic nervesLumbar splanchnic nervesLeast splanchnic nervesLesser splanchnic nervesGreater splanchnic nervesPrevertebral plexusand gangliaParavertebralsympathetic trunkAbdominalandpelvic visceraAortaT5 to T9T12T9 to T10(T10 to T11)L1 to L2
Fig. 1.47 Parasympathetic part of the autonomic division of the PNS.
Thoracic visceral plexusPrevertebral plexusAbdominal visceraSynapse with nerve cellsof enteric systemErectile tissues of penisand clitorisS2 to S4Sacral parasympatheticoutflow via pelvicsplanchnic nervesCranial parasympatheticoutflow via cranial nervesHeartSubmandibularganglionPterygopalatineganglionOtic ganglionCiliary ganglion[III][VII][IX][X]Pelvic visceraPupillary constrictionTransition from supply by [X]to pelvic splanchnic nervesSalivary glandsLacrimal glandParotid gland
Fig. 1.48 Enteric part of the nervous system.
Fig. 1.49 Nerve plexuses.
C7C6C5C4C3C2C1T1T2T3T4T5T6T7T8T9T10T11T12L1S1S2S3S4S5L2L3L4L5C8GreaterLeastLesserSOMATIC PLEXUSESVISCERAL PLEXUSESCervical plexusanterior rami C1 to C4Brachial plexusanterior rami C5 to T1Lumbar plexusanterior rami L1 to L4Sacral plexusanterior ramiL4 to S4Parasympathetic [X]S2 to S4 pelvic splanchnic nerves(parasympathetic)Pulmonary branchPulmonary branchesCardiac branchesCardiac plexusThoracic aortic plexusEsophageal plexusPrevertebral plexusVagal trunkGanglion imparSacral splanchnic nervesSplanchnicnervesLumbar splanchnicnerves
Fig. 1.50 Mechanism for referred pain from an inflamed appendix to the T10 dermatome.
Table 1.1 The approximate dosage of radiation exposure as an order of magnitude
In the clinic
These are extra bones that are not usually found as part of the normal skeleton, but can exist as a normal variant in many people. They are typically found in multiple locations in the wrist and hands, ankles and feet (Fig. 1.13). These should not be mistaken for fractures on imaging.
Sesamoid bones are embedded within tendons, the largest of which is the patella. There are many other sesamoids in the body particularly in tendons of the hands and feet, and most frequently in flexor tendons of the thumb and big toe.
Degenerative and inflammatory changes of, as well as mechanical stresses on, the accessory bones and sesamoids can cause pain, which can be treated with physiotherapy and targeted steroid injections, but in some severe cases it may be necessary to surgically remove the bone.
In the clinic
Determination of skeletal age
Throughout life the bones develop in a predictable way to form the skeletally mature adult at the end of puberty. In western countries skeletal maturity tends to occur between the ages of 20 and 25 years. However, this may well vary according to geography and socioeconomic conditions. Skeletal maturity will also be determined by genetic factors and disease states. | Gray's Anatomy |
Up until the age of skeletal maturity, bony growth and development follows a typically predictable ordered state, which can be measured through either ultrasound, plain radiographs, or MRI scanning. Typically, the nondominant (left) hand is radiographed, and the radiograph is compared to a series of standard radiographs. From these images the bone age can be determined (Fig. 1.14).
In certain disease states, such as malnutrition and hypothyroidism, bony maturity may be slow. If the skeletal bone age is significantly reduced from the patient’s true age, treatment may be required.
In the healthy individual the bone age accurately represents the true age of the patient. This is important in determining the true age of the subject. This may also have medicolegal importance.
In the clinic
The bone marrow serves an important function. There are two types of bone marrow, red marrow (otherwise known as myeloid tissue) and yellow marrow. Red blood cells, platelets, and most white blood cells arise from within the red marrow. In the yellow marrow a few white cells are made; however, this marrow is dominated by large fat globules (producing its yellow appearance) (Fig. 1.15).
From birth most of the body’s marrow is red; however, as the subject ages, more red marrow is converted into yellow marrow within the medulla of the long and flat bones.
Bone marrow contains two types of stem cells. Hemopoietic stem cells give rise to the white blood cells, red blood cells, and platelets. Mesenchymal stem cells differentiate into structures that form bone, cartilage, and muscle.
There are a number of diseases that may involve the bone marrow, including infection and malignancy. In patients who develop a bone marrow malignancy (e.g., leukemia) it may be possible to harvest nonmalignant cells from the patient’s bone marrow or cells from another person’s bone marrow. The patient’s own marrow can be destroyed with chemotherapy or radiation and the new cells infused. This treatment is bone marrow transplantation.
In the clinic
Fractures occur in normal bone because of abnormal load or stress, in which the bone gives way (Fig. 1.16A). Fractures may also occur in bone that is of poor quality (osteoporosis); in such cases a normal stress is placed upon a bone that is not of sufficient quality to withstand this force and subsequently fractures.
In children whose bones are still developing, fractures may occur across the growth plate or across the shaft. These shaft fractures typically involve partial cortical disruption, similar to breaking a branch of a young tree; hence they are termed “greenstick” fractures.
After a fracture has occurred, the natural response is to heal the fracture. Between the fracture margins a blood clot is formed into which new vessels grow. A jelly-like matrix is formed, and further migration of collagen-producing cells occurs. On this soft tissue framework, calcium hydroxyapatite is produced by osteoblasts and forms insoluble crystals, and then bone matrix is laid down. As more bone is produced, a callus can be demonstrated forming across the fracture site.
Treatment of fractures requires a fracture line reduction. If this cannot be maintained in a plaster of Paris cast, it may require internal or external fixation with screws and metal rods (Fig. 1.16B).
In the clinic
Avascular necrosis is cellular death of bone resulting from a temporary or permanent loss of blood supply to that bone. Avascular necrosis may occur in a variety of medical conditions, some of which have an etiology that is less than clear. A typical site for avascular necrosis is a fracture across the femoral neck in an elderly patient. In these patients there is loss of continuity of the cortical medullary blood flow with loss of blood flow deep to the retinacular fibers. This essentially renders the femoral head bloodless; it subsequently undergoes necrosis and collapses (Fig. 1.17). In these patients it is necessary to replace the femoral head with a prosthesis.
In the clinic | Gray's Anatomy |
As the skeleton develops, there are stages of intense growth typically around the ages of 7 to 10 years and later in puberty. These growth spurts are associated with increased cellular activity around the growth plate between the head and shaft of a bone. This increase in activity renders the growth plates more vulnerable to injuries, which may occur from dislocation across a growth plate or fracture through a growth plate. Occasionally an injury may result in growth plate compression, destroying that region of the growth plate, which may result in asymmetrical growth across that joint region. All fractures across the growth plate must be treated with care and expediency, requiring fracture reduction.
In the clinic
Degenerative joint disease is commonly known as osteoarthritis or osteoarthrosis. The disorder is related to aging but not caused by aging. Typically there are decreases in water and proteoglycan content within the cartilage. The cartilage becomes more fragile and more susceptible to mechanical disruption (Fig. 1.22). As the cartilage wears, the underlying bone becomes fissured and also thickens. Synovial fluid may be forced into small cracks that appear in the bone’s surface, which produces large cysts. Furthermore, reactive juxta-articular bony nodules are formed (osteophytes) (Fig. 1.23). As these processes occur, there is slight deformation, which alters the biomechanical forces through the joint. This in turn creates abnormal stresses, which further disrupt the joint.
In the United States, osteoarthritis accounts for up to one-quarter of primary health care visits and is regarded as a significant problem.
The etiology of osteoarthritis is not clear; however, osteoarthritis can occur secondary to other joint diseases, such as rheumatoid arthritis and infection. Overuse of joints and abnormal strains, such as those experienced by people who play sports, often cause one to be more susceptible to chronic joint osteoarthritis.
Various treatments are available, including weight reduction, proper exercise, anti-inflammatory drug treatment, and joint replacement (Fig. 1.24).
Arthroscopy is a technique of visualizing the inside of a joint using a small telescope placed through a tiny incision in the skin. Arthroscopy can be performed in most joints. However, it is most commonly performed in the knee, shoulder, ankle, and hip joints.
Arthroscopy allows the surgeon to view the inside of the joint and its contents. Notably, in the knee, the menisci and the ligaments are easily seen, and it is possible using separate puncture sites and specific instruments to remove the menisci and replace the cruciate ligaments. The advantages of arthroscopy are that it is performed through small incisions, it enables patients to quickly recover and return to normal activity, and it only requires either a light anesthetic or regional anesthesia during the procedure.
In the clinic
Joint replacement is undertaken for a variety of reasons. These predominantly include degenerative joint disease and joint destruction. Joints that have severely degenerated or lack their normal function are painful. In some patients, the pain may be so severe that it prevents them from leaving the house and undertaking even the smallest of activities without discomfort.
Large joints are commonly affected, including the hip, knee, and shoulder. However, with ongoing developments in joint replacement materials and surgical techniques, even small joints of the fingers can be replaced.
Typically, both sides of the joint are replaced; in the hip joint the acetabulum will be reamed, and a plastic or metal cup will be introduced. The femoral component will be fitted precisely to the femur and cemented in place (Fig. 1.25).
Most patients derive significant benefit from joint replacement and continue to lead an active life afterward. In a minority of patients who have been fitted with a metal acetabular cup and metal femoral component, an aseptic lymphocyte-dominated vasculitis-associated lesion (ALVAL) may develop, possibly caused by a hypersensitivity response to the release of metal ions in adjacent tissues. These patients often have chronic pain and might need additional surgery to replace these joint replacements with safer models. | Gray's Anatomy |
In the clinic
The importance of fascias
A fascia is a thin band of tissue that surrounds muscles, bones, organs, nerves, and blood vessels and often remains uninterrupted as a 3D structure between tissues. It provides important support for tissues and can provide a boundary between structures.
Clinically, fascias are extremely important because they often limit the spread of infection and malignant disease. When infections or malignant diseases cross a fascial plain, a primary surgical clearance may require a far more extensive dissection to render the area free of tumor or infection.
A typical example of the clinical importance of a fascial layer would be of that covering the psoas muscle. Infection within an intervertebral body secondary to tuberculosis can pass laterally into the psoas muscle. Pus fills the psoas muscle but is limited from further spread by the psoas fascia, which surrounds the muscle and extends inferiorly into the groin pointing below the inguinal ligament.
In the clinic
Placement of skin incisions and scarring
Surgical skin incisions are ideally placed along or parallel to Langer’s lines, which are lines of skin tension that correspond to the orientation of the dermal collagen fibers. They tend to run in the same direction as the underlying muscle fibers and incisions that are made along these lines tend to heal better with less scarring. In contrast, incisions made perpendicular to Langer’s lines are more likely to heal with a prominent scar and in some severe cases can lead to raised, firm, hypertrophic, or keloid, scars.
In the clinic
Muscle paralysis is the inability to move a specific muscle or muscle group and may be associated with other neurological abnormalities, including loss of sensation. Major causes include stroke, trauma, poliomyelitis, and iatrogenic factors. Paralysis may be due to abnormalities in the brain, the spinal cord, and the nerves supplying the muscles.
In the long term, muscle paralysis will produce secondary muscle wasting and overall atrophy of the region due to disuse.
In the clinic
Muscle atrophy is a wasting disorder of muscle. It can be produced by a variety of causes, which include nerve damage to the muscle and disuse.
Muscle atrophy is an important problem in patients who have undergone long-term rest or disuse, requiring extensive rehabilitation and muscle building exercises to maintain normal activities of daily living.
In the clinic
Muscle injuries and strains tend to occur in specific muscle groups and usually are related to a sudden exertion and muscle disruption. They typically occur in athletes.
Muscle tears may involve a small interstitial injury up to a complete muscle disruption (Fig. 1.26). It is important to identify which muscle groups are affected and the extent of the tear to facilitate treatment and obtain a prognosis, which will determine the length of rehabilitation necessary to return to normal activity.
In the clinic
Atherosclerosis is a disease that affects arteries. There is a chronic inflammatory reaction in the walls of the arteries, with deposition of cholesterol and fatty proteins. This may in turn lead to secondary calcification, with reduction in the diameter of the vessels impeding distal flow. The plaque itself may be a site for attraction of platelets that may “fall off” (embolize) distally. Plaque fissuring may occur, which allows fresh clots to form and occlude the vessel.
The importance of atherosclerosis and its effects depend upon which vessel is affected. If atherosclerosis occurs in the carotid artery, small emboli may form and produce a stroke. In the heart, plaque fissuring may produce an acute vessel thrombosis, producing a myocardial infarction (heart attack). In the legs, chronic narrowing of vessels may limit the ability of the patient to walk and ultimately cause distal ischemia and gangrene of the toes.
In the clinic
Varicose veins are tortuous dilated veins that typically occur in the legs, although they may occur in the superficial veins of the arm and in other organs. | Gray's Anatomy |
In normal individuals the movement of adjacent leg muscles pumps the blood in the veins to the heart. Blood is also pumped from the superficial veins through the investing layer of fascia of the leg into the deep veins. Valves in these perforating veins may become damaged, allowing blood to pass in the opposite direction. This increased volume and pressure produces dilatation and tortuosity of the superficial veins (Fig. 1.27). Apart from the unsightliness of larger veins, the skin may become pigmented and atrophic with a poor response to tissue trauma. In some patients even small trauma may produce skin ulceration, which requires elevation of the limb and application of pressure bandages to heal.
Treatment of varicose veins depends on their location, size, and severity. Typically the superficial varicose veins can be excised and stripped, allowing blood only to drain into the deep system.
In the clinic
All organs require a blood supply from the arteries and drainage by veins. Within most organs there are multiple ways of perfusing the tissue such that if the main vessel feeding the organ or vein draining the organ is blocked, a series of smaller vessels (collateral vessels) continue to supply and drain the organ.
In certain circumstances, organs have more than one vessel perfusing them, such as the hand, which is supplied by the radial and ulnar arteries. Loss of either the radial or the ulnar artery may not produce any symptoms of reduced perfusion to the hand.
There are circumstances in which loss of a vein produces significant venous collateralization. Some of these venous collaterals become susceptible to bleeding. This is a considerable problem in patients who have undergone portal vein thrombosis or occlusion, where venous drainage from the gut bypasses the liver through collateral veins to return to the systemic circulation.
Normal vascular anastomoses associated with an organ are important. Some organs, such as the duodenum, have a dual blood supply arising from the branches of the celiac trunk and also from the branches of the superior mesenteric artery. Should either of these vessels be damaged, blood supply will be maintained to the organ. The brain has multiple vessels supplying it, dominated by the carotid arteries and the vertebral arteries. Vessels within the brain are end arteries and have a poor collateral circulation; hence any occlusion will produce long-term cerebral damage.
In the clinic
Lymph nodes are efficient filters and have an internal honeycomb of reticular connective tissue filled with lymphocytes. These lymphocytes act on bacteria, viruses, and other bodily cells to destroy them. Lymph nodes tend to drain specific areas, and if infection occurs within a drainage area, the lymph node will become active. The rapid cell turnover and production of local inflammatory mediators may cause the node to enlarge and become tender. Similarly, in patients with malignancy the lymphatics may drain metastasizing cells to the lymph nodes. These can become enlarged and inflamed and will need to be removed if clinically symptomatic.
Lymph nodes may become diffusely enlarged in certain systemic illnesses (e.g., viral infection), or local groups may become enlarged with primary lymph node malignancies, such as lymphoma (Fig. 1.31).
In the clinic
A knowledge of dermatomes and myotomes is absolutely fundamental to carrying out a neurological examination. A typical dermatome map is shown in Fig. 1.38.
Clinically, a dermatome is that area of skin supplied by a single spinal nerve or spinal cord level. A myotome is that region of skeletal muscle innervated by a single spinal nerve or spinal cord level. Most individual muscles of the body are innervated by more than one spinal cord level, so the evaluation of myotomes is usually accomplished by testing movements of joints or muscle groups.
In the clinic | Gray's Anatomy |
Referred pain occurs when sensory information comes to the spinal cord from one location but is interpreted by the CNS as coming from another location innervated by the same spinal cord level. Usually, this happens when the pain information comes from a region, such as the gut, which has a low amount of sensory output. These afferents converge on neurons at the same spinal cord level that receive information from the skin, which is an area with a high amount of sensory output. As a result, pain from the normally low output region is interpreted as coming from the normally high output region.
Pain is most often referred from a region innervated by the visceral part of the nervous system to a region innervated, at the same spinal cord level, by the somatic side of the nervous system.
to another. For example, irritation of the peritoneum on the inferior surface of the diaphragm, which is innervated by the phrenic nerve, can be referred to the skin on the top of the shoulder, which is innervated by other somatic nerves arising at the same spinal cord level.
A young man sought medical care because of central abdominal pain that was diffuse and colicky. After some hours, the pain began to localize in the right iliac fossa and became constant. He was referred to an abdominal surgeon, who removed a grossly inflamed appendix. The patient made an uneventful recovery.
When the appendix becomes inflamed, the visceral sensory fibers are stimulated. These fibers enter the spinal cord with the sympathetic fibers at spinal cord level T10. The pain is referred to the dermatome of T10, which is in the umbilical region (Fig. 1.50). The pain is diffuse, not focal; every time a peristaltic wave passes through the ileocecal region, the pain recurs. This intermittent type of pain is referred to as colic.
In the later stages of the disease, the appendix contacts and irritates the parietal peritoneum in the right iliac fossa, which is innervated by somatic sensory nerves. This produces a constant focal pain, which predominates over the colicky pain that the patient felt some hours previously. The patient no longer interprets the referred pain from the T10 dermatome.
Although this is a typical history for appendicitis, it should always be borne in mind that the patient’s symptoms and signs may vary. The appendix is situated in a retrocecal position in approximately 70% of patients; therefore it may never contact the parietal peritoneum anteriorly in the right iliac fossa. It is also possible that the appendix is long and may directly contact other structures. As a consequence, the patient may have other symptoms (e.g., the appendix may contact the ureter, and the patient may then develop urological symptoms).
Although appendicitis is common, other disorders, for example of the bowel and pelvis, may produce similar symptoms.
The Body
In the clinic—cont’d
The back consists of the posterior aspect of the body and provides the musculoskeletal axis of support for the trunk. Bony elements consist mainly of the vertebrae, although proximal elements of the ribs, superior aspects of the pelvic bones, and posterior basal regions of the skull contribute to the back’s skeletal framework (Fig. 2.1).
Associated muscles interconnect the vertebrae and ribs with each other and with the pelvis and skull. The back contains the spinal cord and proximal parts of the spinal nerves, which send and receive information to and from most of the body.
The skeletal and muscular elements of the back support the body’s weight, transmit forces through the pelvis to the lower limbs, carry and position the head, and brace and help maneuver the upper limbs. The vertebral column is positioned posteriorly in the body at the midline. When viewed laterally, it has a number of curvatures (Fig. 2.2):
The primary curvature of the vertebral column is concave anteriorly, reflecting the original shape of the embryo, and is retained in the thoracic and sacral regions in adults. | Gray's Anatomy |
Secondary curvatures, which are concave posteriorly, form in the cervical and lumbar regions and bring the center of gravity into a vertical line, which allows the body’s weight to be balanced on the vertebral column in a way that expends the least amount of muscular energy to maintain an upright bipedal stance.
As stresses on the back increase from the cervical to lumbar regions, lower back problems are common.
Muscles of the back consist of extrinsic and intrinsic groups:
The extrinsic muscles of the back move the upper limbs and the ribs.
The intrinsic muscles of the back maintain posture and move the vertebral column; these movements include flexion (anterior bending), extension, lateral flexion, and rotation (Fig. 2.3).
Although the amount of movement between any two vertebrae is limited, the effects between vertebrae are additive along the length of the vertebral column. Also, freedom of movement and extension are limited in the thoracic region relative to the lumbar part of the vertebral column. Muscles in more anterior regions flex the vertebral column.
In the cervical region, the first two vertebrae and associated muscles are specifically modified to support and position the head. The head flexes and extends, in the nodding motion, on vertebra CI, and rotation of the head occurs as vertebra CI moves on vertebra CII (Fig. 2.3).
Protection of the nervous system
The vertebral column and associated soft tissues of the back contain the spinal cord and proximal parts of the spinal nerves (Fig. 2.4). The more distal parts of the spinal nerves pass into all other regions of the body, including certain regions of the head.
The major bones of the back are the 33 vertebrae (Fig. 2.5). The number and specific characteristics of the vertebrae vary depending on the body region with which they are associated. There are seven cervical, twelve thoracic, five lumbar, five sacral, and three to four coccygeal vertebrae. The sacral vertebrae fuse into a single bony element, the sacrum. The coccygeal vertebrae are rudimentary in structure, vary in number from three to four, and often fuse into a single coccyx.
A typical vertebra consists of a vertebral body and a vertebral arch (Fig. 2.6).
The vertebral body is anterior and is the major weightbearing component of the bone. It increases in size from vertebra CII to vertebra LV. Fibrocartilaginous intervertebral discs separate the vertebral bodies of adjacent vertebrae.
The vertebral arch is firmly anchored to the posterior surface of the vertebral body by two pedicles, which form the lateral pillars of the vertebral arch. The roof of the vertebral arch is formed by right and left laminae, which fuse at the midline.
The vertebral arches of the vertebrae are aligned to form the lateral and posterior walls of the vertebral canal, which extends from the first cervical vertebra (CI) to the last sacral vertebra (vertebra SV). This bony canal contains the spinal cord and its protective membranes, together with blood vessels, connective tissue, fat, and proximal parts of spinal nerves.
The vertebral arch of a typical vertebra has a number of characteristic projections, which serve as: attachments for muscles and ligaments, levers for the action of muscles, and sites of articulation with adjacent vertebrae.
A spinous process projects posteriorly and generally inferiorly from the roof of the vertebral arch.
On each side of the vertebral arch, a transverse process extends laterally from the region where a lamina meets a pedicle. From the same region, a superior articular process and an inferior articular process articulate with similar processes on adjacent vertebrae.
Each vertebra also contains rib elements. In the thorax, these costal elements are large and form ribs, which articulate with the vertebral bodies and transverse processes.
In all other regions, these rib elements are small and are incorporated into the transverse processes. Occasionally, they develop into ribs in regions other than the thorax, usually in the lower cervical and upper lumbar regions. | Gray's Anatomy |
Muscles in the back can be classified as extrinsic or intrinsic based on their embryological origin and type of innervation (Fig. 2.7).
The extrinsic muscles are involved with movements of the upper limbs and thoracic wall and, in general, are innervated by anterior rami of spinal nerves. The superficial group of these muscles is related to the upper limbs, while the intermediate layer of muscles is associated with the thoracic wall.
All of the intrinsic muscles of the back are deep in position and are innervated by the posterior rami of spinal nerves. They support and move the vertebral column and participate in moving the head. One group of intrinsic muscles also moves the ribs relative to the vertebral column.
The spinal cord lies within a bony canal formed by adjacent vertebrae and soft tissue elements (the vertebral canal) (Fig. 2.8):
The anterior wall is formed by the vertebral bodies of the vertebrae, intervertebral discs, and associated ligaments.
The lateral walls and roof are formed by the vertebral arches and ligaments.
Within the vertebral canal, the spinal cord is surrounded by a series of three connective tissue membranes (the meninges):
The pia mater is the innermost membrane and is intimately associated with the surface of the spinal cord.
The second membrane, the arachnoid mater, is separated from the pia by the subarachnoid space, which contains cerebrospinal fluid.
The thickest and most external of the membranes, the dura mater, lies directly against, but is not attached to, the arachnoid mater.
In the vertebral canal, the dura mater is separated from surrounding bone by an extradural (epidural) space containing loose connective tissue, fat, and a venous plexus.
The 31 pairs of spinal nerves are segmental in distribution and emerge from the vertebral canal between the pedicles of adjacent vertebrae. There are eight pairs of cervical nerves (C1 to C8), twelve thoracic (T1 to T12), five lumbar (L1 to L5), five sacral (S1 to S5), and one coccygeal (Co). Each nerve is attached to the spinal cord by a posterior root and an anterior root (Fig. 2.9).
After exiting the vertebral canal, each spinal nerve branches into: a posterior ramus—collectively, the small posterior rami innervate the back; and an anterior ramus—the much larger anterior rami innervate most other regions of the body except the head, which is innervated predominantly, but not exclusively, by cranial nerves.
The anterior rami form the major somatic plexuses (cervical, brachial, lumbar, and sacral) of the body. Major visceral components of the PNS (sympathetic trunk and prevertebral plexus) of the body are also associated mainly with the anterior rami of spinal nerves.
Cervical regions of the back constitute the skeletal and much of the muscular framework of the neck, which in turn supports and moves the head (Fig. 2.10).
The brain and cranial meninges are continuous with the spinal cord meninges at the foramen magnum of the skull. The paired vertebral arteries ascend, one on each side, through foramina in the transverse processes of cervical vertebrae and pass through the foramen magnum to participate, with the internal carotid arteries, in supplying blood to the brain.
Thorax, abdomen, and pelvis
The different regions of the vertebral column contribute to the skeletal framework of the thorax, abdomen, and pelvis (Fig. 2.10). In addition to providing support for each of these parts of the body, the vertebrae provide attachments for muscles and fascia, and articulation sites for other bones. The anterior rami of spinal nerves associated with the thorax, abdomen, and pelvis pass into these parts of the body from the back. | Gray's Anatomy |
The bones of the back provide extensive attachments for muscles associated with anchoring and moving the upper limbs on the trunk. This is less true of the lower limbs, which are firmly anchored to the vertebral column through articulation of the pelvic bones with the sacrum. The upper and lower limbs are innervated by anterior rami of spinal nerves that emerge from cervical and lumbosacral levels, respectively, of the vertebral column.
During development, the vertebral column grows much faster than the spinal cord. As a result, the spinal cord does not extend the entire length of the vertebral canal (Fig. 2.11).
In the adult, the spinal cord typically ends between vertebrae LI and LII, although it can end as high as vertebra TXII and as low as the disc between vertebrae LII and LIII.
Spinal nerves originate from the spinal cord at increasingly oblique angles from vertebrae CI to Co, and the nerve roots pass in the vertebral canal for increasingly longer distances. Their spinal cord level of origin therefore becomes increasingly dissociated from their vertebral column level of exit. This is particularly evident for lumbar and sacral spinal nerves.
Each spinal nerve exits the vertebral canal laterally through an intervertebral foramen (Fig. 2.12). The foramen is formed between adjacent vertebral arches and is closely related to intervertebral joints:
The superior and inferior margins are formed by notches in adjacent pedicles.
The posterior margin is formed by the articular processes of the vertebral arches and the associated joint.
The anterior border is formed by the intervertebral disc between the vertebral bodies of the adjacent vertebrae.
Any pathology that occludes or reduces the size of an intervertebral foramen, such as bone loss, herniation of the intervertebral disc, or dislocation of the zygapophysial joint (the joint between the articular processes), can affect the function of the associated spinal nerve.
Innervation of the back
Posterior branches of spinal nerves innervate the intrinsic muscles of the back and adjacent skin. The cutaneous distribution of these posterior rami extends into the gluteal region of the lower limb and the posterior aspect of the head. Parts of dermatomes innervated by the posterior rami of spinal nerves are shown in Fig. 2.13.
Skeletal components of the back consist mainly of the vertebrae and associated intervertebral discs. The skull, scapulae, pelvic bones, and ribs also contribute to the bony framework of the back and provide sites for muscle attachment.
There are approximately 33 vertebrae, which are subdivided into five groups based on morphology and location (Fig. 2.14):
The seven cervical vertebrae between the thorax and skull are characterized mainly by their small size and the presence of a foramen in each transverse process (Figs. 2.14 and 2.15).
The 12 thoracic vertebrae are characterized by their articulated ribs (Figs. 2.14 and 2.16); although all vertebrae have rib elements, these elements are small and are incorporated into the transverse processes in regions other than the thorax; but in the thorax, the ribs are separate bones and articulate via synovial joints with the vertebral bodies and transverse processes of the associated vertebrae.
Inferior to the thoracic vertebrae are five lumbar vertebrae, which form the skeletal support for the posterior abdominal wall and are characterized by their large size (Figs. 2.14 and 2.17).
Next are five sacral vertebrae fused into one single bone called the sacrum, which articulates on each side with a pelvic bone and is a component of the pelvic wall.
Inferior to the sacrum is a variable number, usually four, of coccygeal vertebrae, which fuse into a single small triangular bone called the coccyx. | Gray's Anatomy |
In the embryo, the vertebrae are formed intersegmentally from cells called sclerotomes, which originate from adjacent somites (Fig. 2.18). Each vertebra is derived from the cranial parts of the two somites below, one on each side, and the caudal parts of the two somites above. The spinal nerves develop segmentally and pass between the forming vertebrae.
A typical vertebra consists of a vertebral body and a posterior vertebral arch (Fig. 2.19). Extending from the vertebral arch are a number of processes for muscle attachment and articulation with adjacent bone.
The vertebral body is the weight-bearing part of the vertebra and is linked to adjacent vertebral bodies by intervertebral discs and ligaments. The size of vertebral bodies increases inferiorly as the amount of weight supported increases.
The vertebral arch forms the lateral and posterior parts of the vertebral foramen.
The vertebral foramina of all the vertebrae together form the vertebral canal, which contains and protects the spinal cord. Superiorly, the vertebral canal is continuous, through the foramen magnum of the skull, with the cranial cavity of the head.
The vertebral arch of each vertebra consists of pedicles and laminae (Fig. 2.19):
The two pedicles are bony pillars that attach the vertebral arch to the vertebral body.
The two laminae are flat sheets of bone that extend from each pedicle to meet in the midline and form the roof of the vertebral arch.
A spinous process projects posteriorly and inferiorly from the junction of the two laminae and is a site for muscle and ligament attachment.
A transverse process extends posterolaterally from the junction of the pedicle and lamina on each side and is a site for muscle and ligament attachment, and for articulation with ribs in the thoracic region.
Also projecting from the region where the pedicles join the laminae are superior and inferior articular processes (Fig. 2.19), which articulate with the inferior and superior articular processes, respectively, of adjacent vertebrae.
Between the vertebral body and the origin of the articular processes, each pedicle is notched on its superior and inferior surfaces. These superior and inferior vertebral notches participate in forming intervertebral foramina.
The seven cervical vertebrae are characterized by their small size and by the presence of a foramen in each transverse process. A typical cervical vertebra has the following features (Fig. 2.20A):
The vertebral body is short in height and square shaped when viewed from above and has a concave superior surface and a convex inferior surface.
Each transverse process is trough shaped and perforated by a round foramen transversarium.
The spinous process is short and bifid.
The vertebral foramen is triangular.
The first and second cervical vertebrae—the atlas and axis—are specialized to accommodate movement of the head.
Vertebra CI (the atlas) articulates with the head (Fig. 2.21). Its major distinguishing feature is that it lacks a vertebral body (Fig. 2.20B). In fact, the vertebral body of CI fuses onto the body of CII during development to become the dens of CII. As a result, there is no intervertebral disc between CI and CII. When viewed from above, the atlas is ring shaped and composed of two lateral masses interconnected by an anterior arch and a posterior arch.
Each lateral mass articulates above with an occipital condyle of the skull and below with the superior articular process of vertebra CII (the axis). The superior articular surfaces are bean shaped and concave, whereas the inferior articular surfaces are almost circular and flat.
The atlanto-occipital joint allows the head to nod up and down on the vertebral column.
The posterior surface of the anterior arch has an articular facet for the dens, which projects superiorly from the vertebral body of the axis. The dens is held in position by a strong transverse ligament of atlas posterior to it and spanning the distance between the oval attachment facets on the medial surfaces of the lateral masses of the atlas. | Gray's Anatomy |
The dens acts as a pivot that allows the atlas and attached head to rotate on the axis, side to side.
The transverse processes of the atlas are large and protrude further laterally than those of the other cervical vertebrae and act as levers for muscle action, particularly for muscles that move the head at the atlanto-axial joints.
The axis is characterized by the large tooth-like dens, which extends superiorly from the vertebral body (Figs. 2.20B and 2.21). The anterior surface of the dens has an oval facet for articulation with the anterior arch of the atlas.
The two superolateral surfaces of the dens possess circular impressions that serve as attachment sites for strong alar ligaments, one on each side, which connect the dens to the medial surfaces of the occipital condyles. These alar ligaments check excessive rotation of the head and atlas relative to the axis.
The twelve thoracic vertebrae are all characterized by their articulation with ribs. A typical thoracic vertebra has two partial facets (superior and inferior costal facets) on each side of the vertebral body for articulation with the head of its own rib and the head of the rib below (Fig. 2.20C). The superior costal facet is much larger than the inferior costal facet.
Each transverse process also has a facet (transverse costal facet) for articulation with the tubercle of its own rib. The vertebral body of the vertebra is somewhat heart shaped when viewed from above, and the vertebral foramen is circular.
The five lumbar vertebrae are distinguished from vertebrae in other regions by their large size (Fig. 2.20D). Also, they lack facets for articulation with ribs. The transverse processes are generally thin and long, with the exception of those on vertebra LV, which are massive and somewhat cone shaped for the attachment of iliolumbar ligaments to connect the transverse processes to the pelvic bones.
The vertebral body of a typical lumbar vertebra is cylindrical and the vertebral foramen is triangular in shape and larger than in the thoracic vertebrae.
The sacrum is a single bone that represents the five fused sacral vertebrae (Fig. 2.20E). It is triangular in shape with the apex pointed inferiorly, and is curved so that it has a concave anterior surface and a correspondingly convex posterior surface. It articulates above with vertebra LV and below with the coccyx. It has two large L-shaped facets, one on each lateral surface, for articulation with the pelvic bones.
The posterior surface of the sacrum has four pairs of posterior sacral foramina, and the anterior surface has four pairs of anterior sacral foramina for the passage of the posterior and anterior rami, respectively, of S1 to S4 spinal nerves.
The posterior wall of the vertebral canal may be incomplete near the inferior end of the sacrum.
The coccyx is a small triangular bone that articulates with the inferior end of the sacrum and represents three to four fused coccygeal vertebrae (Fig. 2.20F). It is characterized by its small size and by the absence of vertebral arches and therefore a vertebral canal.
Intervertebral foramina are formed on each side between adjacent parts of vertebrae and associated intervertebral discs (Fig. 2.22). The foramina allow structures, such as spinal nerves and blood vessels, to pass in and out of the vertebral canal.
An intervertebral foramen is formed by the inferior vertebral notch on the pedicle of the vertebra above and the superior vertebral notch on the pedicle of the vertebra below. The foramen is bordered: posteriorly by the zygapophysial joint between the articular processes of the two vertebrae, and anteriorly by the intervertebral disc and adjacent vertebral bodies.
Each intervertebral foramen is a confined space surrounded by bone and ligament, and by joints. Pathology in any of these structures, and in the surrounding muscles, can affect structures within the foramen. | Gray's Anatomy |
In most regions of the vertebral column, the laminae and spinous processes of adjacent vertebrae overlap to form a reasonably complete bony dorsal wall for the vertebral canal. However, in the lumbar region, large gaps exist between the posterior components of adjacent vertebral arches (Fig. 2.23). These gaps between adjacent laminae and spinous processes become increasingly wide from vertebra LI to vertebra LV. The spaces can be widened further by flexion of the vertebral column. These gaps allow relatively easy access to the vertebral canal for clinical procedures.
Joints between vertebrae in the back
The two major types of joints between vertebrae are: symphyses between vertebral bodies (Fig. 2.31), and synovial joints between articular processes (Fig. 2.32).
A typical vertebra has a total of six joints with adjacent vertebrae: four synovial joints (two above and two below) and two symphyses (one above and one below). Each symphysis includes an intervertebral disc.
Although the movement between any two vertebrae is limited, the summation of movement among all vertebrae results in a large range of movement by the vertebral column.
Movements by the vertebral column include flexion, extension, lateral flexion, rotation, and circumduction.
Movements by vertebrae in a specific region (cervical, thoracic, and lumbar) are determined by the shape and orientation of joint surfaces on the articular processes and on the vertebral bodies.
The symphysis between adjacent vertebral bodies is formed by a layer of hyaline cartilage on each vertebral body and an intervertebral disc, which lies between the layers.
The intervertebral disc consists of an outer anulus fibrosus, which surrounds a central nucleus pulposus (Fig. 2.31).
The anulus fibrosus consists of an outer ring of collagen surrounding a wider zone of fibrocartilage arranged in a lamellar configuration. This arrangement of fibers limits rotation between vertebrae.
The nucleus pulposus fills the center of the intervertebral disc, is gelatinous, and absorbs compression forces between vertebrae.
Degenerative changes in the anulus fibrosus can lead to herniation of the nucleus pulposus. Posterolateral herniation can impinge on the roots of a spinal nerve in the intervertebral foramen.
The synovial joints between superior and inferior articular processes on adjacent vertebrae are the zygapophysial joints (Fig. 2.32). A thin articular capsule attached to the margins of the articular facets encloses each joint.
In cervical regions, the zygapophysial joints slope inferiorly from anterior to posterior and their shape facilitates flexion and extension. In thoracic regions, the joints are oriented vertically and their shape limits flexion and extension, but facilitates rotation. In lumbar regions, the joint surfaces are curved and adjacent processes interlock, thereby limiting range of movement, though flexion and extension are still major movements in the lumbar region.
The lateral margins of the upper surfaces of typical cervical vertebrae are elevated into crests or lips termed uncinate processes. These may articulate with the body of the vertebra above to form small “uncovertebral” synovial joints (Fig. 2.33).
Joints between vertebrae are reinforced and supported by numerous ligaments, which pass between vertebral bodies and interconnect components of the vertebral arches.
The anterior and posterior longitudinal ligaments are on the anterior and posterior surfaces of the vertebral bodies and extend along most of the vertebral column (Fig. 2.35).
The anterior longitudinal ligament is attached superiorly to the base of the skull and extends inferiorly to attach to the anterior surface of the sacrum. Along its length it is attached to the vertebral bodies and intervertebral discs. | Gray's Anatomy |
The posterior longitudinal ligament is on the posterior surfaces of the vertebral bodies and lines the anterior surface of the vertebral canal. Like the anterior longitudinal ligament, it is attached along its length to the vertebral bodies and intervertebral discs. The upper part of the posterior longitudinal ligament that connects CII to the intracranial aspect of the base of the skull is termed the tectorial membrane (see Fig. 2.20B).
The ligamenta flava, on each side, pass between the laminae of adjacent vertebrae (Fig. 2.36). These thin, broad ligaments consist predominantly of elastic tissue and form part of the posterior surface of the vertebral canal. Each ligamentum flavum runs between the posterior surface of the lamina on the vertebra below to the anterior surface of the lamina of the vertebra above. The ligamenta flava resist separation of the laminae in flexion and assist in extension back to the anatomical position.
The supraspinous ligament connects and passes along the tips of the vertebral spinous processes from vertebra CVII to the sacrum (Fig. 2.37). From vertebra CVII to the skull, the ligament becomes structurally distinct from more caudal parts of the ligament and is called the ligamentum nuchae.
The ligamentum nuchae is a triangular, sheet-like structure in the median sagittal plane:
The base of the triangle is attached to the skull, from the external occipital protuberance to the foramen magnum.
The apex is attached to the tip of the spinous process of vertebra CVII.
The deep side of the triangle is attached to the posterior tubercle of vertebra CI and the spinous processes of the other cervical vertebrae.
The ligamentum nuchae supports the head. It resists flexion and facilitates returning the head to the anatomical position. The broad lateral surfaces and the posterior edge of the ligament provide attachment for adjacent muscles.
Interspinous ligaments pass between adjacent vertebral spinous processes (Fig. 2.38). They attach from the base to the apex of each spinous process and blend with the supraspinous ligament posteriorly and the ligamenta flava anteriorly on each side.
Muscles of the back are organized into superficial, intermediate, and deep groups.
Muscles in the superficial and intermediate groups are extrinsic muscles because they originate embryologically from locations other than the back. They are innervated by anterior rami of spinal nerves:
The superficial group consists of muscles related to and involved in movements of the upper limb.
The intermediate group consists of muscles attached to the ribs and may serve a respiratory function.
Muscles of the deep group are intrinsic muscles because they develop in the back. They are innervated by posterior rami of spinal nerves and are directly related to movements of the vertebral column and head.
Superficial group of back muscles
The muscles in the superficial group are immediately deep to the skin and superficial fascia (Figs. 2.42 to 2.45). They attach the superior part of the appendicular skeleton (clavicle, scapula, and humerus) to the axial skeleton (skull, ribs, and vertebral column). Because these muscles are primarily involved with movements of this part of the appendicular skeleton, they are sometimes referred to as the appendicular group.
Muscles in the superficial group include the trapezius, latissimus dorsi, rhomboid major, rhomboid minor, and levator scapulae. The rhomboid major, rhomboid minor, and levator scapulae muscles are located deep to the trapezius muscle in the superior part of the back.
Each trapezius muscle is flat and triangular, with the base of the triangle situated along the vertebral column (the muscle’s origin) and the apex pointing toward the tip of the shoulder (the muscle’s insertion) (Fig. 2.43 and Table 2.1). The muscles on both sides together form a trapezoid. | Gray's Anatomy |
The superior fibers of the trapezius, from the skull and upper portion of the vertebral column, descend to attach to the lateral third of the clavicle and to the acromion of the scapula. Contraction of these fibers elevates the scapula. In addition, the superior and inferior fibers work together to rotate the lateral aspect of the scapula upward, which needs to occur when raising the upper limb above the head.
Motor innervation of the trapezius is by the accessory nerve [XI], which descends from the neck onto the deep surface of the muscle (Fig. 2.44). Proprioceptive fibers from the trapezius pass in the branches of the cervical plexus and enter the spinal cord at spinal cord levels C3 and C4.
The blood supply to the trapezius is from the superficial branch of the transverse cervical artery, the acromial branch of the suprascapular artery, and the dorsal branches of posterior intercostal arteries.
Latissimus dorsi is a large, flat triangular muscle that begins in the lower portion of the back and tapers as it ascends to a narrow tendon that attaches to the humerus anteriorly (Figs. 2.42 to 2.45 and Table 2.1). As a result, movements associated with this muscle include extension, adduction, and medial rotation of the upper limb. The latissimus dorsi can also depress the shoulder, preventing its upward movement.
The thoracodorsal nerve of the brachial plexus innervates the latissimus dorsi muscle. Associated with this nerve is the thoracodorsal artery, which is the primary blood supply of the muscle. Additional small arteries come from dorsal branches of posterior intercostal and lumbar arteries.
Levator scapulae is a slender muscle that descends from the transverse processes of the upper cervical vertebrae to the upper portion of the scapula on its medial border at the superior angle (Figs. 2.43 and 2.45 and Table 2.1). It elevates the scapula and may assist other muscles in rotating the lateral aspect of the scapula inferiorly.
The levator scapulae is innervated by branches from the anterior rami of spinal nerves C3 and C4 and the dorsal scapular nerve, and its arterial supply consists of branches primarily from the transverse and ascending cervical arteries.
The two rhomboid muscles are inferior to levator scapulae (Fig. 2.45 and Table 2.1). Rhomboid minor is superior to rhomboid major, and is a small, cylindrical muscle that arises from the ligamentum nuchae of the neck and the spinous processes of vertebrae CVII and TI and attaches to the medial scapular border opposite the root of the spine of the scapula.
The larger rhomboid major originates from the spinous processes of the upper thoracic vertebrae and attaches to the medial scapular border inferior to rhomboid minor.
The two rhomboid muscles work together to retract or pull the scapula toward the vertebral column. With other muscles they may also rotate the lateral aspect of the scapula inferiorly.
The dorsal scapular nerve, a branch of the brachial plexus, innervates both rhomboid muscles (Fig. 2.46).
Intermediate group of back muscles
The muscles in the intermediate group of back muscles consist of two thin muscular sheets in the superior and inferior regions of the back, immediately deep to the muscles in the superficial group (Fig. 2.47 and Table 2.2). Fibers from these two serratus posterior muscles (serratus posterior superior and serratus posterior inferior) pass obliquely outward from the vertebral column to attach to the ribs. This positioning suggests a respiratory function, and at times, these muscles have been referred to as the respiratory group. | Gray's Anatomy |
Serratus posterior superior is deep to the rhomboid muscles, whereas serratus posterior inferior is deep to the latissimus dorsi. Both serratus posterior muscles are attached to the vertebral column and associated structures medially, and either descend (the fibers of the serratus posterior superior) or ascend (the fibers of the serratus posterior inferior) to attach to the ribs. These two muscles therefore elevate and depress the ribs.
The serratus posterior muscles are innervated by segmental branches of anterior rami of intercostal nerves. Their vascular supply is provided by a similar segmental pattern through the intercostal arteries.
Deep group of back muscles
The deep or intrinsic muscles of the back extend from the pelvis to the skull and are innervated by segmental branches of the posterior rami of spinal nerves. They include: the extensors and rotators of the head and neck— the splenius capitis and cervicis (spinotransversales muscles), the extensors and rotators of the vertebral column—the erector spinae and transversospinales, and the short segmental muscles—the interspinales and intertransversarii.
The vascular supply to this deep group of muscles is through branches of the vertebral, deep cervical, occipital, transverse cervical, posterior intercostal, subcostal, lumbar, and lateral sacral arteries.
The thoracolumbar fascia covers the deep muscles of the back and trunk (Fig. 2.48). This fascial layer is critical to the overall organization and integrity of the region:
Superiorly, it passes anteriorly to the serratus posterior muscle and is continuous with deep fascia in the neck.
In the thoracic region, it covers the deep muscles and separates them from the muscles in the superficial and intermediate groups.
Medially, it attaches to the spinous processes of the thoracic vertebrae and, laterally, to the angles of the ribs.
The medial attachments of the latissimus dorsi and serratus posterior inferior muscles blend into the thoracolumbar fascia. In the lumbar region, the thoracolumbar fascia consists of three layers:
The posterior layer is thick and is attached to the spinous processes of the lumbar vertebrae and sacral vertebrae and to the supraspinous ligament—from these attachments, it extends laterally to cover the erector spinae.
The middle layer is attached medially to the tips of the transverse processes of the lumbar vertebrae and intertransverse ligaments—inferiorly, it is attached to the iliac crest and, superiorly, to the lower border of rib XII.
The anterior layer covers the anterior surface of the quadratus lumborum muscle (a muscle of the posterior abdominal wall) and is attached medially to the transverse processes of the lumbar vertebrae—inferiorly, it is attached to the iliac crest and, superiorly, it forms the lateral arcuate ligament for attachment of the diaphragm.
The posterior and middle layers of the thoracolumbar fascia come together at the lateral margin of the erector spinae (Fig. 2.48). At the lateral border of the quadratus lumborum, the anterior layer joins them and forms the aponeurotic origin for the transversus abdominis muscle of the abdominal wall.
The two spinotransversales muscles run from the spinous processes and ligamentum nuchae upward and laterally (Fig. 2.49 and Table 2.3):
The splenius capitis is a broad muscle attached to the occipital bone and mastoid process of the temporal bone.
The splenius cervicis is a narrow muscle attached to the transverse processes of the upper cervical vertebrae.
Together the spinotransversales muscles draw the head backward, extending the neck. Individually, each muscle rotates the head to one side—the same side as the contracting muscle. | Gray's Anatomy |
The erector spinae is the largest group of intrinsic back muscles. The muscles lie posterolaterally to the vertebral column between the spinous processes medially and the angles of the ribs laterally. They are covered in the thoracic and lumbar regions by thoracolumbar fascia and the serratus posterior inferior, rhomboid, and splenius muscles. The mass arises from a broad, thick tendon attached to the sacrum, the spinous processes of the lumbar and lower thoracic vertebrae, and the iliac crest (Fig. 2.50 and Table 2.4). It divides in the upper lumbar region into three vertical columns of muscle, each of which is further subdivided regionally (lumborum, thoracis, cervicis, and capitis), depending on where the muscles attach superiorly.
The outer or most laterally placed column of the erector spinae muscles is the iliocostalis, which is associated with the costal elements and passes from the common tendon of origin to multiple insertions into the angles of the ribs and the transverse processes of the lower cervical vertebrae.
The middle or intermediate column is the longissimus, which is the largest of the erector spinae subdivision extending from the common tendon of origin to the base of the skull. Throughout this vast expanse, the lateral positioning of the longissimus muscle is in the area of the transverse processes of the various vertebrae.
The most medial muscle column is the spinalis, which is the smallest of the subdivisions and interconnects the spinous processes of adjacent vertebrae. The spinalis is most constant in the thoracic region and is generally absent in the cervical region. It is associated with a deeper muscle (the semispinalis capitis) as the erector spinae group approaches the skull.
The muscles in the erector spinae group are the primary extensors of the vertebral column and head. Acting bilaterally, they straighten the back, returning it to the upright position from a flexed position, and pull the head posteriorly. They also participate in controlling vertebral column flexion by contracting and relaxing in a coordinated fashion. Acting unilaterally, they bend the vertebral column laterally. In addition, unilateral contractions of muscles attached to the head turn the head to the actively contracting side.
The transversospinales muscles run obliquely upward and medially from transverse processes to spinous processes, filling the groove between these two vertebral projections (Fig. 2.51 and Table 2.5). They are deep to the erector spinae and consist of three major subgroups—the semispinalis, multifidus, and rotatores muscles.
The semispinalis muscles are the most superficial collection of muscle fibers in the transversospinales group. These muscles begin in the lower thoracic region and end by attaching to the skull, crossing between four and six vertebrae from their point of origin to point of attachment. Semispinalis muscles are found in the thoracic and cervical regions, and attach to the occipital bone at the base of the skull.
Deep to the semispinalis is the second group of muscles, the multifidus. Muscles in this group span the length of the vertebral column, passing from a lateral point of origin upward and medially to attach to spinous processes and spanning between two and four vertebrae. The multifidus muscles are present throughout the length of the vertebral column but are best developed in the lumbar region.
The small rotatores muscles are the deepest of the transversospinales group. They are present throughout the length of the vertebral column but are best developed in the thoracic region. Their fibers pass upward and medially from transverse processes to spinous processes crossing two vertebrae (long rotators) or attaching to an adjacent vertebra (short rotators).
When muscles in the transversospinales group contract bilaterally, they extend the vertebral column, an action similar to that of the erector spinae group. However, when muscles on only one side contract, they pull the spinous processes toward the transverse processes on that side, causing the trunk to turn or rotate in the opposite direction. | Gray's Anatomy |
One muscle in the transversospinales group, the semispinalis capitis, has a unique action because it attaches to the skull. Contracting bilaterally, this muscle pulls the head posteriorly, whereas unilateral contraction pulls the head posteriorly and turns it, causing the chin to move superiorly and turn toward the side of the contracting muscle. These actions are similar to those of the upper erector spinae.
The two groups of segmental muscles (Fig. 2.51 and Table 2.6) are deeply placed in the back and innervated by posterior rami of spinal nerves.
The first group of segmental muscles are the levatores costarum muscles, which arise from the transverse processes of vertebrae CVII and TI to TXI. They have an oblique lateral and downward direction and insert into the rib below the vertebra of origin in the area of the tubercle. Contraction elevates the ribs.
The second group of segmental muscles are the true segmental muscles of the back—the interspinales, which pass between adjacent spinous processes, and the intertransversarii, which pass between adjacent transverse processes. These postural muscles stabilize adjoining vertebrae during movements of the vertebral column to allow more effective action of the large muscle groups.
A small group of deep muscles in the upper cervical region at the base of the occipital bone move the head. They connect vertebra CI (the atlas) to vertebra CII (the axis) and connect both vertebrae to the base of the skull. Because of their location they are sometimes referred to as suboccipital muscles (Figs. 2.51 and 2.52 and Table 2.7). They include, on each side: rectus capitis posterior major, rectus capitis posterior minor, obliquus capitis inferior, and obliquus capitis superior.
Contraction of the suboccipital muscles extends and rotates the head at the atlanto-occipital and atlanto-axial joints, respectively.
The suboccipital muscles are innervated by the posterior ramus of the first cervical nerve, which enters the area between the vertebral artery and the posterior arch of the atlas (Fig. 2.52). The vascular supply to the muscles in this area is from branches of the vertebral and occipital arteries.
The suboccipital muscles form the boundaries of the suboccipital triangle, an area that contains several important structures (Fig. 2.52):
The rectus capitis posterior major muscle forms the medial border of the triangle.
The obliquus capitis superior muscle forms the lateral border.
The obliquus capitis inferior muscle forms the inferior border.
The contents of the suboccipital triangle include: posterior ramus of CI, vertebral artery, and veins
The spinal cord extends from the foramen magnum to approximately the level of the disc between vertebrae LI and LII in adults, although it can end as high as vertebra TXII or as low as the disc between vertebrae LII and LIII (Fig. 2.53). In neonates, the spinal cord extends approximately to vertebra LIII but can reach as low as vertebra LIV. The distal end of the cord (the conus medullaris) is cone shaped. A fine filament of connective tissue (the pial part of the filum terminale) continues inferiorly from the apex of the conus medullaris.
The spinal cord is not uniform in diameter along its length. It has two major swellings or enlargements in regions associated with the origin of spinal nerves that innervate the upper and lower limbs. A cervical enlargement occurs in the region associated with the origins of spinal nerves C5 to T1, which innervate the upper limbs. A lumbosacral enlargement occurs in the region associated with the origins of spinal nerves L1 to S3, which innervate the lower limbs.
The external surface of the spinal cord is marked by a number of fissures and sulci (Fig. 2.54):
The anterior median fissure extends the length of the anterior surface.
The posterior median sulcus extends along the posterior surface. | Gray's Anatomy |
The posterolateral sulcus on each side of the posterior surface marks where the posterior rootlets of spinal nerves enter the cord.
Internally, the cord has a small central canal surrounded by gray and white matter:
The gray matter is rich in nerve cell bodies, which form longitudinal columns along the cord, and in cross section these columns form a characteristic H-shaped appearance in the central regions of the cord.
The white matter surrounds the gray matter and is rich in nerve cell processes, which form large bundles or tracts that ascend and descend in the cord to other spinal cord levels or carry information to and from the brain.
The arterial supply to the spinal cord comes from two sources (Fig. 2.55). It consists of: longitudinally oriented vessels, arising superior to the cervical portion of the cord, which descend on the surface of the cord; and feeder arteries that enter the vertebral canal through the intervertebral foramina at every level; these feeder vessels, or segmental spinal arteries, arise predominantly from the vertebral and deep cervical arteries in the neck, the posterior intercostal arteries in the thorax, and the lumbar arteries in the abdomen.
After entering an intervertebral foramen, the segmental spinal arteries give rise to anterior and posterior radicular arteries (Fig. 2.55). This occurs at every vertebral level. The radicular arteries follow, and supply, the anterior and posterior roots. At various vertebral levels, the segmental spinal arteries also give off segmental medullary arteries (Fig. 2.55). These vessels pass directly to the longitudinally oriented vessels, reinforcing these.
The longitudinal vessels consist of: a single anterior spinal artery, which originates within the cranial cavity as the union of two vessels that arise from the vertebral arteries—the resulting single anterior spinal artery passes inferiorly, approximately parallel to the anterior median fissure, along the surface of the spinal cord; and two posterior spinal arteries, which also originate in the cranial cavity, usually arising directly from a terminal branch of each vertebral artery (the posterior inferior cerebellar artery)—the right and left posterior spinal arteries descend along the spinal cord, each as two branches that bracket the posterolateral sulcus and the connection of posterior roots with the spinal cord.
The anterior and posterior spinal arteries are reinforced along their length by eight to ten segmental medullary arteries (Fig. 2.55). The largest of these is the arteria radicularis magna or the artery of Adamkiewicz (Fig. 2.55). This vessel arises in the lower thoracic or upper lumbar region, usually on the left side, and reinforces the arterial supply to the lower portion of the spinal cord, including the lumbar enlargement.
Veins that drain the spinal cord form a number of longitudinal channels (Fig. 2.56):
Two pairs of veins on each side bracket the connections of the posterior and anterior roots to the cord.
One midline channel parallels the anterior median fissure.
One midline channel passes along the posterior median sulcus.
These longitudinal channels drain into an extensive internal vertebral plexus in the extradural (epidural) space of the vertebral canal, which then drains into segmentally arranged vessels that connect with major systemic veins, such as the azygos system in the thorax. The internal vertebral plexus also communicates with intracranial veins.
The spinal dura mater is the outermost meningeal membrane and is separated from the bones forming the vertebral canal by an extradural space (Fig. 2.59). Superiorly, it is continuous with the inner meningeal layer of cranial dura mater at the foramen magnum of the skull. Inferiorly, the dural sac dramatically narrows at the level of the lower border of vertebra SII and forms an investing sheath for the pial part of the filum terminale of the spinal cord. This terminal cord-like extension of dura mater (the dural part of the filum terminale) attaches to the posterior surface of the vertebral bodies of the coccyx. | Gray's Anatomy |
As spinal nerves and their roots pass laterally, they are surrounded by tubular sleeves of dura mater, which merge with and become part of the outer covering (epineurium) of the nerves.
The arachnoid mater is a thin delicate membrane against, but not adherent to, the deep surface of the dura mater (Fig. 2.59). It is separated from the pia mater by the subarachnoid space. The arachnoid mater ends at the level of vertebra SII (see Fig. 2.53).
The subarachnoid space between the arachnoid and pia mater contains CSF (Fig. 2.59). The subarachnoid space around the spinal cord is continuous at the foramen magnum with the subarachnoid space surrounding the brain. Inferiorly, the subarachnoid space terminates at approximately the level of the lower border of vertebra SII (see Fig. 2.53).
Delicate strands of tissue (arachnoid trabeculae) are continuous with the arachnoid mater on one side and the pia mater on the other; they span the subarachnoid space and interconnect the two adjacent membranes. Large blood vessels are suspended in the subarachnoid space by similar strands of material, which expand over the vessels to form a continuous external coat.
The subarachnoid space extends farther inferiorly than the spinal cord. The spinal cord ends at approximately the disc between vertebrae LI and LII, whereas the subarachnoid space extends to approximately the lower border of vertebra SII (see Fig. 2.53). The subarachnoid space is largest in the region inferior to the terminal end of the spinal cord, where it surrounds the cauda equina. As a consequence, CSF can be withdrawn from the subarachnoid space in the lower lumbar region without endangering the spinal cord.
The spinal pia mater is a vascular membrane that firmly adheres to the surface of the spinal cord (Fig. 2.59). It extends into the anterior median fissure and reflects as sleeve-like coatings onto posterior and anterior rootlets and roots as they cross the subarachnoid space. As the roots exit the space, the sleeve-like coatings reflect onto the arachnoid mater.
On each side of the spinal cord, a longitudinally oriented sheet of pia mater (the denticulate ligament) extends laterally from the cord toward the arachnoid and dura mater (Fig. 2.59).
Medially, each denticulate ligament is attached to the spinal cord in a plane that lies between the origins of the posterior and anterior rootlets.
Laterally, each denticulate ligament forms a series of triangular extensions along its free border, with the apex of each extension being anchored through the arachnoid mater to the dura mater.
The lateral attachments of the denticulate ligaments generally occur between the exit points of adjacent posterior and anterior rootlets. The ligaments function to position the spinal cord in the center of the subarachnoid space.
Arrangement of structures in the vertebral canal
The vertebral canal is bordered: anteriorly by the bodies of the vertebrae, intervertebral discs, and posterior longitudinal ligament (Fig. 2.60); laterally, on each side by the pedicles and intervertebral foramina; and posteriorly by the laminae and ligamenta flava, and in the median plane the roots of the interspinous ligaments and vertebral spinous processes.
Between the walls of the vertebral canal and the dural sac is an extradural space containing a vertebral plexus of veins embedded in fatty connective tissue.
The vertebral spinous processes can be palpated through the skin in the midline in thoracic and lumbar regions of the back. Between the skin and spinous processes is a layer of superficial fascia. In lumbar regions, the adjacent spinous processes and the associated laminae on either side of the midline do not overlap, resulting in gaps between adjacent vertebral arches. | Gray's Anatomy |
When carrying out a lumbar puncture (spinal tap), the needle passes between adjacent vertebral spinous processes, through the supraspinous and interspinous ligaments, and enters the extradural space. The needle continues through the dura and arachnoid mater and enters the subarachnoid space, which contains CSF.
Each spinal nerve is connected to the spinal cord by posterior and anterior roots (Fig. 2.61):
The posterior root contains the processes of sensory neurons carrying information to the CNS—the cell bodies of the sensory neurons, which are derived embryologically from neural crest cells, are clustered in a spinal ganglion at the distal end of the posterior root, usually in the intervertebral foramen.
The anterior root contains motor nerve fibers, which carry signals away from the CNS—the cell bodies of the primary motor neurons are in anterior regions of the spinal cord.
Medially, the posterior and anterior roots divide into rootlets, which attach to the spinal cord.
A spinal segment is the area of the spinal cord that gives rise to the posterior and anterior rootlets, which will form a single pair of spinal nerves. Laterally, the posterior and anterior roots on each side join to form a spinal nerve.
Each spinal nerve divides, as it emerges from an intervertebral foramen, into two major branches: a small posterior ramus and a much larger anterior ramus (Fig. 2.61):
The posterior rami innervate only intrinsic back muscles (the epaxial muscles) and an associated narrow strip of skin on the back.
The anterior rami innervate most other skeletal muscles (the hypaxial muscles) of the body, including those of the limbs and trunk, and most remaining areas of the skin, except for certain regions of the head.
Near the point of division into anterior and posterior rami, each spinal nerve gives rise to two to four small recurrent meningeal (sinuvertebral) nerves (see Fig. 2.59). These nerves reenter the intervertebral foramen to supply dura, ligaments, intervertebral discs, and blood vessels.
All major somatic plexuses (cervical, brachial, lumbar, and sacral) are formed by anterior rami.
Because the spinal cord is much shorter than the vertebral column, the roots of spinal nerves become longer and pass more obliquely from the cervical to coccygeal regions of the vertebral canal (Fig. 2.62).
In adults, the spinal cord terminates at a level approximately between vertebrae LI and LII, but this can range between vertebra TXII and the disc between vertebrae LII and LIII. Consequently, posterior and anterior roots forming spinal nerves emerging between vertebrae in the lower regions of the vertebral column are connected to the spinal cord at higher vertebral levels.
Below the end of the spinal cord, the posterior and anterior roots of lumbar, sacral, and coccygeal nerves pass inferiorly to reach their exit points from the vertebral canal. This terminal cluster of roots is the cauda equina.
Nomenclature of spinal nerves
There are approximately 31 pairs of spinal nerves (Fig. 2.62), named according to their position with respect to associated vertebrae: eight cervical nerves—C1 to C8, twelve thoracic nerves—T1 to T12, five lumbar nerves—L1 to L5, five sacral nerves—S1 to S5, one coccygeal nerve—Co.
The first cervical nerve (C1) emerges from the vertebral canal between the skull and vertebra CI (Fig. 2.63). Therefore cervical nerves C2 to C7 also emerge from the vertebral canal above their respective vertebrae. Because there are only seven cervical vertebrae, C8 emerges between vertebrae CVII and TI. As a consequence, all remaining spinal nerves, beginning with T1, emerge from the vertebral canal below their respective vertebrae. | Gray's Anatomy |
Surface features of the back are used to locate muscle groups for testing peripheral nerves, to determine regions of the vertebral column, and to estimate the approximate position of the inferior end of the spinal cord. They are also used to locate organs that occur posteriorly in the thorax and abdomen.
Absence of lateral curvatures
When viewed from behind, the normal vertebral column has no lateral curvatures. The vertical skin furrow between muscle masses on either side of the midline is straight (Fig. 2.64).
in the sagittal plane
When viewed from the side, the normal vertebral column has primary curvatures in the thoracic and sacral/coccygeal regions and secondary curvatures in the cervical and lumbar regions (Fig. 2.65). The primary curvatures are concave anteriorly. The secondary curvatures are concave posteriorly.
A number of readily palpable bony features provide useful landmarks for defining muscles and for locating structures associated with the vertebral column. Among these features are the external occipital protuberance, the scapula, and the iliac crest (Fig. 2.66).
The external occipital protuberance is palpable in the midline at the back of the head just superior to the hairline.
The spine, medial border, and inferior angle of the scapula are often visible and are easily palpable.
The iliac crest is palpable along its entire length, from the anterior superior iliac spine at the lower lateral margin of the anterior abdominal wall to the posterior superior iliac spine near the base of the back. The position of the posterior superior iliac spine is often visible as a “sacral dimple” just lateral to the midline.
How to identify specific vertebral
Identification of vertebral spinous processes (Fig. 2.67A) can be used to differentiate between regions of the vertebral column and facilitate visualizing the position of deeper structures, such as the inferior ends of the spinal cord and subarachnoid space.
The spinous process of vertebra CII can be identified through deep palpation as the most superior bony protuberance in the midline inferior to the skull.
Most of the other spinous processes, except for that of vertebra CVII, are not readily palpable because they are obscured by soft tissue.
The spinous process of CVII is usually visible as a prominent eminence in the midline at the base of the neck (Fig. 2.67B), particularly when the neck is flexed.
Extending between CVII and the external occipital protuberance of the skull is the ligamentum nuchae, which is readily apparent as a longitudinal ridge when the neck is flexed (Fig. 2.67C).
Inferior to the spinous process of CVII is the spinous process of TI, which is also usually visible as a midline protuberance. Often it is more prominent than the spinous process of CVII (Fig. 2.67A,B).
The root of the spine of the scapula is at the same level as the spinous process of vertebra TIII, and the inferior angle of the scapula is level with the spinous process of vertebra TVII (Fig. 2.67A).
The spinous process of vertebra TXII is level with the midpoint of a vertical line between the inferior angle of the scapula and the iliac crest (Fig. 2.67A).
A horizontal line between the highest point of the iliac crest on each side crosses through the spinous process of vertebra LIV. The LIII and LV vertebral spinous processes can be palpated above and below the LIV spinous process, respectively (Fig. 2.67A).
The sacral dimples that mark the position of the posterior superior iliac spine are level with the SII vertebral spinous process (Fig. 2.67A).
The tip of the coccyx is palpable at the base of the vertebral column between the gluteal masses (Fig. 2.67A). | Gray's Anatomy |
The tips of the vertebral spinous processes do not always lie in the same horizontal plane as their corresponding vertebral bodies. In thoracic regions, the spinous processes are long and sharply sloped downward so that their tips lie at the level of the vertebral body below. In other words, the tip of the TIII vertebral spinous process lies at vertebral level TIV.
In lumbar and sacral regions, the spinous processes are generally shorter and less sloped than in thoracic regions, and their palpable tips more closely reflect the position of their corresponding vertebral bodies. As a consequence, the palpable end of the spinous process of vertebra LIV lies at approximately the LIV vertebral level.
Visualizing the inferior ends of the spinal cord and subarachnoid space
The spinal cord does not occupy the entire length of the vertebral canal. Normally in adults, it terminates at the level of the disc between vertebrae LI and LII; however, it may end as high as TXII or as low as the disc between vertebrae LII and LIII. The subarachnoid space ends at approximately the level of vertebra SII (Fig. 2.68A).
Because the subarachnoid space can be accessed in the lower lumbar region without endangering the spinal cord, it is important to be able to identify the position of the lumbar vertebral spinous processes. The LIV vertebral spinous process is level with a horizontal line between the highest points on the iliac crests. In the lumbar region, the palpable ends of the vertebral spinous processes lie opposite their corresponding vertebral bodies. The subarachnoid space can be accessed between vertebral levels LIII and LIV and between LIV and LV without endangering the spinal cord (Fig. 2.68B). The subarachnoid space ends at vertebral level SII, which is level with the sacral dimples marking the posterior superior iliac spines.
A number of intrinsic and extrinsic muscles of the back can readily be observed and palpated. The largest of these are the trapezius and latissimus dorsi muscles (Fig. 2.69A and 2.69B). Retracting the scapulae toward the midline can accentuate the rhomboid muscles (Fig. 2.69C), which lie deep to the trapezius muscle. The erector spinae muscles are visible as two longitudinal columns separated by a furrow in the midline (Fig. 2.69A).
Fig. 2.1 Skeletal framework of the back.
Fig. 2.2 Curvatures of the vertebral column.
Cervical curvature(secondary curvature)Thoracic curvature(primary curvature)Lumbar curvature(secondary curvature)Sacral/coccygeal curvature(primary curvature)Gravity lineConcave primarycurvature of backEarly embryoAdultSomites
Fig. 2.3 Back movements.
Fig. 2.4 Nervous system.
Fig. 2.5 Vertebrae.
Fig. 2.6 A typical vertebra. A. Superior view. B. Lateral view.
Fig. 2.7 Back muscles. A. Extrinsic muscles. B. Intrinsic muscles.
Deep groupSerratus posteriorinferiorSerratus posteriorsuperiorSuboccipitalLevator scapulaeSpleniusRhomboid minorSuperficial groupABIntermediate groupIntrinsic musclesTrue back muscles innervated by posterior rami of spinal nervesRhomboid majorSpinalisIliocostalisErector spinaeLongissimusLatissimusdorsiTrapeziusExtrinsic musclesInnervated by anterior rami of spinal nerves or cranial nerve XI (trapezius)
Fig. 2.8 Vertebral canal.
Spinal cordPia materSubarachnoid spaceDura materArachnoid materAnterior ramusPosterior ramusPosition of spinal ganglionTransverseprocessSpinousprocessPosterior longitudinalligamentAnterior internal vertebralvenous plexusIntervertebral discExtradural spaceExtradural fatVertebral body | Gray's Anatomy |
Fig. 2.9 Spinal nerves (transverse section).
Fig. 2.10 Relationships of the back to other regions.
Cervical region• supports and moves head• transmits spinal cord and vertebral arteries between head and neck Thoracic region• support for thoraxLumbar region• support for abdomenSacral region• transmits weight to lower limbs through pelvic bones• framework for posterior aspect of pelvisVertebral arteries travelin transverse processes ofC6-C1, then pass throughforamen magnum
Fig. 2.11 Vertebral canal, spinal cord, and spinal nerves.
1121110112233445595678412345678123C8T1T2T3T4T5T6T7T8T9T10T11T12L1L2L3L4L5S1S2S3S4S5CoC7C6C5C4Cervicalenlargement(of spinal cord)C2C3C1SubarachnoidspaceLumbosacralenlargement(of spinal cord)Arachnoid materEnd of spinalcord at LI–LIIvertebraeEnd ofsubarachnoidspace–sacralvertebra IIDura materPedicles ofvertebraeSpinalganglion
Fig. 2.12 Intervertebral foramina.
Fig. 2.13 Dermatomes innervated by posterior rami of spinal nerves.
C2C3C4T2T3T4T5T6T7T8T9L5S1S2S4S3S5, Co*The dorsal rami of L4 and L5 may not have cutaneousbranches and may therefore not be represented asdermatomes on the backL4L3L2L1T11T12T10
Fig. 2.14 Vertebrae.
Fig. 2.15 Radiograph of cervical region of vertebral column. A. Anteroposterior view. B. Lateral view.
ARib IICIISpinous process of CVII
Vertebralbody of CIIILocation ofintervertebral discVertebra prominens(spinous process of CVII)Posterior tubercleof CI (atlas)B
Fig. 2.16 Radiograph of thoracic region of vertebral column. A. Anteroposterior view. B. Lateral view.
RibPedicleLocation of intervertebral discSpinous processTransverse processVertebral bodyA
BIntervertebral foramenVertebral bodyLocation of intervertebral disc
Fig. 2.17 Radiograph of lumbar region of vertebral column. A. Anteroposterior view. B. Lateral view.
RibTransverse processPedicleSpinous process of LIVA
Location ofintervertebral discVertebral body of LIIIIntervertebral foramenB
Fig. 2.18 Development of the vertebrae.
Fig. 2.19 Typical vertebra.
Fig. 2.20 Regional vertebrae. A. Typical cervical vertebra.
B. Atlas and axis. C. Typical thoracic vertebra. D. Typical lumbar vertebra.
E. Sacrum. F. Coccyx. | Gray's Anatomy |
Transverse processDensDensForamen transversariumSuperior viewSuperior viewSuperior viewPosterior viewPosterosuperior viewBAnterior tuberclePosterior tubercleAnterior archLateral massPosterior archFacet for densFacet for occipital condyleImpressionsfor alarligamentsAlarligamentsTectorial membrane (upper partof posterior longitudinal ligament)PosteriorlongitudinalligamentFacets forattachment ofalar ligamentsAtlas (CI vertebra)Atlas (CI vertebra) and Axis (CII vertebra)Atlas (CIvertebra)and Axis(CII vertebra)and baseof skullAxis (CII vertebra)Transverse ligament of atlasTransverse ligament of atlasVertebral bodyTransverse processTransverseprocessSpinousprocessMammillaryprocessSpinousprocessSuperior viewLateral viewSuperior viewFacet for articulationwith tubercle ofits own ribDemifacet for articulationwith head of rib belowDemifacet for articulationwith head of its own ribCDApical ligamentof densInferior longitudinalband of cruciformligament
Anterior viewDorsolateral viewPosterior viewFacet for articulation with pelvic boneEFAnterior sacral foraminaPosterior sacral foraminaCoccygeal cornuIncomplete sacral canal
Fig. 2.21 Radiograph showing CI (atlas) and CII (axis) vertebrae. Open mouth, anteroposterior (odontoid peg) view.
Superior articularfacet of CIIDensInferior articular faceton lateral mass of CI
Fig. 2.22 Intervertebral foramen.
Fig. 2.23 Spaces between adjacent vertebral arches in the lumbar region.
Fig. 2.24 T1-weighted MR image in the sagittal plane demonstrating a lumbosacral myelomeningocele. There is an absence of laminae and spinous processes in the lumbosacral region.
Fig. 2.25 Radiograph of the lumbar region of the vertebral column demonstrating a wedge fracture of the L1 vertebra. This condition is typically seen in patients with osteoporosis.
Fig. 2.26 Radiograph of the lumbar region of the vertebral column demonstrating three intrapedicular needles, all of which have been placed into the middle of the vertebral bodies. The high-density material is radiopaque bone cement, which has been injected as a liquid that will harden.
Fig. 2.27 Severe scoliosis. A. Radiograph, anteroposterior view. B. Volume-rendered CT, anterior view.
Fig. 2.28 Sagittal CT showing kyphosis.
Fig. 2.29 Variations in vertebral number. A. Fused vertebral bodies of cervical vertebrae. B. Hemivertebra. C. Axial slice MRI through the LV vertebra. The iliolumbar ligament runs from the tip of the LV vertebra transverse process to the iliac crest.
Fused bodies of cervical vertebraeA
HemivertebraPartial lumbarization of first sacral vertebraB
Fig. 2.30 A. MRI of a spine with multiple collapsed vertebrae due to diffuse metastatic myeloma infiltration. B1, B2. Positron emission tomography CT (PETCT) study detecting cancer cells in the spine that have high glucose metabolism.
Fig. 2.31 Intervertebral joints.
Anulus fibrosusNucleus pulposusLayer of hyalinecartilage
Fig. 2.32 Zygapophysial joints.
Fig. 2.33 Uncovertebral joint.
Fig. 2.34 Disc protrusion. T2-weighted magnetic resonance images of the lumbar region of the vertebral column. A. Sagittal plane.
B. Axial plane.
Fig. 2.35 Anterior and posterior longitudinal ligaments of vertebral column. | Gray's Anatomy |
Fig. 2.36 Ligamenta flava.
Fig. 2.37 Supraspinous ligament and ligamentum nuchae.
Fig. 2.38 Interspinous ligaments.
Fig. 2.39 Axial slice MRI through the lumbar spine demonstrating bilateral hypertrophy of the ligamentum flavum.
Fig. 2.40 Radiograph of lumbar region of vertebral column, oblique view (“Scottie dog”). A. Normal radiograph of lumbar region of vertebral column, oblique view. In this view, the transverse process (nose), pedicle (eye), superior articular process (ear), inferior articular process (front leg), and pars interarticularis (neck) resemble a dog. A fracture of the pars interarticularis is visible as a break in the neck of the dog, or the appearance of a collar. B. Fracture of pars interarticularis. C. CT of lumbar spine shows fracture of the LV pars interarticularis.
Fig. 2.41 A. Anterior lumbar interbody fusion (ALIF). B. Posterior lumbar interbody fusion (PLIF).
Fig. 2.42 Superficial group of back muscles—trapezius and latissimus dorsi.
Spinous process of CVIIAcromionSpine of scapulaIliac crestGreater occipital nerve(posterior ramus of C2)Third occipital nerve(posterior ramus of C3)Medial branches of posterior ramiLateral branches of posterior ramiTrapeziusLatissimus dorsiThoracolumbar fascia
Fig. 2.43 Superficial group of back muscles—trapezius and latissimus dorsi, with rhomboid major, rhomboid minor, and levator scapulae located deep to trapezius in the superior part of the back.
Fig. 2.44 Innervation and blood supply of trapezius.
TrapeziusLatissimus dorsiRhomboid minorRhomboid majorLevator scapulaeAccessory nerve [XI]Superficial branch of transverse cervical artery
Fig. 2.45 Rhomboid muscles and levator scapulae.
Fig. 2.46 Innervation and blood supply of the rhomboid muscles.
Dorsal scapular nerveTrapeziusLatissimus dorsiRhomboid minorRhomboid majorLevator scapulaeSuperficial branch of transverse cervical arteryDeep branch of transverse cervical artery
Fig. 2.47 Intermediate group of back muscles—serratus posterior muscles.
Fig. 2.48 Thoracolumbar fascia and the deep back muscles (transverse section).
Fig. 2.49 Deep group of back muscles—spinotransversales muscles (splenius capitis and splenius cervicis).
Fig. 2.50 Deep group of back muscles—erector spinae muscles.
Spinous process of CVIIIliac crestSplenius capitisLongissimus capitis Ligamentum nuchaeLongissimus thoracisLongissimus cervicisSpinalis thoracisSpinalisIliocostalis lumborum Iliocostalis thoracisIliocostalis cervicisIliocostalisLongissimus
Fig. 2.51 Deep group of back muscles—transversospinales and segmental muscles.
Spinous process of CVIIObliquus capitis inferiorObliquus capitis superiorRectus capitis posterior minorRectus capitis posterior majorSemispinalis thoracisIntertransversariusErector spinaeRotatores thoracis(short, long)Levatores costarum(short, long)Semispinalis capitisMultifidus
Fig. 2.52 Deep group of back muscles—suboccipital muscles. This also shows the borders of the suboccipital triangle. | Gray's Anatomy |
Spinous process of CIIPosterior ramus of C1Obliquus capitis superior Rectus capitis posterior minorObliquus capitis inferiorRectus capitis posterior majorSplenius capitisSplenius capitisLongissimus capitisSemispinalis cervicisSemispinalis capitisSemispinalis capitisVertebral artery
Fig. 2.53 Spinal cord.
End of spinalcord LI–LIIConus medullarisInferior part ofarachnoid materEnd of subarachnoidspace SIICervicalenlargement(of spinal cord)Lumbosacralenlargement(of spinal cord)FilumterminalePial partDural partPedicles ofvertebrae
Fig. 2.54 Features of the spinal cord.
Fig. 2.55 Arteries that supply the spinal cord. A. Anterior view of spinal cord (not all segmental spinal arteries are shown).
B. Segmental supply of spinal cord.
Posterior spinal arteryADeep cervical arteryCostocervical trunkThyrocervical trunkSubclavian arteryPosterior intercostalarterySegmentalspinal arteryArtery of Adamkiewicz(branch fromsegmentalspinal artery)Ascending cervicalarteryVertebral arterySegmental medullaryarteriesAnterior spinal arterySegmental medullaryarteries (branch fromsegmental spinalartery)Lateral sacral arterySegmentalspinal artery
Fig. 2.56 Veins that drain the spinal cord.
Fig. 2.57 MRI of the spine. There is discitis of the T10-T11 intervertebral disc with destruction of the adjacent endplates. There is also a prevertebral abscess and an epidural abscess, which impinges the cord.
Fig. 2.58 CT at the level of CI demonstrates two breaks in the closed ring of the atlas following an axial-loading injury.
Fig. 2.59 Meninges.
Fig. 2.60 Arrangement of structures in the vertebral canal and the back (lumbar region).
Crura of diaphragmAortaPsoasDuraQuadratus lumborumInternal vertebral plexus of veinsin extradural spaceErector spinae musclesLigamenta flavaSupraspinous ligamentInterspinous ligamentLumbar arteryVeinCauda equinaSkinVertebraIntervertebral discIntervertebral foramenLaminaPediclePosterior longitudinal ligament
Fig. 2.61 Basic organization of a spinal nerve.
Fig. 2.62 Course of spinal nerves in the vertebral canal.
1121110112233445595678412345678123C8T1T2T3T4T5T6T7T8T9T10T11T12L1L2L3L4L5S1S2S3S4S5CoC7C6C5C4Cervical enlargement(of spinal cord)C2C3C1Lumbosacral enlargement(of spinal cord)Cauda equinaPedicles of vertebraeSpinal ganglion
Fig. 2.63 Nomenclature of the spinal nerves.
Nerve C1 emerges betweenskull and CI vertebraNerve C8 emerges inferior topedicle of CVII vertebraNerves C2 to C7 emergesuperior to pediclesNerves T1 to Co emergeinferior to pedicles oftheir respective vertebraeC2C1C3C4C5C6C7C8T1CICVIITIPedicleTransition innomenclatureof nervesT2
Fig. 2.64 Normal appearance of the back. A. In women. B. In men.
Fig. 2.65 Normal curvatures of the vertebral column.
Fig. 2.66 Back of a woman with major palpable bony landmarks indicated. | Gray's Anatomy |
Spine of scapulaInferior angle of scapulaMedial border of scapulaPosition of externaloccipital protuberancePosterior superior iliac spineIliac crest
Fig. 2.67 The back with the positions of vertebral spinous processes and associated structures indicated. A. In a man. B. In a woman with neck flexed. The prominent CVII and TI vertebral spinous processes are labeled. C. In a woman with neck flexed to accentuate the ligamentum nuchae.
Tip of coccyxSII vertebral spinous processTXII vertebral spinous processTVII vertebral spinous processTIII vertebral spinous processTI vertebral spinous processRoot of spine of scapulaInferior angle of scapulaHighest point of iliac crestIliac crestSacral dimpleCVII vertebral spinous processCII vertebral spinous processPosition of externaloccipital protuberanceLIV vertebral spinous processA
Fig. 2.68 Back with the ends of the spinal cord and subarachnoid space indicated. A. In a man.
Back with the ends of the spinal cord and subarachnoid space indicated. B. In a woman lying on her side in a fetal position, which accentuates the lumbar vertebral spinous processes and opens the spaces between adjacent vertebral arches. Cerebrospinal fluid can be withdrawn from the subarachnoid space in lower lumbar regions without endangering the spinal cord.
Tip of coccyxSII vertebral spinous processTXII vertebral spinous processInferior end of spinal cord(normally betweenLI and LII vertebra)Inferior end ofsubarachnoid spaceALIV vertebral spinous process
LIV vertebral spinous processNeedleLV vertebral spinous processTip of coccyxB
Fig. 2.69 Back muscles. A. In a man with latissimus dorsi, trapezius, and erector spinae muscles outlined.
Back muscles. B. In a man with arms abducted to accentuate the lateral margins of the latissimus dorsi muscles. C. In a woman with scapulae externally rotated and forcibly retracted to accentuate the rhomboid muscles.
Fig. 2.70 MRI of the lumbar spine reveals posterior herniation of the L2-3 disc resulting in compression of the cauda equina filaments.
Table 2.1 Superficial (appendicular) group of back muscles
Table 2.2 Intermediate (respiratory) group of back muscles
Table 2.3 Spinotransversales muscles
Table 2.4 Erector spinae group of back muscles
Table 2.5 Transversospinales group of back muscles
Table 2.6 Segmental back muscles
Table 2.7 Suboccipital group of back muscles
In the clinic
Spina bifida is a disorder in which the two sides of vertebral arches, usually in lower vertebrae, fail to fuse during development, resulting in an “open” vertebral canal (Fig. 2.24). There are two types of spina bifida.
The commonest type is spina bifida occulta, in which there is a defect in the vertebral arch of LV or SI. This defect occurs in as many as 10% of individuals and results in failure of the posterior arch to fuse in the midline. Clinically, the patient is asymptomatic, although physical examination may reveal a tuft of hair over the spinous processes.
The more severe form of spina bifida involves complete failure of fusion of the posterior arch at the lumbosacral junction, with a large outpouching of the meninges. This may contain cerebrospinal fluid (a meningocele) or a portion of the spinal cord (a myelomeningocele). These abnormalities may result in a variety of neurological deficits, including problems with walking and bladder function.
In the clinic | Gray's Anatomy |
Vertebroplasty is a relatively new technique in which the body of a vertebra can be filled with bone cement (typically methyl methacrylate). The indications for the technique include vertebral body collapse and pain from the vertebral body, which may be secondary to tumor infiltration. The procedure is most commonly performed for osteoporotic wedge fractures, which are a considerable cause of morbidity and pain in older patients.
Osteoporotic wedge fractures (Fig. 2.25) typically occur in the thoracolumbar region, and the approach to performing vertebroplasty is novel and relatively straightforward. The procedure is performed under sedation or light general anesthetic. Using X-ray guidance the pedicle is identified on the anteroposterior image. A metal cannula is placed through the pedicle into the vertebral body. Liquid bone cement is injected via the cannula into the vertebral body (Fig. 2.26). The function of the bone cement is two-fold. First, it increases the strength of the vertebral body and prevents further loss of height. Furthermore, as the bone cement sets, there is a degree of heat generated that is believed to disrupt pain nerve endings. Kyphoplasty is a similar technique that aims to restore some or all of the lost vertebral body height from the wedge fracture by injecting liquid bone cement into the vertebral body.
In the clinic
Scoliosis is an abnormal lateral curvature of the vertebral column (Fig. 2.27).
A true scoliosis involves not only the curvature (rightor left-sided) but also a rotational element of one vertebra upon another.
The commonest types of scoliosis are those for which we have little understanding about how or why they occur and are termed idiopathic scoliosis. It is thought that there is some initial axial rotation of the vertebrae, which then alters the locations of the mechanical compressive and distractive forces applied through the vertebral growth plates, leading to changes in speed of bone growth and ultimately changes to spinal curvature. These are never present at birth and tend to occur in either the infantile, juvenile, or adolescent age groups. The vertebral bodies and posterior elements (pedicles and laminae) are normal in these patients.
When a scoliosis is present from birth (congenital scoliosis) it is usually associated with other developmental abnormalities. In these patients, there is a strong association with other abnormalities of the chest wall, genitourinary tract, and heart disease. This group of patients needs careful evaluation by many specialists.
A rare but important group of scoliosis is that in which the muscle is abnormal. Muscular dystrophy is the commonest example. The abnormal muscle does not retain the normal alignment of the vertebral column, and curvature develops as a result. A muscle biopsy is needed to make the diagnosis.
Other disorders that can produce scoliosis include bone tumors, spinal cord tumors, and localized disc protrusions.
In the clinic
Kyphosis is abnormal curvature of the vertebral column in the thoracic region, producing a “hunchback” deformity. This condition occurs in certain disease states, the most dramatic of which is usually secondary to tuberculosis infection of a thoracic vertebral body, where the kyphosis becomes angulated at the site of the lesion. This produces the gibbus deformity, a deformity that was prevalent before the use of antituberculous medication (Fig. 2.28).
In the clinic
Lordosis is abnormal curvature of the vertebral column in the lumbar region, producing a swayback deformity.
In the clinic
There are usually seven cervical vertebrae, although in certain diseases these may be fused. Fusion of cervical vertebrae (Fig. 2.29A) can be associated with other abnormalities, for example Klippel-Feil syndrome, in which there is fusion of vertebrae CI and CII or CV and CVI, and may be associated with a high-riding abnormalities.
Variations in the number of thoracic vertebrae also are well described. | Gray's Anatomy |
One of the commonest abnormalities in the lumbar vertebrae is a partial fusion of vertebra LV with the sacrum (sacralization of the lumbar vertebra). Partial separation of vertebra SI from the sacrum (lumbarization of first sacral vertebra) may also occur (Fig. 2.29B). The LV vertebra can usually be identified by the iliolumbar ligament, which is a band of connective tissue that runs from the tip of the transverse process of LV to the iliac crest bilaterally (Fig. 2.29C).
A hemivertebra occurs when a vertebra develops only on one side (Fig. 2.29B).
In the clinic
The vertebrae and cancer
The vertebrae are common sites for metastatic disease (secondary spread of cancer cells). When cancer cells grow within the vertebral bodies and the posterior elements, they interrupt normal bone cell turnover, leading to either bone destruction or formation and destroying the mechanical properties of the bone. A minor injury may therefore lead to vertebral collapse (Fig. 2.30A). Cancer cells have a much higher glucose metabolism compared with normal adjacent bone cells. These metastatic cancer cells can therefore be detected by administering radioisotope-labeled glucose to a patient and then tracing where the labeled glucose has been metabolized (Fig. 2.30B). Importantly, vertebrae that contain extensive metastatic disease may extrude fragments of tumor into the vertebral canal, compressing nerves and the spinal cord.
In the clinic
Osteoporosis is a pathophysiologic condition in which bone quality is normal but the quantity of bone is deficient. It is a metabolic bone disorder that commonly occurs in women in their 50s and 60s and in men in their 70s.
Many factors influence the development of osteoporosis, including genetic predetermination, level of activity and nutritional status, and, in particular, estrogen levels in women.
Typical complications of osteoporosis include “crush” vertebral body fractures, distal fractures of the radius, and hip fractures.
With increasing age and poor-quality bone, patients are more susceptible to fracture. Healing tends to be impaired in these elderly patients, who consequently require long hospital stays and prolonged rehabilitation.
Patients likely to develop osteoporosis can be identified by dual-photon X-ray absorptiometry (DXA) scanning. Low-dose X-rays are passed through the bone, and by counting the number of photons detected and knowing the dose given, the number of X-rays absorbed by the bone can be calculated. The amount of X-ray absorption can be directly correlated with the bone mass, and this can be used to predict whether a patient is at risk for osteoporotic fractures.
In the clinic
Back pain is an extremely common disorder. It can be related to mechanical problems or to disc protrusion impinging on a nerve. In cases involving discs, it may be necessary to operate and remove the disc that is pressing on the nerve.
Not infrequently, patients complain of pain and no immediate cause is found; the pain is therefore attributed to mechanical discomfort, which may be caused by degenerative disease. One of the treatments is to pass a needle into the facet joint and inject it with local anesthetic and corticosteroid.
In the clinic
Herniation of intervertebral discs
The discs between the vertebrae are made up of a central portion (the nucleus pulposus) and a complex series of fibrous rings (anulus fibrosus). A tear can occur within the anulus fibrosus through which the material of the nucleus pulposus can track. After a period of time, this material may track into the vertebral canal or into the intervertebral foramen to impinge on neural structures (Fig. 2.34). This is a common cause of back pain. A disc may protrude posteriorly to directly impinge on the cord or the roots of the lumbar nerves, depending on the level, or may protrude posterolaterally adjacent to the pedicle and impinge on the descending root. | Gray's Anatomy |
In cervical regions of the vertebral column, cervical disc protrusions often become ossified and are termed disc osteophyte bars.
In the clinic
Some diseases have a predilection for synovial joints rather than symphyses. A typical example is rheumatoid arthritis, which primarily affects synovial joints and synovial bursae, resulting in destruction of the joint and its lining. Symphyses are usually preserved.
In the clinic
The ligamenta flava are important structures associated with the vertebral canal (Fig. 2.39). In degenerative conditions of the vertebral column, the ligamenta flava may hypertrophy. This is often associated with hypertrophy and arthritic change of the zygapophysial joints. In combination, zygapophysial joint hypertrophy, ligamenta flava hypertrophy, and a mild disc protrusion can reduce the dimensions of the vertebral canal, producing the syndrome of spinal stenosis.
In the clinic
Vertebral fractures can occur anywhere along the vertebral column. In most instances, the fracture will heal under appropriate circumstances. At the time of injury, it is not the fracture itself but related damage to the contents of the vertebral canal and the surrounding tissues that determines the severity of the patient’s condition.
Vertebral column stability is divided into three arbitrary clinical “columns”: the anterior column consists of the vertebral bodies and the anterior longitudinal ligament; the middle column comprises the vertebral body and the posterior longitudinal ligament; and the posterior column is made up of the ligamenta flava, interspinous ligaments, supraspinous ligaments, and the ligamentum nuchae in the cervical vertebral column.
Destruction of one of the clinical columns is usually a stable injury requiring little more than rest and appropriate analgesia. Disruption of two columns is highly likely to be unstable and requires fixation and immobilization. A three-column spinal injury usually results in a significant neurological event and requires fixation to prevent further extension of the neurological defect and to create vertebral column stability.
At the craniocervical junction, a complex series of ligaments create stability. If the traumatic incident disrupts craniocervical stability, the chances of a significant spinal cord injury are extremely high. The consequences are quadriplegia. In addition, respiratory function may be compromised by paralysis of the phrenic nerve (which arises from spinal nerves C3 to C5), and severe hypotension (low blood pressure) may result from central disruption of the sympathetic part of the autonomic division of the nervous system.
Mid and lower cervical vertebral column disruption may produce a range of complex neurological problems involving the upper and lower limbs, although below the level of C5, respiratory function is unlikely to be compromised.
Lumbar vertebral column injuries are rare. When they occur, they usually involve significant force. Knowing that a significant force is required to fracture a vertebra, one must assess the abdominal organs and the rest of the axial skeleton for further fractures and visceral rupture.
Vertebral injuries may also involve the soft tissues and supporting structures between the vertebrae. Typical examples of this are the unifacetal and bifacetal cervical vertebral dislocations that occur in hyperflexion injuries.
The pars interarticularis is a clinical term to describe the specific region of a vertebra between the superior and inferior facet (zygapophysial) joints (Fig. 2.40A). This region is susceptible to trauma, especially in athletes.
If a fracture occurs around the pars interarticularis, the vertebral body may slip anteriorly and compress the vertebral canal. | Gray's Anatomy |
The most common sites for pars interarticularis fractures are the LIV and LV levels (Fig. 2.40B,C). (Clinicians often refer to parts of the back in shorthand terms that are not strictly anatomical; for example, facet joints and apophyseal joints are terms used instead of zygapophysial joints, and spinal column is used instead of vertebral column.)
It is possible for a vertebra to slip anteriorly upon its inferior counterpart without a pars interarticularis fracture. Usually this is related to abnormal anatomy of the facet joints, facet joint degenerative change. This disorder is termed spondylolisthesis.
In the clinic
Surgical procedures on the back
A prolapsed intervertebral disc may impinge upon the meningeal (thecal) sac, cord, and most commonly the nerve root, producing symptoms attributable to that level. In some instances the disc protrusion will undergo a degree of involution that may allow symptoms to resolve without intervention. In some instances pain, loss of function, and failure to resolve may require surgery to remove the disc protrusion.
It is of the utmost importance that the level of the disc protrusion is identified before surgery. This may require MRI scanning and on-table fluoroscopy to prevent operating on the wrong level. A midline approach to the right or to the left of the spinous processes will depend upon the most prominent site of the disc bulge. In some instances removal of the lamina will increase the potential space and may relieve symptoms. Some surgeons perform a small fenestration (windowing) within the ligamentum flavum. This provides access to the canal. The meningeal sac and its contents are gently retracted, exposing the nerve root and the offending disc. The disc is dissected free, removing its effect on the nerve root and the canal.
Spinal fusion is performed when it is necessary to fuse one vertebra with the corresponding superior or inferior vertebra, and in some instances multilevel fusion may be necessary. Indications are varied, though they include stabilization after fracture, stabilization related to tumor infiltration, and stabilization when mechanical pain is produced either from the disc or from the posterior elements.
There are a number of surgical methods in which a fusion can be performed, through either a posterior approach and fusing the posterior elements, an anterior approach by removal of the disc and either disc replacement or anterior fusion, or in some instances a 360° fusion where the posterior elements and the vertebral bodies are fused (Fig. 2.41A,B).
In the clinic
Weakness in the trapezius, caused by an interruption of the accessory nerve [XI], may appear as drooping of the shoulder, inability to raise the arm above the head because of impaired rotation of the scapula, or weakness in attempting to raise the shoulder (i.e., shrug the shoulder against resistance).
A weakness in, or an inability to use, the latissimus dorsi, resulting from an injury to the thoracodorsal nerve, diminishes the capacity to pull the body upward while climbing or doing a pull-up.
An injury to the dorsal scapular nerve, which innervates the rhomboids, may result in a lateral shift in the position of the scapula on the affected side (i.e., the normal position of the scapula is lost because of the affected muscle’s inability to prevent antagonistic muscles from pulling the scapula laterally).
In the clinic
The intervertebral discs are poorly vascularized; however, infection within the bloodstream can spread to the discs from the terminal branches of the spinal arteries within the vertebral body endplates, which lie immediately adjacent to the discs (Fig. 2.57). Common sources of infection include the lungs and urinary tract.
In the clinic
Fractures of the atlas and axis | Gray's Anatomy |
Fractures of vertebra CI (the atlas) and vertebra CII (the axis) can potentially lead to the worst types of spinal cord injury including death and paralysis due to injury of the brainstem, which contains the cardiac and respiratory centers. The atlas is a closed ring with no vertebral body. Axial-loading injuries, such as hitting the head while diving into shallow water or hitting the head on the roof of a car in a motor vehicle accident, can cause a “burst” type of fracture, where the ring breaks at more than one site (Fig. 2.58). The British neurosurgeon, Geoffrey Jefferson, first described this fracture pattern in 1920, so these types of fractures are often called Jefferson fractures.
Fractures of the axis usually occur due to severe hyperextension and flexion, which can result in fracture of the tip of the dens, base of the dens, or through the body of the atlas. In judicial hangings, there is hyperextension and distraction injury causing fracture through the atlas pedicles and spondylolisthesis of C2 on C3. This type of fracture is often called a hangman’s fracture.
In many cases of upper neck injuries, even in the absence of fractures to the atlas or axis, there may be injury to the atlanto-axial ligaments, which can render the neck unstable and pose severe risk to the brainstem and upper spinal cord.
In the clinic
An injury to the spinal cord in the cervical portion of the vertebral column can lead to varying degrees of impairment of sensory and motor function (paralysis) in all 4 limbs, termed quadriplegia or tetraplegia. An injury in upper levels of the cervical vertebral column can result in death because of loss of innervation to the diaphragm. An injury to the spinal cord below the level of TI can lead to varying degrees of impairment in motor and sensory function (paralysis) in the lower limbs, termed paraplegia.
In the clinic
A lumbar tap (puncture) is carried out to obtain a sample of CSF for examination. In addition, passage of a needle or conduit into the subarachnoid space (CSF space) is used to inject antibiotics, chemotherapeutic agents, and anesthetics.
The lumbar region is an ideal site to access the subarachnoid space because the spinal cord terminates around the level of the disc between vertebrae LI and LII in the adult. The subarachnoid space extends to the region of the lower border of the SII vertebra. There is therefore a large CSF-filled space containing lumbar and sacral nerve roots but no spinal cord.
Depending on the clinician’s preference, the patient is placed in the lateral or prone position. A needle is passed in the midline in between the spinous processes into the extradural space. Further advancement punctures the dura and arachnoid mater to enter the subarachnoid space. Most needles push the roots away from the tip without causing the patient any symptoms. Once the needle is in the subarachnoid space, fluid can be aspirated. In some situations, it is important to measure CSF pressure.
Local anesthetics can be injected into the extradural space or the subarachnoid space to anesthetize the sacral and lumbar nerve roots. Such anesthesia is useful for operations on the pelvis and the legs, which can then be carried out without the need for general anesthesia. When procedures are carried out, the patient must be in the erect position and not lying on his or her side or in the head-down position. If a patient lies on his or her side, the anesthesia is likely to be unilateral. If the patient is placed in the head-down position, the anesthetic can pass cranially and potentially depress respiration. | Gray's Anatomy |
In some instances, anesthesiologists choose to carry out extradural anesthesia. A needle is placed through the skin, supraspinous ligament, interspinous ligament, and ligamenta flava into the areolar tissue and fat around the dura mater. Anesthetic agent is introduced and diffuses around the vertebral canal to anesthetize the exiting nerve roots and diffuse into the subarachnoid space.
In the clinic
Herpes zoster is the virus that produces chickenpox in children. In some patients the virus remains dormant in the cells of the spinal ganglia. Under certain circumstances, the virus becomes activated and travels along the neuronal bundles to the areas supplied by that nerve (the dermatome). A rash ensues, which is characteristically exquisitely painful. Importantly, this typical dermatomal distribution is characteristic of this disorder.
In the clinic
Back pain is an extremely common condition affecting almost all individuals at some stage during their life. It is of key clinical importance to identify whether the back pain relates to the vertebral column and its attachments or relates to other structures.
The failure to consider other potential structures that may produce back pain can lead to significant mortality and morbidity. Pain may refer to the back from a number of organs situated in the retroperitoneum. Pancreatic pain in particular refers to the back and may be associated with pancreatic cancer and pancreatitis. Renal pain, which may be produced by stones in the renal collecting system or renal tumors, also typically refers to the back. More often than not this is usually unilateral; however, it can produce central posterior back pain. Enlarged lymph nodes in the preand para-aortic region may produce central posterior back pain and may be a sign of solid tumor malignancy, infection, or Hodgkin’s lymphoma. An enlarging abdominal aorta (abdominal aortic aneurysm) may cause back pain as it enlarges without rupture. Therefore it is critical to think of this structure as a potential cause of back pain, because treatment will be lifesaving. Moreover, a ruptured abdominal aortic aneurysm may also cause acute back pain in the first instance.
In all patients back pain requires careful assessment not only of the vertebral column but also of the chest and abdomen in order not to miss other important anatomical structures that may produce signs and symptoms radiating to the back.
A 50-year-old man was brought to the emergency department with severe lower back pain that had started several days ago. In the past 24 hours he has had two episodes of fecal incontinence and inability to pass urine and now reports numbness and weakness in both his legs.
The attending physician performed a physical examination and found that the man had reduced strength during knee extension and when dorsiflexing his feet and toes. He also had reduced reflexes in his knees and ankles, numbness in the perineal (saddle) region, as well as reduced anal sphincter tone.
The patient’s symptoms and physical examination findings raised serious concern for compression of multiple lumbar and sacral nerve roots in the spine, affecting both motor and sensory pathways. His reduced power in extending his knees and reduced knee reflexes was suggestive of compression of the L4 nerve roots. His reduced ability to dorsiflex his feet and toes was suggestive of compression of the L5 nerve roots. His reduced ankle reflexes was suggestive of compression of the S1 and S2 nerve roots, and his perineal numbness was suggestive of compression of the S3, S4, and S5 nerve roots.
A diagnosis of cauda equina syndrome was made, and the patient was transferred for an urgent MRI scan, which confirmed the presence of a severely herniating L2-3 disc compressing the cauda equina, giving rise to the cauda equina syndrome (Fig. 2.70). The patient underwent surgical decompression of the cauda equina and made a full recovery. | Gray's Anatomy |
The collection of lumbar and sacral nerve roots beyond the conus medullaris has a horsetail-like appearance, from which it derives its name “cauda equina.” Compression of the cauda equina may be caused by a herniating disc (as in this case), fracture fragments following traumatic injury, tumor, abscess, or severe degenerative stenosis of the central canal.
Cauda equina syndrome is classed as a surgical emergency to prevent permanent and irreversible damage to the compressed nerve roots.
A 45-year-old man was involved in a serious car accident. On examination he had a severe injury to the cervical region of his vertebral column with damage to the spinal cord. In fact, his breathing became erratic and stopped.
If the cervical spinal cord injury is above the level of C5, breathing is likely to stop. The phrenic nerve takes origin from C3, C4, and C5 and supplies the diaphragm. Breathing may not cease immediately if the lesion is just below C5, but does so as the cord becomes edematous and damage progresses superiorly. In addition, some respiratory and ventilatory exchange may occur by using neck muscles plus the sternocleidomastoid and trapezius muscles, which are innervated by the accessory nerve [XI].
The patient was unable to sense or move his upper and lower limbs.
The patient has paralysis of the upper and lower limbs and is therefore quadriplegic. If breathing is unaffected, the lesion is below the level of C5 or at the level of C5. The nerve supply to the upper limbs is via the brachial plexus, which begins at the C5 level. The site of the spinal cord injury is at or above the C5 level.
It is important to remember that although the cord has been transected in the cervical region, the cord below this level is intact. Reflex activity may therefore occur below the injury, but communication with the brain is lost.
A 25-year-old woman complained of increasing lumbar back pain. Over the ensuing weeks she was noted to have an enlarging lump in the right groin, which was mildly tender to touch. On direct questioning, the patient also complained of a productive cough with sputum containing mucus and blood, and she had a mild temperature.
The chest radiograph revealed a cavitating apical lung mass, which explains the pulmonary history.
Given the age of the patient a primary lung cancer is unlikely. The hemoptysis (coughing up blood in the sputum) and the rest of the history suggest the patient has a lung infection. Given the chest radiographic findings of a cavity in the apex of the lung, a diagnosis of tuberculosis (TB) was made. This was confirmed by bronchoscopy and aspiration of pus, which was cultured.
During the patient’s pulmonary infection, the tuberculous bacillus had spread via the blood to vertebra LI. The bone destruction began in the cancellous bone of the vertebral body close to the intervertebral discs. This disease progressed and eroded into the intervertebral disc, which became infected. The disc was destroyed, and the infected disc material extruded around the disc anteriorly and passed into the psoas muscle sheath. This is not an uncommon finding for a tuberculous infection of the lumbar portion of the vertebral column.
As the infection progressed, the pus spread within the psoas muscle sheath beneath the inguinal ligament to produce a hard mass in the groin. This is a typical finding for a psoas abscess.
Fortunately for the patient, there was no evidence of any damage within the vertebral canal.
The patient underwent a radiologically guided drainage of the psoas abscess and was treated for over 6 months with a long-term antibiotic regimen. She made an excellent recovery with no further symptoms, although the cavities within the lungs remain. It healed with sclerosis. | Gray's Anatomy |
A 72-year-old fit and healthy man was brought to the emergency department with severe back pain beginning at the level of the shoulder blades and extending to the midlumbar region. The pain was of relatively acute onset and was continuous. The patient was able to walk to the gurney as he entered the ambulance; however, at the emergency department the patient complained of inability to use both legs.
The attending physician examined the back thoroughly and found no significant abnormality. He noted that there was reduced sensation in both legs, and there was virtually no power in extensor or flexor groups. The patient was tachycardic, which was believed to be due to pain, and the blood pressure obtained in the ambulance measured 120/80 mm Hg. It was noted that the patient’s current blood pressure was 80/40 mm Hg; however, the patient did not complain of typical clinical symptoms of hypotension.
On first inspection, it is difficult to “add up” these clinical symptoms and signs. In essence we have a progressive paraplegia associated with severe back pain and an anomaly in blood pressure measurements, which are not compatible with the clinical state of the patient.
It was deduced that the blood pressure measurements were obtained in different arms, and both were reassessed.
The blood pressure measurements were true. In the right arm the blood pressure measured 120/80 mm Hg and in the left arm the blood pressure measured 80/40 mm Hg. This would imply a deficiency of blood to the left arm.
The patient was transferred from the emergency department to the CT scanner, and a scan was performed that included the chest, abdomen, and pelvis.
The CT scan demonstrated a dissecting thoracic aortic aneurysm. Aortic dissection occurs when the tunica intima and part of the tunica media of the wall of the aorta become separated from the remainder of the tunica media and the tunica adventitia of the aorta wall. This produces a false lumen. Blood passes not only in the true aortic lumen but also through a small hole into the wall of the aorta and into the false lumen. It often reenters the true aortic lumen inferiorly. This produces two channels through which blood may flow. The process of the aortic dissection produces considerable pain for the patient and is usually of rapid onset. Typically the pain is felt between the shoulder blades and radiating into the back, and although the pain is not from the back musculature or the vertebral column, careful consideration of structures other than the back should always be sought.
The difference in the blood pressure between the two arms indicates the level at which the dissection has begun. The “point of entry” is proximal to the left subclavian artery. At this level a small flap has been created, which limits the blood flow to the left upper limb, giving the low blood pressure recording. The brachiocephalic trunk has not been affected by the aortic dissection, and hence blood flow remains appropriate to the right upper limb.
The paraplegia was caused by ischemia to the spinal cord.
The blood supply to the spinal cord is from a single anterior spinal artery and two posterior spinal arteries. These arteries are fed via segmental spinal arteries at every vertebral level. There are a number of reinforcing arteries (segmental medullary arteries) along the length of the spinal cord—the largest of which is the artery of Adamkiewicz. This artery of Adamkiewicz, a segmental medullary artery, typically arises from the lower thoracic or upper lumbar region, and unfortunately during this patient’s aortic dissection, the origin of this vessel was disrupted. This produces acute spinal cord ischemia and has produced the paraplegia in the patient.
Unfortunately, the dissection extended, the aorta ruptured, and the patient succumbed. | Gray's Anatomy |
A 55-year-old woman came to her physician with sensory alteration in the right gluteal (buttock) region and in the intergluteal (natal) cleft. Examination also demonstrated low-grade weakness of the muscles of the foot and subtle weakness of the extensor hallucis longus, extensor digitorum longus, and fibularis tertius on the right. The patient also complained of some mild pain symptoms posteriorly in the right gluteal region.
A lesion was postulated in the left sacrum.
Pain in the right sacro-iliac region could easily be attributed to the sacro-iliac joint, which is often very sensitive to pain. The weakness of the intrinsic muscles of the foot and the extensor hallucis longus, extensor digitorum longus, and fibularis tertius muscles raises the possibility of an abnormality affecting the nerves exiting the sacrum and possibly the lumbosacral junction. The altered sensation around the gluteal region toward the anus would also support these anatomical localizing features.
An X-ray was obtained of the pelvis.
The X-ray appeared on first inspection unremarkable. However, the patient underwent further investigation, including CT and MRI, which demonstrated a large destructive lesion involving the whole of the left sacrum extending into the anterior sacral foramina at the S1, S2, and S3 levels. Interestingly, plain radiographs of the sacrum may often appear normal on first inspection, and further imaging should always be sought in patients with a suspected sacral abnormality.
The lesion was expansile and lytic.
Most bony metastases are typically nonexpansile. They may well erode the bone, producing lytic type of lesions, or may become very sclerotic (prostate metastases and breast metastases). From time to time we see a mixed pattern of lytic and sclerotic.
There are a number of uncommon instances in which certain metastases are expansile and lytic. These typically occur in renal metastases and may be seen in multiple myeloma. The anatomical importance of these specific tumors is that they often expand and impinge upon other structures. The expansile nature of this patient’s tumor within the sacrum was the cause for compression of the sacral nerve roots, producing her symptoms.
The patient underwent a course of radiotherapy, had the renal tumor excised, and is currently undergoing a course of chemoimmunotherapy.
122.e1 122.e2
Conceptual Overview • Relationship to Other Regions
Fig. 2.20, cont’d
Fig. 2.20, cont’d
In the clinic—cont’d
Fig. 2.55, cont’d
Fig. 2.68, cont’d
Fig. 2.69, cont’d
The thorax is an irregularly shaped cylinder with a narrow opening (superior thoracic aperture) superiorly and a relatively large opening (inferior thoracic aperture) inferiorly (Fig. 3.1). The superior thoracic aperture is open, allowing continuity with the neck; the inferior thoracic aperture is closed by the diaphragm.
The musculoskeletal wall of the thorax is flexible and consists of segmentally arranged vertebrae, ribs, and muscles and the sternum.
The thoracic cavity enclosed by the thoracic wall and the diaphragm is subdivided into three major compartments: a left and a right pleural cavity, each surrounding a lung, and the mediastinum.
The mediastinum is a thick, flexible soft tissue partition oriented longitudinally in a median sagittal position. It contains the heart, esophagus, trachea, major nerves, and major systemic blood vessels.
The pleural cavities are completely separated from each other by the mediastinum. Therefore abnormal events in one pleural cavity do not necessarily affect the other cavity. This also means that the mediastinum can be entered surgically without opening the pleural cavities. | Gray's Anatomy |
Another important feature of the pleural cavities is that they extend above the level of rib I. The apex of each lung actually extends into the root of the neck. As a consequence, abnormal events in the root of the neck can involve the adjacent pleura and lung, and events in the adjacent pleura and lung can involve the root of the neck.
One of the most important functions of the thorax is breathing. The thorax not only contains the lungs but also provides the machinery necessary—the diaphragm, thoracic wall, and ribs—for effectively moving air into and out of the lungs.
Up and down movements of the diaphragm and changes in the lateral and anterior dimensions of the thoracic wall, caused by movements of the ribs, alter the volume of the thoracic cavity and are key elements in breathing.
Protection of vital organs
The thorax houses and protects the heart, lungs, and great vessels. Because of the upward domed shape of the diaphragm, the thoracic wall also offers protection to some important abdominal viscera.
Much of the liver lies under the right dome of the diaphragm, and the stomach and spleen lie under the left. The posterior aspects of the superior poles of the kidneys lie on the diaphragm and are anterior to rib XII, on the right, and to ribs XI and XII, on the left.
The mediastinum acts as a conduit for structures that pass completely through the thorax from one body region to another and for structures that connect organs in the thorax to other body regions.
The esophagus, vagus nerves, and thoracic duct pass through the mediastinum as they course between the abdomen and neck.
The phrenic nerves, which originate in the neck, also pass through the mediastinum to penetrate and supply the diaphragm.
Other structures such as the trachea, thoracic aorta, and superior vena cava course within the mediastinum en route to and from major visceral organs in the thorax.
The thoracic wall consists of skeletal elements and muscles (Fig. 3.1):
Posteriorly, it is made up of twelve thoracic vertebrae and their intervening intervertebral discs;
Laterally, the wall is formed by ribs (twelve on each side) and three layers of flat muscles, which span the intercostal spaces between adjacent ribs, move the ribs, and provide support for the intercostal spaces;
Anteriorly, the wall is made up of the sternum, which consists of the manubrium of sternum, body of sternum, and xiphoid process.
The manubrium of sternum, angled posteriorly on the body of sternum at the manubriosternal joint, forms the sternal angle, which is a major surface landmark used by clinicians in performing physical examinations of the thorax.
The anterior (distal) end of each rib is composed of costal cartilage, which contributes to the mobility and elasticity of the wall.
All ribs articulate with thoracic vertebrae posteriorly. Most ribs (from rib II to IX) have three articulations with the vertebral column. The head of each rib articulates with the body of its own vertebra and with the body of the vertebra above (Fig. 3.2). As these ribs curve posteriorly, each also articulates with the transverse process of its vertebra.
Anteriorly, the costal cartilages of ribs I to VII articulate with the sternum.
The costal cartilages of ribs VIII to X articulate with the inferior margins of the costal cartilages above them. Ribs XI and XII are called floating ribs because they do not articulate with other ribs, costal cartilages, or the sternum. Their costal cartilages are small, only covering their tips.
The skeletal framework of the thoracic wall provides extensive attachment sites for muscles of the neck, abdomen, back, and upper limbs.
A number of these muscles attach to ribs and function as accessory respiratory muscles; some of them also stabilize the position of the first and last ribs. | Gray's Anatomy |
Completely surrounded by skeletal elements, the superior thoracic aperture consists of the body of vertebra TI posteriorly, the medial margin of rib I on each side, and the manubrium anteriorly.
The superior margin of the manubrium is in approximately the same horizontal plane as the intervertebral disc between vertebrae TII and TIII.
The first ribs slope inferiorly from their posterior articulation with vertebra TI to their anterior attachment to the manubrium. Consequently, the plane of the superior thoracic aperture is at an oblique angle, facing somewhat anteriorly.
At the superior thoracic aperture, the superior aspects of the pleural cavities, which surround the lungs, lie on either side of the entrance to the mediastinum (Fig. 3.3).
Structures that pass between the upper limb and thorax pass over rib I and the superior part of the pleural cavity as they enter and leave the mediastinum. Structures that pass between the neck and head and the thorax pass more vertically through the superior thoracic aperture.
The inferior thoracic aperture is large and expandable. Bone, cartilage, and ligaments form its margin (Fig. 3.4A).
The inferior thoracic aperture is closed by the diaphragm, and structures passing between the abdomen and thorax pierce or pass posteriorly to the diaphragm.
Skeletal elements of the inferior thoracic aperture are: the body of vertebra TXII posteriorly, rib XII and the distal end of rib XI posterolaterally, the distal cartilaginous ends of ribs VII to X, which unite to form the costal margin anterolaterally, and the xiphoid process anteriorly.
The joint between the costal margin and sternum lies roughly in the same horizontal plane as the intervertebral disc between vertebrae TIX and TX. In other words, the posterior margin of the inferior thoracic aperture is inferior to the anterior margin.
When viewed anteriorly, the inferior thoracic aperture is tilted superiorly.
The musculotendinous diaphragm seals the inferior thoracic aperture (Fig. 3.4B).
Generally, muscle fibers of the diaphragm arise radially, from the margins of the inferior thoracic aperture, and converge into a large central tendon.
Because of the oblique angle of the inferior thoracic aperture, the posterior attachment of the diaphragm is inferior to the anterior attachment.
The diaphragm is not flat; rather, it “balloons” superiorly, on both the right and left sides, to form domes. The right dome is higher than the left, reaching as far as rib V.
As the diaphragm contracts, the height of the domes decreases and the volume of the thorax increases.
The esophagus and inferior vena cava penetrate the diaphragm; the aorta passes posterior to the diaphragm.
The mediastinum is a thick midline partition that extends from the sternum anteriorly to the thoracic vertebrae posteriorly, and from the superior thoracic aperture to the inferior thoracic aperture.
A horizontal plane passing through the sternal angle and the intervertebral disc between vertebrae TIV and TV separates the mediastinum into superior and inferior parts (Fig. 3.5). The inferior part is further subdivided by the pericardium, which encloses the pericardial cavity surrounding the heart. The pericardium and heart constitute the middle mediastinum.
The anterior mediastinum lies between the sternum and the pericardium; the posterior mediastinum lies between the pericardium and thoracic vertebrae.
The two pleural cavities are situated on either side of the mediastinum (Fig. 3.6).
Each pleural cavity is completely lined by a mesothelial membrane called the pleura.
During development, the lungs grow out of the mediastinum, becoming surrounded by the pleural cavities. As a result, the outer surface of each organ is covered by pleura.
Each lung remains attached to the mediastinum by a root formed by the airway, pulmonary blood vessels, lymphatic tissues, and nerves. | Gray's Anatomy |
The pleura lining the walls of the cavity is the parietal pleura, whereas that reflected from the mediastinum at the roots and onto the surfaces of the lungs is the visceral pleura. Only a potential space normally exists between the visceral pleura covering lung and the parietal pleura lining the wall of the thoracic cavity.
The lung does not completely fill the potential space of the pleural cavity, resulting in recesses, which do not contain lung and are important for accommodating changes in lung volume during breathing. The costodiaphragmatic recess, which is the largest and clinically most important recess, lies inferiorly between the thoracic wall and diaphragm.
The superior thoracic aperture opens directly into the root of the neck (Fig. 3.7).
The superior aspect of each pleural cavity extends approximately 2 to 3 cm above rib I and the costal cartilage into the neck. Between these pleural extensions, major visceral structures pass between the neck and superior mediastinum. In the midline, the trachea lies immediately anterior to the esophagus. Major blood vessels and nerves pass in and out of the thorax at the superior thoracic aperture anteriorly and laterally to these structures.
An axillary inlet, or gateway to the upper limb, lies on each side of the superior thoracic aperture. These two axillary inlets and the superior thoracic aperture communicate superiorly with the root of the neck (Fig. 3.7).
Each axillary inlet is formed by: the superior margin of the scapula posteriorly, the clavicle anteriorly, and the lateral margin of rib I medially.
The apex of each triangular inlet is directed laterally and is formed by the medial margin of the coracoid process, which extends anteriorly from the superior margin of the scapula.
The base of the axillary inlet’s triangular opening is the lateral margin of rib I.
Large blood vessels passing between the axillary inlet and superior thoracic aperture do so by passing over rib I.
Proximal parts of the brachial plexus also pass between the neck and upper limb by passing through the axillary inlet.
The diaphragm separates the thorax from the abdomen. Structures that pass between the thorax and abdomen either penetrate the diaphragm or pass posteriorly to it (Fig. 3.8):
The inferior vena cava pierces the central tendon of the diaphragm to enter the right side of the mediastinum near vertebral level TVIII.
The esophagus penetrates the muscular part of the diaphragm to leave the mediastinum and enter the abdomen just to the left of the midline at vertebral level TX.
The aorta passes posteriorly to the diaphragm at the midline at vertebral level TXII.
Numerous other structures that pass between the thorax and abdomen pass through or posterior to the diaphragm.
The breasts, consisting of mammary glands, superficial fascia, and overlying skin, are in the pectoral region on each side of the anterior thoracic wall (Fig. 3.9).
Vessels, lymphatics, and nerves associated with the breast are as follows:
Branches from the internal thoracic arteries and veins perforate the anterior chest wall on each side of the sternum to supply anterior aspects of the thoracic wall. Those branches associated mainly with the second to fourth intercostal spaces also supply the anteromedial parts of each breast.
Lymphatic vessels from the medial part of the breast accompany the perforating arteries and drain into the parasternal nodes on the deep surface of the thoracic wall.
Vessels and lymphatics associated with lateral parts of the breast emerge from or drain into the axillary region of the upper limb.
Lateral and anterior branches of the fourth to sixth intercostal nerves carry general sensation from the skin of the breast.
When working with patients, physicians use vertebral levels to determine the position of important anatomical structures within body regions. | Gray's Anatomy |
The horizontal plane passing through the disc that separates thoracic vertebrae TIV and TV is one of the most significant planes in the body (Fig. 3.10) because it: passes through the sternal angle anteriorly, marking the position of the anterior articulation of the costal cartilage of rib II with the sternum. The sternal angle is used to find the position of rib II as a reference for counting ribs (because of the overlying clavicle, rib I is not palpable); separates the superior mediastinum from the inferior mediastinum and marks the position of the superior limit of the pericardium; marks where the arch of the aorta begins and ends; passes through the site where the superior vena cava penetrates the pericardium to enter the heart; is the level at which the trachea bifurcates into right and left main bronchi; and marks the superior limit of the pulmonary trunk.
Venous shunts from left to right
The right atrium is the chamber of the heart that receives deoxygenated blood returning from the body. It lies on the right side of the midline, and the two major veins, the superior and inferior venae cavae, that drain into it are also located on the right side of the body. This means that, to get to the right side of the body, all blood coming from the left side has to cross the midline. This left-to-right shunting is carried out by a number of important and, in some cases, very large veins, several of which are in the thorax (Fig. 3.11).
In adults, the left brachiocephalic vein crosses the midline immediately posterior to the manubrium and delivers blood from the left side of the head and neck, the left upper limb, and part of the left thoracic wall into the superior vena cava.
The hemiazygos and accessory hemiazygos veins drain posterior and lateral parts of the left thoracic wall, pass immediately anterior to the bodies of thoracic vertebrae, and flow into the azygos vein on the right side, which ultimately connects with the superior vena cava.
The arrangement of vessels and nerves that supply the thoracic wall reflects the segmental organization of the wall. Arteries to the wall arise from two sources: the thoracic aorta, which is in the posterior mediastinum, and a pair of vessels, the internal thoracic arteries, which run along the deep aspect of the anterior thoracic wall on either side of the sternum.
the wall, mainly along the inferior margin of each rib (Fig. 3.12A). Running with these vessels are intercostal nerves (the anterior rami of thoracic spinal nerves), which innervate the wall, related parietal pleura, and associated skin. The position of these nerves and vessels relative to the ribs must be considered when passing objects, such as chest tubes, through the thoracic wall.
Dermatomes of the thorax generally reflect the segmental organization of the thoracic spinal nerves (Fig. 3.12B). The exception occurs, anteriorly and superiorly, with the first thoracic dermatome, which is located mostly in the upper limb, and not on the trunk.
The anterosuperior region of the trunk receives branches from the anterior ramus of C4 via supraclavicular branches of the cervical plexus.
The highest thoracic dermatome on the anterior chest wall is T2, which also extends into the upper limb.
In the midline, skin over the xiphoid process is innervated by T6.
Dermatomes of T7 to T12 follow the contour of the ribs onto the anterior abdominal wall (Fig. 3.12C).
All preganglionic nerve fibers of the sympathetic system are carried out of the spinal cord in spinal nerves T1 to L2 (Fig. 3.13). This means that sympathetic fibers found anywhere in the body ultimately emerge from the spinal cord as components of these spinal nerves. Preganglionic sympathetic fibers destined for the head are carried out of the spinal cord in spinal nerve T1. | Gray's Anatomy |
The thoracic wall is expandable because most ribs articulate with other components of the wall by true joints that allow movement, and because of the shape and orientation of the ribs (Fig. 3.14).
A rib’s posterior attachment is superior to its anterior attachment. Therefore, when a rib is elevated, it moves the anterior thoracic wall forward relative to the posterior wall, which is fixed. In addition, the middle part of each rib is inferior to its two ends, so that when this region of the rib is elevated, it expands the thoracic wall laterally. Finally, because the diaphragm is muscular, it changes the volume of the thorax in the vertical direction.
Changes in the anterior, lateral, and vertical dimensions of the thoracic cavity are important for breathing.
Innervation of the diaphragm
The diaphragm is innervated by two phrenic nerves that originate, one on each side, as branches of the cervical plexus in the neck (Fig. 3.15). They arise from the anterior rami of cervical nerves C3, C4, and C5, with the major contribution coming from C4.
The phrenic nerves pass vertically through the neck, the superior thoracic aperture, and the mediastinum to supply motor innervation to the entire diaphragm, including the crura (muscular extensions that attach the diaphragm to the upper lumbar vertebrae). In the mediastinum, the phrenic nerves pass anteriorly to the roots of the lungs.
The tissues that initially give rise to the diaphragm are in an anterior position on the embryological disc before the head fold develops, which explains the cervical origin of the nerves that innervate the diaphragm. In other words, the tissue that gives rise to the diaphragm originates superior to the ultimate location of the diaphragm.
Spinal cord injuries below the level of the origin of the phrenic nerve do not affect movement of the diaphragm.
The cylindrical thorax consists of: a wall, two pleural cavities, the lungs, and the mediastinum.
The thorax houses the heart and lungs, acts as a conduit for structures passing between the neck and the abdomen, and plays a principal role in breathing. In addition, the thoracic wall protects the heart and lungs and provides support for the upper limbs. Muscles anchored to the anterior thoracic wall provide some of this support, and together with their associated connective tissues, nerves, and vessels, and the overlying skin and superficial fascia, define the pectoral region.
The pectoral region is external to the anterior thoracic wall and helps anchor the upper limb to the trunk. It consists of: a superficial compartment containing skin, superficial fascia, and breasts; and a deep compartment containing muscles and associated structures.
Nerves, vessels, and lymphatics in the superficial compartment emerge from the thoracic wall, the axilla, and the neck.
The breasts consist of mammary glands and associated skin and connective tissues. The mammary glands are modified sweat glands in the superficial fascia anterior to the pectoral muscles and the anterior thoracic wall (Fig. 3.16).
The mammary glands consist of a series of ducts and associated secretory lobules. These converge to form 15 to 20 lactiferous ducts, which open independently onto the nipple. The nipple is surrounded by a circular pigmented area of skin termed the areola.
A well-developed, connective tissue stroma surrounds the ducts and lobules of the mammary gland. In certain regions, this condenses to form well-defined ligaments, the suspensory ligaments of breast, which are continuous with the dermis of the skin and support the breast. Carcinoma of the breast creates tension on these ligaments, causing pitting of the skin.
In nonlactating women, the predominant component of the breasts is fat, while glandular tissue is more abundant in lactating women. | Gray's Anatomy |
The breast lies on deep fascia related to the pectoralis major muscle and other surrounding muscles. A layer of loose connective tissue (the retromammary space) separates the breast from the deep fascia and provides some degree of movement over underlying structures.
The base, or attached surface, of each breast extends vertically from ribs II to VI, and transversely from the sternum to as far laterally as the midaxillary line.
The breast is related to the thoracic wall and to structures associated with the upper limb; therefore, vascular supply and drainage can occur by multiple routes (Fig. 3.16): laterally, vessels from the axillary artery—superior thoracic, thoraco-acromial, lateral thoracic, and subscapular arteries; medially, branches from the internal thoracic artery; the second to fourth intercostal arteries via branches that perforate the thoracic wall and overlying muscle.
Veins draining the breast parallel the arteries and ultimately drain into the axillary, internal thoracic, and intercostal veins.
Innervation of the breast is via anterior and lateral cutaneous branches of the second to sixth intercostal nerves. The nipple is innervated by the fourth intercostal nerve.
Lymphatic drainage of the breast is as follows:
Approximately 75% is via lymphatic vessels that drain laterally and superiorly into axillary nodes (Fig. 3.16).
Most of the remaining drainage is into parasternal nodes deep to the anterior thoracic wall and associated with the internal thoracic artery.
Some drainage may occur via lymphatic vessels that follow the lateral branches of posterior intercostal arteries and connect with intercostal nodes situated near the heads and necks of ribs.
Axillary nodes drain into the subclavian trunks, parasternal nodes drain into the bronchomediastinal trunks, and intercostal nodes drain either into the thoracic duct or into the bronchomediastinal trunks.
The breast in men is rudimentary and consists only of small ducts, often composed of cords of cells, that normally do not extend beyond the areola. Breast cancer can occur in men.
Muscles of the pectoral region
Each pectoral region contains the pectoralis major, pectoralis minor, and subclavius muscles (Fig. 3.17 and Table 3.1). All originate from the anterior thoracic wall and insert into bones of the upper limb.
The pectoralis major muscle is the largest and most superficial of the pectoral region muscles. It directly underlies the breast and is separated from it by deep fascia and the loose connective tissue of the retromammary space.
The pectoralis major has a broad origin that includes the anterior surfaces of the medial half of the clavicle, the sternum, and related costal cartilages. The muscle fibers converge to form a flat tendon, which inserts into the lateral lip of the intertubercular sulcus of the humerus.
The pectoralis major adducts, flexes, and medially rotates the arm.
The subclavius and pectoralis minor muscles underlie the pectoralis major:
The subclavius is small and passes laterally from the anterior and medial part of rib I to the inferior surface of the clavicle.
The pectoralis minor passes from the anterior surfaces of ribs III to V to the coracoid process of the scapula.
Both the subclavius and pectoralis minor pull the tip of the shoulder inferiorly.
A continuous layer of deep fascia, the clavipectoral fascia, encloses the subclavius and pectoralis minor and attaches to the clavicle above and to the floor of the axilla below.
The muscles of the pectoral region form the anterior wall of the axilla, a region between the upper limb and the neck through which all major structures pass. Nerves, vessels, and lymphatics that pass between the pectoral region and the axilla pass through the clavipectoral fascia between the subclavius and pectoralis minor or pass under the inferior margins of the pectoralis major and minor. | Gray's Anatomy |
The thoracic wall is segmental in design and composed of skeletal elements and muscles. It extends between: the superior thoracic aperture, bordered by vertebra TI, rib I, and the manubrium of the sternum; and the inferior thoracic aperture, bordered by vertebra TXII, rib XII, the end of rib XI, the costal margin, and the xiphoid process of the sternum.
The skeletal elements of the thoracic wall consist of the thoracic vertebrae, intervertebral discs, ribs, and sternum.
There are twelve thoracic vertebrae, each of which is characterized by articulations with ribs.
A typical thoracic vertebra has a heart-shaped vertebral body, with roughly equal dimensions in the transverse and anteroposterior directions, and a long spinous process (Fig. 3.18). The vertebral foramen is generally circular and the laminae are broad and overlap with those of the vertebra below. The superior articular processes are flat, with their articular surfaces facing almost directly posteriorly, while the inferior articular processes project from the laminae and their articular facets face anteriorly. The transverse processes are club shaped and project posterolaterally.
Articulation with ribs
A typical thoracic vertebra has three sites on each side for articulation with ribs.
Two demifacets (i.e., partial facets) are located on the superior and inferior aspects of the body for articulation with corresponding sites on the heads of adjacent ribs. The superior costal facet articulates with part of the head of its own rib, and the inferior costal facet articulates with part of the head of the rib below.
An oval facet (transverse costal facet) at the end of the transverse process articulates with the tubercle of its own rib.
Not all vertebrae articulate with ribs in the same fashion (Fig. 3.19):
The superior costal facets on the body of vertebra TI are complete and articulate with a single facet on the head of its own rib—in other words, the head of rib I does not articulate with vertebra CVII.
Similarly, vertebra TX (and often TIX) articulates only with its own ribs and therefore lacks inferior demifacets on the body.
Vertebrae TXI and TXII articulate only with the heads of their own ribs—they lack transverse costal facets and have only a single complete facet on each side of their bodies.
There are twelve pairs of ribs, each terminating anteriorly in a costal cartilage (Fig. 3.20).
Although all ribs articulate with the vertebral column, only the costal cartilages of the upper seven ribs, known as true ribs, articulate directly with the sternum. The remaining five pairs of ribs are false ribs:
The costal cartilages of ribs VIII to X articulate anteriorly with the costal cartilages of the ribs above.
Ribs XI and XII have no anterior connection with other ribs or with the sternum and are often called floating ribs.
A typical rib consists of a curved shaft with anterior and posterior ends (Fig. 3.21). The anterior end is continuous with its costal cartilage. The posterior end articulates with the vertebral column and is characterized by a head, neck, and tubercle.
The head is somewhat expanded and typically presents two articular surfaces separated by a crest. The smaller superior surface articulates with the inferior costal facet on the body of the vertebra above, whereas the larger inferior facet articulates with the superior costal facet of its own vertebra.
The neck is a short flat region of bone that separates the head from the tubercle.
The tubercle projects posteriorly from the junction of the neck with the shaft and consists of two regions, an articular part and a nonarticular part:
The articular part is medial and has an oval facet for articulation with a corresponding facet on the transverse process of the associated vertebra.
The raised nonarticular part is roughened by ligament attachments.
The shaft is generally thin and flat with internal and external surfaces. | Gray's Anatomy |
The superior margin is smooth and rounded, whereas the inferior margin is sharp. The shaft bends forward just laterally to the tubercle at a site termed the angle. It also has a gentle twist around its longitudinal axis so that the external surface of the anterior part of the shaft faces somewhat superiorly relative to the posterior part. The inferior margin of the internal surface is marked by a distinct costal groove.
Distinct features of upper and lower ribs
The upper and lower ribs have distinct features (Fig. 3.22).
Rib I is flat in the horizontal plane and has broad superior and inferior surfaces. From its articulation with vertebra TI, it slopes inferiorly to its attachment to the manubrium of the sternum. The head articulates only with the body of vertebra TI and therefore has only one articular surface. Like other ribs, the tubercle has a facet for articulation with the transverse process. The superior surface of the rib is characterized by a distinct tubercle, the scalene tubercle, which separates two smooth grooves that cross the rib approximately midway along the shaft. The anterior groove is caused by the subclavian vein, and the posterior groove is caused by the subclavian artery. Anterior and posterior to these grooves, the shaft is roughened by muscle and ligament attachments.
Rib II, like rib I, is flat but twice as long. It articulates with the vertebral column in a way typical of most ribs.
The head of rib X has a single facet for articulation with its own vertebra.
Ribs XI and XII articulate only with the bodies of their own vertebrae and have no tubercles or necks. Both ribs are short, have little curve, and are pointed anteriorly.
The adult sternum consists of three major elements: the broad and superiorly positioned manubrium of the sternum, the narrow and longitudinally oriented body of the sternum, and the small and inferiorly positioned xiphoid process (Fig. 3.23).
Manubrium of the sternum
The manubrium of the sternum forms part of the bony framework of the neck and the thorax.
The superior surface of the manubrium is expanded laterally and bears a distinct and palpable notch, the jugular notch (suprasternal notch), in the midline.
On either side of this notch is a large oval fossa for articulation with the clavicle. Immediately inferior to this fossa, on each lateral surface of the manubrium, is a facet for the attachment of the first costal cartilage. At the lower end of the lateral border is a demifacet for articulation with the upper half of the anterior end of the second costal cartilage.
Body of the sternum
The body of the sternum is flat.
The anterior surface of the body of the sternum is often marked by transverse ridges that represent lines of fusion between the segmental elements called sternebrae, from which this part of the sternum arises embryologically.
The lateral margins of the body of the sternum have articular facets for costal cartilages. Superiorly, each lateral margin has a demifacet for articulation with the inferior aspect of the second costal cartilage. Inferior to this demifacet are four facets for articulation with the costal cartilages of ribs III to VI.
At the inferior end of the body of the sternum is a demifacet for articulation with the upper demifacet on the seventh costal cartilage. The inferior end of the body of the sternum is attached to the xiphoid process.
The xiphoid process is the smallest part of the sternum. Its shape is variable: it may be wide, thin, pointed, bifid, curved, or perforated. It begins as a cartilaginous structure, which becomes ossified in the adult. On each side of its upper lateral margin is a demifacet for articulation with the inferior end of the seventh costal cartilage.
A typical rib articulates with: the bodies of adjacent vertebrae, forming a joint with the head of the rib; and the transverse process of its related vertebra, forming a costotransverse joint (Fig. 3.24). | Gray's Anatomy |
Together, the costovertebral joints and related ligaments allow the necks of the ribs either to rotate around their longitudinal axes, which occurs mainly in the upper ribs, or to ascend and descend relative to the vertebral column, which occurs mainly in the lower ribs. The combined movements of all of the ribs on the vertebral column are essential for altering the volume of the thoracic cavity during breathing.
Joint with head of rib
The two facets on the head of the rib articulate with the superior facet on the body of its own vertebra and with the inferior facet on the body of the vertebra above. This joint is divided into two synovial compartments by an intra-articular ligament, which attaches the crest to the adjacent intervertebral disc and separates the two articular surfaces on the head of the rib. The two synovial compartments and the intervening ligament are surrounded by a single joint capsule attached to the outer margins of the combined articular surfaces of the head and vertebral column.
Costotransverse joints are synovial joints between the tubercle of a rib and the transverse process of the related vertebra (Fig. 3.24). The capsule surrounding each joint is thin. The joint is stabilized by two strong extracapsular ligaments that span the space between the transverse process and the rib on the medial and lateral sides of the joint:
The costotransverse ligament is medial to the joint and attaches the neck of the rib to the transverse process.
The lateral costotransverse ligament is lateral to the joint and attaches the tip of the transverse process to the roughened nonarticular part of the tubercle of the rib.
A third ligament, the superior costotransverse ligament, attaches the superior surface of the neck of the rib to the transverse process of the vertebra above.
Slight gliding movements occur at the costotransverse joints.
The sternocostal joints are joints between the upper seven costal cartilages and the sternum (Fig. 3.25).
The joint between rib I and the manubrium is not synovial and consists of a fibrocartilaginous connection between the manubrium and the costal cartilage. The second to seventh joints are synovial and have thin capsules reinforced by surrounding sternocostal ligaments.
The joint between the second costal cartilage and the sternum is divided into two compartments by an intraarticular ligament. This ligament attaches the second costal cartilage to the junction of the manubrium and the body of the sternum.
Interchondral joints occur between the costal cartilages of adjacent ribs (Fig. 3.25), mainly between the costal cartilages of ribs VII to X, but may also involve the costal cartilages of ribs V and VI.
Interchondral joints provide indirect anchorage to the sternum and contribute to the formation of a smooth inferior costal margin. They are usually synovial, and the thin fibrous capsules are reinforced by interchondral ligaments.
The joints between the manubrium and the body of the sternum and between the body of the sternum and the xiphoid process are usually symphyses (Fig. 3.25). Only slight angular movements occur between the manubrium and the body of the sternum during respiration. The joint between the body of the sternum and the xiphoid process often becomes ossified with age.
A clinically useful feature of the manubriosternal joint is that it can be palpated easily. This is because the manubrium normally angles posteriorly on the body of the sternum, forming a raised feature referred to as the sternal angle. This elevation marks the site of articulation of rib II with the sternum. Rib I is not palpable, because it lies inferior to the clavicle and is embedded in tissues at the base of the neck. Therefore, rib II is used as a reference for counting ribs and can be felt immediately lateral to the sternal angle. | Gray's Anatomy |
In addition, the sternal angle lies on a horizontal plane that passes through the intervertebral disc between vertebrae TIV and TV (see Fig. 3.10). This plane separates the superior mediastinum from the inferior mediastinum and marks the superior border of the pericardium. The plane also passes through the end of the ascending aorta and the beginning of the arch of the aorta, the end of the arch of the aorta and the beginning of the thoracic aorta, and the bifurcation of the trachea, and just superior to the pulmonary trunk (see Fig. 3.79 and 3.86).
Intercostal spaces lie between adjacent ribs and are filled by intercostal muscles (Fig. 3.26).
Intercostal nerves and associated major arteries and veins lie in the costal groove along the inferior margin of the superior rib and pass in the plane between the inner two layers of muscles.
In each space, the vein is the most superior structure and is therefore highest in the costal groove. The artery is inferior to the vein, and the nerve is inferior to the artery and often not protected by the groove. Therefore, the nerve is the structure most at risk when objects perforate the upper aspect of an intercostal space.
Small collateral branches of the major intercostal nerves and vessels are often present superior to the inferior rib below.
Deep to the intercostal spaces and ribs, and separating these structures from the underlying pleura, is a layer of loose connective tissue, called endothoracic fascia, which contains variable amounts of fat.
Superficial to the spaces are deep fascia, superficial fascia, and skin. Muscles associated with the upper limbs and back overlie the spaces.
Muscles of the thoracic wall include those that fill and support the intercostal spaces, those that pass between the sternum and the ribs, and those that cross several ribs between costal attachments (Table 3.2).
The muscles of the thoracic wall, together with muscles between the vertebrae and ribs posteriorly (i.e., the levatores costarum and serratus posterior superior and serratus posterior inferior muscles) alter the position of the ribs and sternum and so change the thoracic volume during breathing. They also reinforce the thoracic wall.
The intercostal muscles are three flat muscles found in each intercostal space that pass between adjacent ribs (Fig. 3.27). Individual muscles in this group are named according to their positions:
The external intercostal muscles are the most superficial.
The internal intercostal muscles are sandwiched between the external and innermost muscles.
The innermost intercostal muscles are the deepest of the three muscles.
The intercostal muscles are innervated by the related intercostal nerves. As a group, the intercostal muscles provide structural support for the intercostal spaces during breathing. They can also move the ribs.
The eleven pairs of external intercostal muscles extend from the inferior margins (lateral edges of costal grooves) of the ribs above to the superior margins of the ribs below. When the thoracic wall is viewed from a lateral position, the muscle fibers pass obliquely anteroinferiorly (Fig. 3.27). The muscles extend around the thoracic wall from the regions of the tubercles of the ribs to the costal cartilages, where each layer continues as a thin connective tissue aponeurosis termed the external intercostal membrane. The external intercostal muscles are most active in inspiration. | Gray's Anatomy |
The eleven pairs of internal intercostal muscles pass between the most inferior lateral edge of the costal grooves of the ribs above, to the superior margins of the ribs below. They extend from parasternal regions, where the muscles course between adjacent costal cartilages, to the angle of the ribs posteriorly (Fig. 3.27). This layer continues medially toward the vertebral column, in each intercostal space, as the internal intercostal membrane. The muscle fibers pass in the opposite direction to those of the external intercostal muscles. When the thoracic wall is viewed from a lateral position, the muscle fibers pass obliquely posteroinferiorly. The internal intercostal muscles are most active during expiration.
The innermost intercostal muscles are the least distinct of the intercostal muscles, and the fibers have the same orientation as the internal intercostals (Fig. 3.27). These muscles are most evident in the lateral thoracic wall. They extend between the inner surfaces of adjacent ribs from the medial edge of the costal groove to the deep surface of the rib below. Importantly, the neurovascular bundles associated with the intercostal spaces pass around the thoracic wall in the costal grooves in a plane between the innermost and internal intercostal muscles.
The subcostales are in the same plane as the innermost intercostals, span multiple ribs, and are more numerous in lower regions of the posterior thoracic wall (Fig. 3.28A). They extend from the internal surfaces of one rib to the internal surface of the second (next) or third rib below. Their fibers parallel the course of the internal intercostal muscles and extend from the angle of the ribs to more medial positions on the ribs below.
The transversus thoracis muscles are found on the deep surface of the anterior thoracic wall (Fig. 3.28B) and in the same plane as the innermost intercostals.
The transversus thoracis muscles originate from the posterior aspect of the xiphoid process, the inferior part of the body of the sternum, and the adjacent costal cartilages of the lower true ribs. They pass superiorly and laterally to insert into the lower borders of the costal cartilages of ribs III to VI. They most likely pull these latter elements inferiorly.
The transversus thoracis muscles lie deep to the internal thoracic vessels and secure these vessels to the wall.
Vessels that supply the thoracic wall consist mainly of posterior and anterior intercostal arteries, which pass around the wall between adjacent ribs in intercostal spaces (Fig. 3.29). These arteries originate from the aorta and internal thoracic arteries, which in turn arise from the subclavian arteries in the root of the neck. Together, the intercostal arteries form a basket-like pattern of vascular supply around the thoracic wall.
Posterior intercostal arteries originate from vessels associated with the posterior thoracic wall. The upper two posterior intercostal arteries on each side are derived from the supreme intercostal artery, which descends into the thorax as a branch of the costocervical trunk in the neck. The costocervical trunk is a posterior branch of the subclavian artery (Fig. 3.29).
The remaining nine pairs of posterior intercostal arteries arise from the posterior surface of the thoracic aorta. Because the aorta is on the left side of the vertebral column, those posterior intercostal vessels passing to the right side of the thoracic wall cross the midline anterior to the bodies of the vertebrae and therefore are longer than the corresponding vessels on the left.
In addition to having numerous branches that supply various components of the wall, the posterior intercostal arteries have branches that accompany lateral cutaneous branches of the intercostal nerves to superficial regions.
The anterior intercostal arteries originate directly or indirectly as lateral branches from the internal thoracic arteries (Fig. 3.29). | Gray's Anatomy |
Each internal thoracic artery arises as a major branch of the subclavian artery in the neck. It passes anteriorly over the cervical dome of the pleura and descends vertically through the superior thoracic aperture and along the deep aspect of the anterior thoracic wall. On each side, the internal thoracic artery lies posterior to the costal cartilages of the upper six ribs and about 1 cm lateral to the sternum. At approximately the level of the sixth intercostal space, it divides into two terminal branches: the superior epigastric artery, which continues inferiorly into the anterior abdominal wall (Fig. 3.29); and the musculophrenic artery, which passes along the costal margin, goes through the diaphragm, and ends near the last intercostal space.
Anterior intercostal arteries that supply the upper six intercostal spaces arise as lateral branches from the internal thoracic artery, whereas those supplying the lower spaces arise from the musculophrenic artery.
In each intercostal space, the anterior intercostal arteries usually have two branches:
One passes below the margin of the upper rib.
The other passes above the margin of the lower rib and meets a collateral branch of the posterior intercostal artery.
The distributions of the anterior and posterior intercostal vessels overlap and can develop anastomotic connections. The anterior intercostal arteries are generally smaller than the posterior vessels.
In addition to anterior intercostal arteries and a number of other branches, the internal thoracic arteries give rise to perforating branches that pass directly forward between the costal cartilages to supply structures external to the thoracic wall. These vessels travel with the anterior cutaneous branches of the intercostal nerves.
Venous drainage from the thoracic wall generally parallels the pattern of arterial supply (Fig. 3.30).
Centrally, the intercostal veins ultimately drain into the azygos system of veins or into internal thoracic veins, which connect with the brachiocephalic veins in the neck.
Often the upper posterior intercostal veins on the left side come together and form the left superior intercostal vein, which empties into the left brachiocephalic vein.
Similarly, the upper posterior intercostal veins on the right side may come together and form the right superior intercostal vein, which empties into the azygos vein.
Lymphatic vessels of the thoracic wall drain mainly into lymph nodes associated with the internal thoracic arteries (parasternal nodes), with the heads and necks of ribs (intercostal nodes), and with the diaphragm (diaphragmatic nodes) (Fig. 3.31). Diaphragmatic nodes are posterior to the xiphoid and at sites where the phrenic nerves penetrate the diaphragm. They also occur in regions where the diaphragm is attached to the vertebral column.
Parasternal nodes drain into bronchomediastinal trunks. Intercostal nodes in the upper thorax also drain into bronchomediastinal trunks, whereas intercostal nodes in the lower thorax drain into the thoracic duct.
Nodes associated with the diaphragm interconnect with parasternal, prevertebral, and juxta-esophageal nodes, brachiocephalic nodes (anterior to the brachiocephalic veins in the superior mediastinum), and lateral aortic/lumbar nodes (in the abdomen).
Superficial regions of the thoracic wall drain mainly into axillary lymph nodes in the axilla or parasternal nodes.
Innervation of the thoracic wall is mainly by the intercostal nerves, which are the anterior rami of spinal nerves T1 to T11 and lie in the intercostal spaces between adjacent ribs. The anterior ramus of spinal nerve T12 (the subcostal nerve) is inferior to rib XII (Fig. 3.32).
A typical intercostal nerve passes laterally around the thoracic wall in an intercostal space. The largest of the branches is the lateral cutaneous branch, which pierces the lateral thoracic wall and divides into an anterior branch and a posterior branch that innervate the overlying skin. | Gray's Anatomy |
The intercostal nerves end as anterior cutaneous branches, which emerge either parasternally, between adjacent costal cartilages, or laterally to the midline, on the anterior abdominal wall, to supply the skin.
In addition to these major branches, small collateral branches can be found in the intercostal space running along the superior border of the lower rib.
In the thorax, the intercostal nerves carry: somatic motor innervation to the muscles of the thoracic wall (intercostal, subcostal, and transversus thoracis muscles), somatic sensory innervation from the skin and parietal pleura, and postganglionic sympathetic fibers to the periphery.
Sensory innervation of the skin overlying the upper thoracic wall is supplied by cutaneous branches (supraclavicular nerves), which descend from the cervical plexus in the neck.
In addition to innervating the thoracic wall, intercostal nerves innervate other regions:
The anterior ramus of T1 contributes to the brachial plexus.
The lateral cutaneous branch of the second intercostal nerve (the intercostobrachial nerve) contributes to cutaneous innervation of the medial surface of the upper arm.
The lower intercostal nerves supply the muscles, skin, and peritoneum of the abdominal wall.
The diaphragm is a thin musculotendinous structure that fills the inferior thoracic aperture and separates the thoracic cavity from the abdominal cavity (Fig. 3.34 and see Chapter 4). It is attached peripherally to the: xiphoid process of the sternum, costal margin of the thoracic wall, ends of ribs XI and XII, ligaments that span across structures of the posterior abdominal wall, and vertebrae of the lumbar region.
From these peripheral attachments, muscle fibers converge to join the central tendon. The pericardium is attached to the middle part of the central tendon.
In the median sagittal plane, the diaphragm slopes inferiorly from its anterior attachment to the xiphoid, approximately at vertebral level TVIII/IX, to its posterior attachment to the median arcuate ligament, crossing anteriorly to the aorta at approximately vertebral level TXII.
Structures traveling between the thorax and abdomen pass through the diaphragm or between the diaphragm and its peripheral attachments:
The inferior vena cava passes through the central tendon at approximately vertebral level TVIII.
The esophagus passes through the muscular part of the diaphragm, just to the left of midline, approximately at vertebral level TX.
The vagus nerves pass through the diaphragm with the esophagus.
The aorta passes behind the posterior attachment of the diaphragm at vertebral level TXII.
The thoracic duct passes behind the diaphragm with the aorta.
The azygos and hemiazygos veins may also pass through the aortic hiatus or through the crura of the diaphragm.
Other structures outside the posterior attachments of the diaphragm lateral to the aortic hiatus include the sympathetic trunks. The greater, lesser, and least splanchnic nerves penetrate the crura.
The arterial supply to the diaphragm is from vessels that arise superiorly and inferiorly to it (see Fig. 3.34). From above, pericardiacophrenic and musculophrenic arteries supply the diaphragm. These vessels are branches of the internal thoracic arteries. Superior phrenic arteries, which arise directly from lower parts of the thoracic aorta, and small branches from intercostal arteries contribute to the supply. The largest arteries supplying the diaphragm arise from below it. These arteries are the inferior phrenic arteries, which branch directly from the abdominal aorta. | Gray's Anatomy |
Venous drainage of the diaphragm is by veins that generally parallel the arteries. The veins drain into: the brachiocephalic veins in the neck, the azygos system of veins, or abdominal veins (left suprarenal vein and inferior vena cava).
The diaphragm is innervated by the phrenic nerves (C3, C4, and C5), which penetrate the diaphragm and innervate it from its abdominal surface.
Contraction of the domes of the diaphragm flattens the diaphragm, thereby increasing thoracic volume. Movements of the diaphragm are essential for normal breathing.
One of the principal functions of the thoracic wall and the diaphragm is to alter the volume of the thorax and thereby move air in and out of the lungs.
During breathing, the dimensions of the thorax change in the vertical, lateral, and anteroposterior directions.
Elevation and depression of the diaphragm significantly alter the vertical dimensions of the thorax. Depression results when the muscle fibers of the diaphragm contract. Elevation occurs when the diaphragm relaxes.
Changes in the anteroposterior and lateral dimensions result from elevation and depression of the ribs (Fig. 3.35). The posterior ends of the ribs articulate with the vertebral column, whereas the anterior ends of most ribs articulate with the sternum or adjacent ribs.
Because the anterior ends of the ribs are inferior to the posterior ends, when the ribs are elevated, they move the sternum upward and forward. Also, the angle between the body of the sternum and the manubrium may become slightly less acute. When the ribs are depressed, the sternum moves downward and backward. This “pump handle” movement changes the dimensions of the thorax in the anteroposterior direction (Fig. 3.35A).
As well as the anterior ends of the ribs being lower than the posterior ends, the middles of the shafts tend to be lower than the two ends. When the shafts are elevated, the middles of the shafts move laterally. This “bucket handle” movement increases the lateral dimensions of the thorax (Fig. 3.35B).
Any muscles attaching to the ribs can potentially move one rib relative to another and therefore act as accessory respiratory muscles. Muscles in the neck and the abdomen can fix or alter the positions of upper and lower ribs.
Two pleural cavities, one on either side of the mediastinum, surround the lungs (Fig. 3.37):
Superiorly, they extend above rib I into the root of the neck.
Inferiorly, they extend to a level just above the costal margin.
The medial wall of each pleural cavity is the mediastinum.
Each pleural cavity is lined by a single layer of flat cells, mesothelium, and an associated layer of supporting connective tissue; together, they form the pleura.
The pleura is divided into two major types, based on location:
Pleura associated with the walls of a pleural cavity is parietal pleura (Fig. 3.37).
Pleura that reflects from the medial wall and onto the surface of the lung is visceral pleura (Fig. 3.37), which adheres to and covers the lung.
Each pleural cavity is the potential space enclosed between the visceral and parietal pleurae. They normally contain only a very thin layer of serous fluid. As a result, the surface of the lung, which is covered by visceral pleura, directly opposes and freely slides over the parietal pleura attached to the wall.
The names given to the parietal pleura correspond to the parts of the wall with which they are associated (Fig. 3.38):
Pleura related to the ribs and intercostal spaces is termed the costal part.
Pleura covering the diaphragm is the diaphragmatic part.
Pleura covering the mediastinum is the mediastinal part.
The dome-shaped layer of parietal pleura lining the cervical extension of the pleural cavity is cervical pleura (dome of pleura or pleural cupola). | Gray's Anatomy |
Covering the superior surface of the cervical pleura is a distinct dome-like layer of fascia, the suprapleural membrane (Fig. 3.38). This connective tissue membrane is attached laterally to the medial margin of the first rib and behind to the transverse process of vertebra CVII. Superiorly, the membrane receives muscle fibers from some of the deep muscles in the neck (scalene muscles) that function to keep the membrane taut. The suprapleural membrane provides apical support for the pleural cavity in the root of the neck.
In the region of vertebrae TV to TVII, the mediastinal pleura reflects off the mediastinum as a tubular, sleeve-like covering for structures (i.e., airway, vessels, nerves, lymphatics) that pass between the lung and mediastinum. This sleeve-like covering and the structures it contains forms the root of the lung. The root joins the medial surface of the lung at an area referred to as the hilum of the lung. Here, the mediastinal pleura is continuous with the visceral pleura.
The parietal pleural is innervated by somatic afferent fibers. The costal pleura is innervated by branches from the intercostal nerves, and pain would be felt in relation to the thoracic wall. The diaphragmatic pleura and the mediastinal pleura are innervated mainly by the phrenic nerves (originating at spinal cord levels C3, C4, and C5). Pain from these areas would refer to the C3, C4, and C5 dermatomes (lateral neck and the supraclavicular region of the shoulder).
The peripheral reflections of parietal pleura mark the extent of the pleural cavities (Fig. 3.39).
Superiorly, the pleural cavity can project as much as 3 to 4 cm above the first costal cartilage but does not extend above the neck of rib I. This limitation is caused by the inferior slope of rib I to its articulation with the manubrium.
Anteriorly, the pleural cavities approach each other posterior to the upper part of the sternum. However, posterior to the lower part of the sternum, the parietal pleura does not come as close to the midline on the left side as it does on the right because the middle mediastinum, containing the pericardium and heart, bulges to the left.
Inferiorly, the costal pleura reflects onto the diaphragm above the costal margin. In the midclavicular line, the pleural cavity extends inferiorly to approximately rib VIII. In the midaxillary line, it extends to rib X. From this point, the inferior margin courses somewhat horizontally, crossing ribs XI and XII to reach vertebra TXII. From the midclavicular line to the vertebral column, the inferior boundary of the pleura can be approximated by a line that runs between rib VIII, rib X, and vertebra TXII.
The visceral pleura is continuous with the parietal pleura at the hilum of each lung, where structures enter and leave the organ. The visceral pleura is firmly attached to the surface of the lung, including both opposed surfaces of the fissures that divide the lungs into lobes.
Although the visceral pleura is innervated by visceral afferent nerves that accompany bronchial vessels, pain is generally not elicited from this tissue.
The lungs do not completely fill the anterior or posterior inferior regions of the pleural cavities (Fig. 3.40). This results in recesses in which two layers of parietal pleura become opposed. Expansion of the lungs into these spaces usually occurs only during forced inspiration; the recesses also provide potential spaces in which fluids can collect and from which fluids can be aspirated.
Anteriorly, a costomediastinal recess occurs on each side where costal pleura is opposed to mediastinal pleura. The largest is on the left side in the region overlying the heart (Fig. 3.40). | Gray's Anatomy |
The largest and clinically most important recesses are the costodiaphragmatic recesses, which occur in each pleural cavity between the costal pleura and diaphragmatic pleura (Fig. 3.40). The costodiaphragmatic recesses are the regions between the inferior margin of the lungs and inferior margin of the pleural cavities. They are deepest after forced expiration and shallowest after forced inspiration.
During quiet respiration, the inferior margin of the lung crosses rib VI in the midclavicular line and rib VIII in the midaxillary line, and then courses somewhat horizontally to reach the vertebral column at vertebral level TX. Thus, from the midclavicular line and around the thoracic wall to the vertebral column, the inferior margin of the lung can be approximated by a line running between rib VI, rib VIII, and vertebra TX. The inferior margin of the pleural cavity at the same points is rib VIII, rib X, and vertebra TXII. The costodiaphragmatic recess is the region between the two margins.
During expiration, the inferior margin of the lung rises and the costodiaphragmatic recess becomes larger.
The two lungs are organs of respiration and lie on either side of the mediastinum surrounded by the right and left pleural cavities. Air enters and leaves the lungs via main bronchi, which are branches of the trachea.
The pulmonary arteries deliver deoxygenated blood to the lungs from the right ventricle of the heart. Oxygenated blood returns to the left atrium via the pulmonary veins.
The right lung is normally a little larger than the left lung because the middle mediastinum, containing the heart, bulges more to the left than to the right.
Each lung has a half-cone shape, with a base, apex, two surfaces, and three borders (Fig. 3.43).
The base sits on the diaphragm.
The apex projects above rib I and into the root of the neck.
The two surfaces—the costal surface lies immediately adjacent to the ribs and intercostal spaces of the thoracic wall. The mediastinal surface lies against the mediastinum anteriorly and the vertebral column posteriorly and contains the comma-shaped hilum of the lung, through which structures enter and leave.
The three borders—the inferior border of the lung is sharp and separates the base from the costal surface. The anterior and posterior borders separate the costal surface from the medial surface. Unlike the anterior and inferior borders, which are sharp, the posterior border is smooth and rounded.
The lungs lie directly adjacent to, and are indented by, structures contained in the overlying area. The heart and major vessels form bulges in the mediastinum that indent the medial surfaces of the lung; the ribs indent the costal surfaces. Pathology, such as tumors, or abnormalities in one structure can affect the related structure.
The root of each lung is a short tubular collection of structures that together attach the lung to structures in the mediastinum (Fig. 3.44). It is covered by a sleeve of mediastinal pleura that reflects onto the surface of the lung as visceral pleura. The region outlined by this pleural reflection on the medial surface of the lung is the hilum, where structures enter and leave.
A thin blade-like fold of pleura projects inferiorly from the root of the lung and extends from the hilum to the mediastinum. This structure is the pulmonary ligament. It may stabilize the position of the inferior lobe and may also accommodate the down-and-up translocation of structures in the root during breathing.
In the mediastinum, the vagus nerves pass immediately posterior to the roots of the lungs, while the phrenic nerves pass immediately anterior to them.
Within each root and located in the hilum are: a pulmonary artery, two pulmonary veins, a main bronchus, bronchial vessels, nerves, and lymphatics.
Generally, the pulmonary artery is superior at the hilum, the pulmonary veins are inferior, and the bronchi are somewhat posterior in position. | Gray's Anatomy |
On the right side, the lobar bronchus to the superior lobe branches from the main bronchus in the root, unlike on the left where it branches within the lung itself, and is superior to the pulmonary artery.
The right lung has three lobes and two fissures (Fig. 3.45A). Normally, the lobes are freely movable against each other because they are separated, almost to the hilum, by invaginations of visceral pleura. These invaginations form the fissures:
The oblique fissure separates the inferior lobe (lower lobe) from the superior lobe and the middle lobe of the right lung.
The horizontal fissure separates the superior lobe (upper lobe) from the middle lobe.
The approximate position of the oblique fissure on a patient, in quiet respiration, can be marked by a curved line on the thoracic wall that begins roughly at the spinous process of the vertebra TIV level of the spine, crosses the fifth interspace laterally, and then follows the contour of rib VI anteriorly (see pp. 241–242).
The horizontal fissure follows the fourth intercostal space from the sternum until it meets the oblique fissure as it crosses rib V.
The orientations of the oblique and horizontal fissures determine where clinicians should listen for lung sounds from each lobe.
The largest surface of the superior lobe is in contact with the upper part of the anterolateral wall and the apex of this lobe projects into the root of the neck. The surface of the middle lobe lies mainly adjacent to the lower anterior and lateral wall. The costal surface of the inferior lobe is in contact with the posterior and inferior walls.
When listening to lung sounds from each of the lobes, it is important to position the stethoscope on those areas of the thoracic wall related to the underlying positions of the lobes (see p. 243).
The medial surface of the right lung lies adjacent to a number of important structures in the mediastinum and the root of the neck (Fig. 3.45B). These include the: heart, inferior vena cava, superior vena cava, azygos vein, and esophagus.
The right subclavian artery and vein arch over and are related to the superior lobe of the right lung as they pass over the dome of the cervical pleura and into the axilla.
The left lung is smaller than the right lung and has two lobes separated by an oblique fissure (Fig. 3.46A). The oblique fissure of the left lung is slightly more oblique than the corresponding fissure of the right lung.
During quiet respiration, the approximate position of the left oblique fissure can be marked by a curved line on the thoracic wall that begins between the spinous processes of vertebrae TIII and TIV, crosses the fifth interspace laterally, and follows the contour of rib VI anteriorly (see pp. 241–242).
As with the right lung, the orientation of the oblique fissure determines where to listen for lung sounds from each lobe.
The largest surface of the superior lobe is in contact with the upper part of the anterolateral wall, and the apex of this lobe projects into the root of the neck. The costal surface of the inferior lobe is in contact with the posterior and inferior walls.
When listening to lung sounds from each of the lobes, the stethoscope should be placed on those areas of the thoracic wall related to the underlying positions of the lobes (see p. 243).
The inferior portion of the medial surface of the left lung, unlike the right lung, is notched because of the heart’s projection into the left pleural cavity from the middle mediastinum.
From the anterior border of the lower part of the superior lobe a tongue-like extension (the lingula of the left lung) projects over the heart bulge.
The medial surface of the left lung lies adjacent to a number of important structures in the mediastinum and root of the neck (Fig. 3.46B). These include the: heart, aortic arch, thoracic aorta, and esophagus. | Gray's Anatomy |
The left subclavian artery and vein arch over and are related to the superior lobe of the left lung as they pass over the dome of the cervical pleura and into the axilla.
The trachea is a flexible tube that extends from vertebral level CVI in the lower neck to vertebral level TIV/V in the mediastinum where it bifurcates into a right and a left main bronchus (Fig. 3.47). The trachea is held open by C-shaped transverse cartilage rings embedded in its wall—the open part of the C facing posteriorly. The lowest tracheal ring has a hook-shaped structure, the carina, that projects backward in the midline between the origins of the two main bronchi. The posterior wall of the trachea is composed mainly of smooth muscle.
Each main bronchus enters the root of a lung and passes through the hilum into the lung itself. The right main bronchus is wider and takes a more vertical course through the root and hilum than the left main bronchus (Fig. 3.47A). Therefore, inhaled foreign bodies tend to lodge more frequently on the right side than on the left.
The main bronchus divides within the lung into lobar bronchi (secondary bronchi), each of which supplies a lobe. On the right side, the lobar bronchus to the superior lobe originates within the root of the lung.
The lobar bronchi further divide into segmental bronchi (tertiary bronchi), which supply bronchopulmonary segments (Fig. 3.47B).
Within each bronchopulmonary segment, the segmental bronchi give rise to multiple generations of divisions and, ultimately, to bronchioles, which further subdivide and supply the respiratory surfaces. The walls of the bronchi are held open by discontinuous elongated plates of cartilage, but these are not present in bronchioles.
A bronchopulmonary segment is the area of lung supplied by a segmental bronchus and its accompanying pulmonary artery branch.
Tributaries of the pulmonary vein tend to pass intersegmentally between and around the margins of segments.
Each bronchopulmonary segment is shaped like an irregular cone, with the apex at the origin of the segmental bronchus and the base projected peripherally onto the surface of the lung.
A bronchopulmonary segment is the smallest functionally independent region of a lung and the smallest area of lung that can be isolated and removed without affecting adjacent regions.
There are ten bronchopulmonary segments in each lung (Fig. 3.48); some of them fuse in the left lung.
The right and left pulmonary arteries originate from the pulmonary trunk and carry deoxygenated blood to the lungs from the right ventricle of the heart (Fig. 3.49).
The bifurcation of the pulmonary trunk occurs to the left of the midline just inferior to vertebral level TIV/V, and anteroinferiorly to the left of the bifurcation of the trachea.
The right pulmonary artery is longer than the left and passes horizontally across the mediastinum (Fig. 3.49).
It passes: anteriorly and slightly inferiorly to the tracheal bifurcation and anteriorly to the right main bronchus, and posteriorly to the ascending aorta, superior vena cava, and upper right pulmonary vein.
The right pulmonary artery enters the root of the lung and gives off a large branch to the superior lobe of the lung. The main vessel continues through the hilum of the lung, gives off a second (recurrent) branch to the superior lobe, and then divides to supply the middle and inferior lobes.
The left pulmonary artery is shorter than the right and lies anterior to the descending aorta and posterior to the superior pulmonary vein (Fig. 3.49). It passes through the root and hilum and branches within the lung.
On each side a superior pulmonary vein and an inferior pulmonary vein carry oxygenated blood from the lungs back to the heart (Fig. 3.49). The veins begin at the hilum of the lung, pass through the root of the lung, and immediately drain into the left atrium. | Gray's Anatomy |
The bronchial arteries (Fig. 3.49) and veins constitute the “nutritive” vascular system of the pulmonary tissues (bronchial walls and glands, walls of large vessels, and visceral pleura). They interconnect within the lung with branches of the pulmonary arteries and veins.
The bronchial arteries originate from the thoracic aorta or one of its branches:
A single right bronchial artery normally arises from the third posterior intercostal artery (but occasionally, it originates from the upper left bronchial artery).
Two left bronchial arteries arise directly from the anterior surface of the thoracic aorta—the superior left bronchial artery arises at vertebral level TV, and the inferior one inferior to the left bronchus.
The bronchial arteries run on the posterior surfaces of the bronchi and ramify in the lungs to supply pulmonary tissues.
The bronchial veins drain into: either the pulmonary veins or the left atrium, and into the azygos vein on the right or into the superior intercostal vein or hemiazygos vein on the left.
Structures of the lung and the visceral pleura are supplied by visceral afferents and efferents distributed through the anterior pulmonary plexus and posterior pulmonary plexus (Fig. 3.50). These interconnected plexuses lie anteriorly and posteriorly to the tracheal bifurcation and main bronchi. The anterior plexus is much smaller than the posterior plexus.
Branches of these plexuses, which ultimately originate from the sympathetic trunks and vagus nerves, are distributed along branches of the airway and vessels.
Visceral efferents from: the vagus nerves constrict the bronchioles; the sympathetic system dilates the bronchioles.
Superficial, or subpleural, and deep lymphatics of the lung drain into lymph nodes called tracheobronchial nodes around the roots of lobar and main bronchi and along the sides of the trachea (Fig. 3.51). As a group, these lymph nodes extend from within the lung, through the hilum and root, and into the posterior mediastinum.
Efferent vessels from these nodes pass superiorly along the trachea to unite with similar vessels from parasternal nodes and brachiocephalic nodes, which are anterior to brachiocephalic veins in the superior mediastinum, to form the right and left bronchomediastinal trunks. These trunks drain directly into deep veins at the base of the neck, or may drain into the right lymphatic trunk or thoracic duct.
The mediastinum is a broad central partition that separates the two laterally placed pleural cavities (Fig. 3.55). It extends: from the sternum to the bodies of the vertebrae, and from the superior thoracic aperture to the diaphragm (Fig. 3.56).
The mediastinum contains the thymus gland, the pericardial sac, the heart, the trachea, and the major arteries and veins.
Additionally, the mediastinum serves as a passageway for structures such as the esophagus, thoracic duct, and various components of the nervous system as they traverse the thorax on their way to the abdomen.
For organizational purposes, the mediastinum is subdivided into several smaller regions. A transverse plane extending from the sternal angle (the junction between the manubrium and the body of the sternum) to the intervertebral disc between vertebrae TIV and TV separates the mediastinum into the: superior mediastinum, and inferior mediastinum, which is further partitioned into the anterior, middle, and posterior mediastinum by the pericardial sac.
The area anterior to the pericardial sac and posterior to the body of the sternum is the anterior mediastinum. The region posterior to the pericardial sac and the diaphragm and anterior to the bodies of the vertebrae is the posterior mediastinum. The area in the middle, which includes the pericardial sac and its contents, is the middle mediastinum (Fig. 3.57). | Gray's Anatomy |
The anterior mediastinum is posterior to the body of the sternum and anterior to the pericardial sac (see Fig. 3.57).
Its superior boundary is a transverse plane passing from the sternal angle to the intervertebral disc between vertebra TIV and TV, separating it from the superior mediastinum.
Its inferior boundary is the diaphragm.
Laterally, it is bordered by the mediastinal part of parietal pleura on either side.
The major structure in the anterior mediastinum is an inferior extension of the thymus gland (Fig. 3.58). Also present are fat, connective tissue, lymph nodes, mediastinal branches of the internal thoracic vessels, and sternopericardial ligaments, which pass from the posterior surface of the body of the sternum to the fibrous pericardium.
The middle mediastinum is centrally located in the thoracic cavity. It contains the pericardium, heart, origins of the great vessels, various nerves, and smaller vessels.
The pericardium is a fibroserous sac surrounding the heart and the roots of the great vessels. It consists of two components, the fibrous pericardium and the serous pericardium (Fig. 3.59).
The fibrous pericardium is a tough connective tissue outer layer that defines the boundaries of the middle mediastinum. The serous pericardium is thin and consists of two parts:
The parietal layer of serous pericardium lines the inner surface of the fibrous pericardium.
The visceral layer (epicardium) of serous pericardium adheres to the heart and forms its outer covering.
The parietal and visceral layers of serous pericardium are continuous at the roots of the great vessels. The narrow space created between the two layers of serous pericardium, containing a small amount of fluid, is the pericardial cavity. This potential space allows for the relatively uninhibited movement of the heart.
The fibrous pericardium is a cone-shaped bag with its base on the diaphragm and its apex continuous with the adventitia of the great vessels (Fig. 3.59). The base is attached to the central tendon of the diaphragm and to a small muscular area of the diaphragm on the left side. Anteriorly, it is attached to the posterior surface of the sternum by sternopericardial ligaments. These attachments help to retain the heart in its position in the thoracic cavity. The sac also limits cardiac distention.
The phrenic nerves, which innervate the diaphragm and originate from spinal cord levels C3 to C5, pass through the fibrous pericardium and innervate the fibrous pericardium as they travel from their point of origin to their final destination (Fig. 3.60). Their location, within the fibrous pericardium, is directly related to the embryological origin of the diaphragm and the changes that occur during the formation of the pericardial cavity. Similarly, the pericardiacophrenic vessels are also located within and supply the fibrous pericardium as they pass through the thoracic cavity.
The parietal layer of serous pericardium is continuous with the visceral layer of serous pericardium around the roots of the great vessels. These reflections of serous pericardium (Fig. 3.61) occur in two locations: one superiorly, surrounding the arteries—the aorta and the pulmonary trunk; the second more posteriorly, surrounding the veins—the superior and inferior vena cava and the pulmonary veins.
The zone of reflection surrounding the veins is J-shaped, and the cul-de-sac formed within the J, posterior to the left atrium, is the oblique pericardial sinus.
A passage between the two sites of reflected serous pericardium is the transverse pericardial sinus. This sinus lies posterior to the ascending aorta and the pulmonary trunk, anterior to the superior vena cava, and superior to the left atrium. | Gray's Anatomy |
When the pericardium is opened anteriorly during surgery, a finger placed in the transverse sinus separates arteries from veins. A hand placed under the apex of the heart and moved superiorly slips into the oblique sinus.
The pericardium is supplied by branches from the internal thoracic, pericardiacophrenic, musculophrenic, and inferior phrenic arteries, and the thoracic aorta.
Veins from the pericardium enter the azygos system of veins and the internal thoracic and superior phrenic veins.
Nerves supplying the pericardium arise from the vagus nerve [X], the sympathetic trunks, and the phrenic nerves.
It is important to note that the source of somatic sensation (pain) from the parietal pericardium is carried by somatic afferent fibers in the phrenic nerves. For this reason, “pain” related to a pericardial problem may be referred to the supraclavicular region of the shoulder or lateral neck area dermatomes for spinal cord segments
C3, C4, and C5.
The general shape and orientation of the heart are that of a pyramid that has fallen over and is resting on one of its sides. Placed in the thoracic cavity, the apex of this pyramid projects forward, downward, and to the left, whereas the base is opposite the apex and faces in a posterior direction (Fig. 3.63). The sides of the pyramid consist of: a diaphragmatic (inferior) surface on which the pyramid rests, an anterior (sternocostal) surface oriented anteriorly, a right pulmonary surface, and a left pulmonary surface.
The base of the heart is quadrilateral and directed posteriorly. It consists of: the left atrium, a small portion of the right atrium, and the proximal parts of the great veins (superior and inferior venae cavae and the pulmonary veins) (Fig. 3.64).
Because the great veins enter the base of the heart, with the pulmonary veins entering the right and left sides of the left atrium and the superior and inferior venae cavae at the upper and lower ends of the right atrium, the base of the heart is fixed posteriorly to the pericardial wall, opposite the bodies of vertebrae TV to TVIII (TVI to TIX when standing). The esophagus lies immediately posterior to the base.
From the base the heart projects forward, downward, and to the left, ending in the apex. The apex of the heart is formed by the inferolateral part of the left ventricle (Fig. 3.65) and is positioned deep to the left fifth intercostal space, 8 to 9 cm from the midsternal line.
Surfaces of the heart
The anterior surface faces anteriorly and consists mostly of the right ventricle, with some of the right atrium on the right and some of the left ventricle on the left (Fig. 3.65).
The heart in the anatomical position rests on the diaphragmatic surface, which consists of the left ventricle and a small portion of the right ventricle separated by the posterior interventricular groove (Fig. 3.66). This surface faces inferiorly, rests on the diaphragm, is separated from the base of the heart by the coronary sinus, and extends from the base to the apex of the heart.
The left pulmonary surface faces the left lung, is broad and convex, and consists of the left ventricle and a portion of the left atrium (Fig. 3.66).
The right pulmonary surface faces the right lung, is broad and convex, and consists of the right atrium (Fig. 3.66).
Some general descriptions of cardiac orientation refer to right, left, inferior (acute), and obtuse margins:
The right and left margins are the same as the right and left pulmonary surfaces of the heart.
The inferior margin is defined as the sharp edge between the anterior and diaphragmatic surfaces of the heart (Figs 3.63 and 3.65)—it is formed mostly by the right ventricle and a small portion of the left ventricle near the apex. | Gray's Anatomy |
The obtuse margin separates the anterior and left pulmonary surfaces (Fig. 3.63)—it is round and extends from the left auricle to the cardiac apex (Fig. 3.65), and is formed mostly by the left ventricle and superiorly by a small portion of the left auricle.
For radiological evaluations, a thorough understanding of the structures defining the cardiac borders is critical. The right border in a standard posteroanterior view consists of the superior vena cava, the right atrium, and the inferior vena cava (Fig. 3.67A). The left border in a similar view consists of the arch of the aorta, the pulmonary trunk, left auricle, and the left ventricle. The inferior border in this radiological study consists of the right ventricle and the left ventricle at the apex. In lateral views, the right ventricle is seen anteriorly, and the left atrium is visualized posteriorly (Fig. 3.67B).
Internal partitions divide the heart into four chambers (i.e., two atria and two ventricles) and produce surface or external grooves referred to as sulci.
The coronary sulcus circles the heart, separating the atria from the ventricles (Fig. 3.68). As it circles the heart, it contains the right coronary artery, the small cardiac vein, the coronary sinus, and the circumflex branch of the left coronary artery.
The anterior and posterior interventricular sulci separate the two ventricles—the anterior interventricular sulcus is on the anterior surface of the heart and contains the anterior interventricular artery and the great cardiac vein, and the posterior interventricular sulcus is on the diaphragmatic surface of the heart and contains the posterior interventricular artery and the middle cardiac vein.
These sulci are continuous inferiorly, just to the right of the apex of the heart.
The heart functionally consists of two pumps separated by a partition (Fig. 3.69A). The right pump receives deoxygenated blood from the body and sends it to the lungs. The left pump receives oxygenated blood from the lungs and sends it to the body. Each pump consists of an atrium and a ventricle separated by a valve.
The thin-walled atria receive blood coming into the heart, whereas the relatively thick-walled ventricles pump blood out of the heart.
More force is required to pump blood through the body than through the lungs, so the muscular wall of the left ventricle is thicker than the right.
Interatrial, interventricular, and atrioventricular septa separate the four chambers of the heart (Fig. 3.69B). The internal anatomy of each chamber is critical to its function.
In the anatomical position, the right border of the heart is formed by the right atrium. This chamber also contributes to the right portion of the heart’s anterior surface.
Blood returning to the right atrium enters through one of three vessels. These are: the superior and inferior venae cavae, which together deliver blood to the heart from the body; and the coronary sinus, which returns blood from the walls of the heart itself.
The superior vena cava enters the upper posterior portion of the right atrium, and the inferior vena cava and coronary sinus enter the lower posterior portion of the right atrium.
From the right atrium, blood passes into the right ventricle through the right atrioventricular orifice. This opening faces forward and medially and is closed during ventricular contraction by the tricuspid valve.
The interior of the right atrium is divided into two continuous spaces. Externally, this separation is indicated by a shallow, vertical groove (the sulcus terminalis cordis), which extends from the right side of the opening of the superior vena cava to the right side of the opening of the inferior vena cava. Internally, this division is indicated by the crista terminalis (Fig. 3.70), which is a smooth, muscular ridge that begins on the roof of the atrium just in front of the opening of the superior vena cava and extends down the lateral wall to the anterior lip of the inferior vena cava. | Gray's Anatomy |
The space posterior to the crista is the sinus of venae cavae and is derived embryologically from the right horn of the sinus venosus. This component of the right atrium has smooth, thin walls, and both venae cavae empty into this space.
The space anterior to the crista, including the right auricle, is sometimes referred to as the atrium proper. This terminology is based on its origin from the embryonic primitive atrium. Its walls are covered by ridges called the musculi pectinati (pectinate muscles), which fan out from the crista like the “teeth of a comb.” These ridges are also found in the right auricle, which is an ear-like, conical, muscular pouch that externally overlaps the ascending aorta.
An additional structure in the right atrium is the opening of the coronary sinus, which receives blood from most of the cardiac veins and opens medially to the opening of the inferior vena cava. Associated with these openings are small folds of tissue derived from the valve of the embryonic sinus venosus (the valve of the coronary sinus and the valve of inferior vena cava, respectively). During development, the valve of the inferior vena cava helps direct incoming oxygenated blood through the foramen ovale and into the left atrium.
Separating the right atrium from the left atrium is the interatrial septum, which faces forward and to the right because the left atrium lies posteriorly and to the left of the right atrium. A depression is clearly visible in the septum just above the orifice of the inferior vena cava. This is the fossa ovalis (oval fossa), with its prominent margin, the limbus fossa ovalis (border of the oval fossa).
The fossa ovalis marks the location of the embryonic foramen ovale, which is an important part of fetal circulation. The foramen ovale allows oxygenated blood entering the right atrium through the inferior vena cava to pass directly to the left atrium and so bypass the lungs, which are nonfunctional before birth.
Finally, numerous small openings—the openings of the smallest cardiac veins (the foramina of the venae cordis minimae)—are scattered along the walls of the right atrium. These are small veins that drain the myocardium directly into the right atrium.
In the anatomical position, the right ventricle forms most of the anterior surface of the heart and a portion of the diaphragmatic surface. The right atrium is to the right of the right ventricle and the right ventricle is located in front of and to the left of the right atrioventricular orifice. Blood entering the right ventricle from the right atrium therefore moves in a horizontal and forward direction.
The outflow tract of the right ventricle, which leads to the pulmonary trunk, is the conus arteriosus (infundibulum). This area has smooth walls and derives from the embryonic bulbus cordis.
The walls of the inflow portion of the right ventricle have numerous muscular, irregular structures called trabeculae carneae (Fig. 3.71). Most of these are either attached to the ventricular walls throughout their length, forming ridges, or attached at both ends, forming bridges.
A few trabeculae carneae (papillary muscles) have only one end attached to the ventricular surface, while the other end serves as the point of attachment for tendon-like fibrous cords (the chordae tendineae), which connect to the free edges of the cusps of the tricuspid valve.
There are three papillary muscles in the right ventricle. Named relative to their point of origin on the ventricular surface, they are the anterior, posterior, and septal papillary muscles:
The anterior papillary muscle is the largest and most constant papillary muscle, and arises from the anterior wall of the ventricle.
The posterior papillary muscle may consist of one, two, or three structures, with some chordae tendineae arising directly from the ventricular wall.
The septal papillary muscle is the most inconsistent papillary muscle, being either small or absent, with chordae tendineae emerging directly from the septal wall. | Gray's Anatomy |
A single specialized trabeculum, the septomarginal trabecula (moderator band), forms a bridge between the lower portion of the interventricular septum and the base of the anterior papillary muscle. The septomarginal trabecula carries a portion of the cardiac conduction system, the right bundle of the atrioventricular bundle, to the anterior wall of the right ventricle.
The right atrioventricular orifice is closed during ventricular contraction by the tricuspid valve (right atrioventricular valve), so named because it usually consists of three cusps or leaflets (Fig. 3.71). The base of each cusp is secured to the fibrous ring that surrounds the atrioventricular orifice. This fibrous ring helps to maintain the shape of the opening. The cusps are continuous with each other near their bases at sites termed commissures.
The naming of the three cusps, the anterior, septal, and posterior cusps, is based on their relative position in the right ventricle. The free margins of the cusps are attached to the chordae tendineae, which arise from the tips of the papillary muscles.
During filling of the right ventricle, the tricuspid valve is open, and the three cusps project into the right ventricle.
Without the presence of a compensating mechanism, when the ventricular musculature contracts, the valve cusps would be forced upward with the flow of blood and blood would move back into the right atrium. However, contraction of the papillary muscles attached to the cusps by chordae tendineae prevents the cusps from being everted into the right atrium.
Simply put, the papillary muscles and associated chordae tendineae keep the valves closed during the dramatic changes in ventricular size that occur during contraction.
In addition, chordae tendineae from two papillary muscles attach to each cusp. This helps prevent separation of the cusps during ventricular contraction. Proper closing of the tricuspid valve causes blood to exit the right ventricle and move into the pulmonary trunk.
Necrosis of a papillary muscle following a myocardial infarction (heart attack) may result in prolapse of the related valve.
At the apex of the infundibulum, the outflow tract of the right ventricle, the opening into the pulmonary trunk is closed by the pulmonary valve (Fig. 3.71), which consists of three semilunar cusps with free edges projecting upward into the lumen of the pulmonary trunk. The free superior edge of each cusp has a middle, thickened portion, the nodule of the semilunar cusp, and a thin lateral portion, the lunula of the semilunar cusp (Fig. 3.72).
The cusps are named the left, right, and anterior semilunar cusps, relative to their fetal position before rotation of the outflow tracts from the ventricles is complete. Each cusp forms a pocket-like sinus (Fig. 3.72)—a dilation in the wall of the initial portion of the pulmonary trunk. After ventricular contraction, the recoil of blood fills these pulmonary sinuses and forces the cusps closed. This prevents blood in the pulmonary trunk from refilling the right ventricle.
The left atrium forms most of the base or posterior surface of the heart.
As with the right atrium, the left atrium is derived embryologically from two structures.
The posterior half, or inflow portion, receives the four pulmonary veins (Fig. 3.73). It has smooth walls and derives from the proximal parts of the pulmonary veins that are incorporated into the left atrium during development.
The anterior half is continuous with the left auricle. It contains musculi pectinati and derives from the embryonic primitive atrium. Unlike the crista terminalis in the right atrium, no distinct structure separates the two components of the left atrium. | Gray's Anatomy |
The interatrial septum is part of the anterior wall of the left atrium. The thin area or depression in the septum is the valve of the foramen ovale and is opposite the floor of the fossa ovalis in the right atrium.
During development, the valve of the foramen ovale prevents blood from passing from the left atrium to the right atrium. This valve may not be completely fused in some adults, leaving a “probe patent” passage between the right atrium and the left atrium.
The left ventricle lies anterior to the left atrium. It contributes to the anterior, diaphragmatic, and left pulmonary surfaces of the heart, and forms the apex.
Blood enters the ventricle through the left atrioventricular orifice and flows in a forward direction to the apex. The chamber itself is conical, is longer than the right ventricle, and has the thickest layer of myocardium. The outflow tract (the aortic vestibule) is posterior to the infundibulum of the right ventricle, has smooth walls, and is derived from the embryonic bulbus cordis.
The trabeculae carneae in the left ventricle are fine and delicate in contrast to those in the right ventricle.
The general appearance of the trabeculae with muscular ridges and bridges is similar to that of the right ventricle (Fig. 3.74).
Papillary muscles, together with chordae tendineae, are also observed and their structure is as described above for the right ventricle. Two papillary muscles, the anterior and posterior papillary muscles, are usually found in the left ventricle and are larger than those of the right ventricle.
In the anatomical position, the left ventricle is somewhat posterior to the right ventricle. The interventricular septum therefore forms the anterior wall and some of the wall on the right side of the left ventricle. The septum is described as having two parts: a muscular part, and a membranous part.
The muscular part is thick and forms the major part of the septum, whereas the membranous part is the thin, upper part of the septum. A third part of the septum may be considered an atrioventricular part because of its position above the septal cusp of the tricuspid valve. This superior location places this part of the septum between the left ventricle and right atrium.
The left atrioventricular orifice opens into the posterior right side of the superior part of the left ventricle. It is closed during ventricular contraction by the mitral valve (left atrioventricular valve), which is also referred to as the bicuspid valve because it has two cusps, the anterior and posterior cusps (Fig. 3.74). The bases of the cusps are secured to a fibrous ring surrounding the opening, and the cusps are continuous with each other at the commissures. The coordinated action of the papillary muscles and chordae tendineae is as described for the right ventricle.
The aortic vestibule, or outflow tract of the left ventricle, is continuous superiorly with the ascending aorta. The opening from the left ventricle into the aorta is closed by the aortic valve. This valve is similar in structure to the pulmonary valve. It consists of three semilunar cusps with the free edge of each projecting upward into the lumen of the ascending aorta (Fig. 3.75).
Between the semilunar cusps and the wall of the ascending aorta are pocket-like sinuses—the right, left, and posterior aortic sinuses. The right and left coronary arteries originate from the right and left aortic sinuses. Because of this, the posterior aortic sinus and cusp are sometimes referred to as the noncoronary sinus and cusp.
The functioning of the aortic valve is similar to that of the pulmonary valve with one important additional process: as blood recoils after ventricular contraction and fills the aortic sinuses, it is automatically forced into the coronary arteries because these vessels originate from the right and left aortic sinuses. | Gray's Anatomy |
The cardiac skeleton is a collection of dense, fibrous connective tissue in the form of four rings with interconnecting areas in a plane between the atria and the ventricles. The four rings of the cardiac skeleton surround the two atrioventricular orifices, the aortic orifice and opening of the pulmonary trunks. They are the anulus fibrosus. The interconnecting areas include: the right fibrous trigone, which is a thickened area of connective tissue between the aortic ring and right atrioventricular ring; and the left fibrous trigone, which is a thickened area of connective tissue between the aortic ring and the left atrioventricular ring (Fig. 3.76).
The cardiac skeleton helps maintain the integrity of the openings it surrounds and provides points of attachment for the cusps. It also separates the atrial musculature from the ventricular musculature. The atrial myocardium originates from the upper border of the rings, whereas the ventricular myocardium originates from the lower border of the rings.
The cardiac skeleton also serves as a dense connective tissue partition that electrically isolates the atria from the ventricles. The atrioventricular bundle, which passes through the anulus, is the single connection between these two groups of myocardium.
Two coronary arteries arise from the aortic sinuses in the initial portion of the ascending aorta and supply the muscle and other tissues of the heart. They circle the heart in the coronary sulcus, with marginal and interventricular branches, in the interventricular sulci, converging toward the apex of the heart (Fig. 3.77).
The returning venous blood passes through cardiac veins, most of which empty into the coronary sinus. This large venous structure is located in the coronary sulcus on the posterior surface of the heart between the left atrium and left ventricle. The coronary sinus empties into the right atrium between the opening of the inferior vena cava and the right atrioventricular orifice.
Right coronary artery. The right coronary artery originates from the right aortic sinus of the ascending aorta. It passes anteriorly and then descends vertically in the coronary sulcus, between the right atrium and right ventricle (Fig. 3.78A). On reaching the inferior margin of the heart, it turns posteriorly and continues in the sulcus onto the diaphragmatic surface and base of the heart. During this course, several branches arise from the main stem of the vessel:
An early atrial branch passes in the groove between the right auricle and ascending aorta, and gives off the sinu-atrial nodal branch (Fig. 3.78A), which passes posteriorly around the superior vena cava to supply the sinu-atrial node.
A right marginal branch is given off as the right coronary artery approaches the inferior (acute) margin of the heart (Fig. 3.78A,B) and continues along this border toward the apex of the heart.
As the right coronary artery continues on the base/ diaphragmatic surface of the heart, it supplies a small branch to the atrioventricular node before giving off its final major branch, the posterior interventricular branch (Fig. 3.78A), which lies in the posterior interventricular sulcus.
The right coronary artery supplies the right atrium and right ventricle, the sinu-atrial and atrioventricular nodes, the interatrial septum, a portion of the left atrium, the posteroinferior one third of the interventricular septum, and a portion of the posterior part of the left ventricle.
Left coronary artery. The left coronary artery originates from the left aortic sinus of the ascending aorta. It passes between the pulmonary trunk and the left auricle before entering the coronary sulcus. Emerging from behind the pulmonary trunk, the artery divides into its two terminal branches, the anterior interventricular and the circumflex (Fig. 3.78A). | Gray's Anatomy |
The anterior interventricular branch (left anterior descending artery—LAD) (Fig. 3.78A,C) continues around the left side of the pulmonary trunk and descends obliquely toward the apex of the heart in the anterior interventricular sulcus (Fig. 3.78A,C). During its course, one or two large diagonal branches may arise and descend diagonally across the anterior surface of the left ventricle.
The circumflex branch (Fig. 3.78A,C) courses toward the left, in the coronary sulcus and onto the base/diaphragmatic surface of the heart, and usually ends before reaching the posterior interventricular sulcus. A large branch, the left marginal artery (Fig. 3.78A,C), usually arises from it and continues across the rounded obtuse margin of the heart.
The distribution pattern of the left coronary artery enables it to supply most of the left atrium and left ventricle, and most of the interventricular septum, including the atrioventricular bundle and its branches.
Variations in the distribution patterns of coronary arteries. Several major variations in the basic distribution patterns of the coronary arteries occur.
The distribution pattern described above for both right and left coronary arteries is the most common and consists of a right dominant coronary artery. This means that the posterior interventricular branch arises from the right coronary artery. The right coronary artery therefore supplies a large portion of the posterior wall of the left ventricle and the circumflex branch of the left coronary artery is relatively small.
In contrast, in hearts with a left dominant coronary artery, the posterior interventricular branch arises from an enlarged circumflex branch and supplies most of the posterior wall of the left ventricle (Fig. 3.79).
Another point of variation relates to the arterial supply to the sinu-atrial and atrioventricular nodes. In most cases, these two structures are supplied by the right coronary artery. However, vessels from the circumflex branch of the left coronary artery occasionally supply these structures.
The coronary sinus receives four major tributaries: the great, middle, small, and posterior cardiac veins.
Great cardiac vein. The great cardiac vein begins at the apex of the heart (Fig. 3.82A). It ascends in the anterior interventricular sulcus, where it is related to the anterior interventricular artery and is often termed the anterior interventricular vein. Reaching the coronary sulcus, the great cardiac vein turns to the left and continues onto the base/diaphragmatic surface of the heart. At this point, it is associated with the circumflex branch of the left coronary artery. Continuing along its path in the coronary sulcus, the great cardiac vein gradually enlarges to form the coronary sinus, which enters the right atrium (Fig. 3.82B).
Middle cardiac vein. The middle cardiac vein (posterior interventricular vein) begins near the apex of the heart and ascends in the posterior interventricular sulcus toward the coronary sinus (Fig. 3.82B). It is associated with the posterior interventricular branch of the right or left coronary artery throughout its course.
Small cardiac vein. The small cardiac vein begins in the lower anterior section of the coronary sulcus between the right atrium and right ventricle (Fig. 3.82A). It continues in this groove onto the base/diaphragmatic surface of the heart where it enters the coronary sinus at its atrial end.
It is a companion of the right coronary artery throughout its course and may receive the right marginal vein (Fig. 3.82A). This small vein accompanies the marginal branch of the right coronary artery along the acute margin of the heart. If the right marginal vein does not join the small cardiac vein, it enters the right atrium directly.
Posterior cardiac vein. The posterior cardiac vein lies on the posterior surface of the left ventricle just to the left of the middle cardiac vein (Fig. 3.82B). It either enters the coronary sinus directly or joins the great cardiac vein. | Gray's Anatomy |
Other cardiac veins. Two additional groups of cardiac veins are also involved in the venous drainage of the heart.
The anterior veins of the right ventricle (anterior cardiac veins) are small veins that arise on the anterior surface of the right ventricle (Fig. 3.82A). They cross the coronary sulcus and enter the anterior wall of the right atrium. They drain the anterior portion of the right ventricle. The right marginal vein may be part of this group if it does not enter the small cardiac vein.
A group of smallest cardiac veins (venae cordis minimae or veins of Thebesius) have also been described. Draining directly into the cardiac chambers, they are numerous in the right atrium and right ventricle, are occasionally associated with the left atrium, and are rarely associated with the left ventricle.
The lymphatic vessels of the heart follow the coronary arteries and drain mainly into: brachiocephalic nodes, anterior to the brachiocephalic veins; and tracheobronchial nodes, at the inferior end of the trachea.
The musculature of the atria and ventricles is capable of contracting spontaneously. The cardiac conduction system initiates and coordinates contraction. The conduction system consists of nodes and networks of specialized cardiac muscle cells organized into four basic components: the sinu-atrial node, the atrioventricular node, the atrioventricular bundle with its right and left bundle branches, and the subendocardial plexus of conduction cells (the Purkinje fibers).
The unique distribution pattern of the cardiac conduction system establishes an important unidirectional pathway of excitation/contraction. Throughout its course, large branches of the conduction system are insulated from the surrounding myocardium by connective tissue. This tends to decrease inappropriate stimulation and contraction of cardiac muscle fibers.
The number of functional contacts between the conduction pathway and cardiac musculature greatly increases in the subendocardial network.
Thus, a unidirectional wave of excitation and contraction is established, which moves from the papillary muscles and apex of the ventricles to the arterial outflow tracts.
Impulses begin at the sinu-atrial node, the cardiac pacemaker. This collection of cells is located at the superior end of the crista terminalis at the junction of the superior vena cava and the right atrium (Fig. 3.83A).
This is also the junction between the parts of the right atrium derived from the embryonic sinus venosus and the atrium proper.
The excitation signals generated by the sinu-atrial node spread across the atria, causing the muscle to contract.
Concurrently, the wave of excitation in the atria stimulates the atrioventricular node, which is located near the opening of the coronary sinus, close to the attachment of the septal cusp of the tricuspid valve, and within the atrioventricular septum (Fig. 3.83A).
The atrioventricular node is a collection of specialized cells that forms the beginning of an elaborate system of conducting tissue, the atrioventricular bundle, which extends the excitatory impulse to all ventricular musculature.
The atrioventricular bundle is a direct continuation of the atrioventricular node (Fig. 3.83A). It follows along the lower border of the membranous part of the interventricular septum before splitting into right and left bundles.
The right bundle branch continues on the right side of the interventricular septum toward the apex of the right ventricle. From the septum it enters the septomarginal trabecula to reach the base of the anterior papillary muscle. At this point, it divides and is continuous with the final component of the cardiac conduction system, the subendocardial plexus of ventricular conduction cells or Purkinje fibers. This network of specialized cells spreads throughout the ventricle to supply the ventricular musculature, including the papillary muscles. | Gray's Anatomy |
The left bundle branch passes to the left side of the muscular interventricular septum and descends to the apex of the left ventricle (Fig. 3.83B). Along its course it gives off branches that eventually become continuous with the subendocardial plexus of conduction cells (Purkinje fibers). As with the right side, this network of specialized cells spreads the excitation impulses throughout the left ventricle.
The autonomic division of the peripheral nervous system is directly responsible for regulating: heart rate, force of each contraction, and cardiac output.
Branches from both the parasympathetic and sympathetic systems contribute to the formation of the cardiac plexus. This plexus consists of a superficial part, inferior to the aortic arch and between it and the pulmonary trunk (Fig. 3.84A), and a deep part, between the aortic arch and the tracheal bifurcation (Fig. 3.84B).
From the cardiac plexus, small branches that are mixed nerves containing both sympathetic and parasympathetic fibers supply the heart. These branches affect nodal tissue and other components of the conduction system, coronary blood vessels, and atrial and ventricular musculature.
Stimulation of the parasympathetic system: decreases heart rate, reduces force of contraction, and constricts the coronary arteries.
The preganglionic parasympathetic fibers reach the heart as cardiac branches from the right and left vagus nerves. They enter the cardiac plexus and synapse in ganglia located either within the plexus or in the walls of the atria.
Stimulation of the sympathetic system: increases heart rate, and increases the force of contraction.
Sympathetic fibers reach the cardiac plexus through the cardiac nerves from the sympathetic trunk. Preganglionic sympathetic fibers from the upper four or five segments of the thoracic spinal cord enter and move through the sympathetic trunk. They synapse in cervical and upper thoracic sympathetic ganglia, and postganglionic fibers proceed as bilateral branches from the sympathetic trunk to the cardiac plexus.
Visceral afferents from the heart are also a component of the cardiac plexus. These fibers pass through the cardiac plexus and return to the central nervous system in the cardiac nerves from the sympathetic trunk and in the vagal cardiac branches.
The afferents associated with the vagal cardiac nerves return to the vagus nerve [X]. They sense alterations in blood pressure and blood chemistry and are therefore primarily concerned with cardiac reflexes.
The afferents associated with the cardiac nerves from the sympathetic trunks return to either the cervical or the thoracic portions of the sympathetic trunk. If they are in the cervical portion of the trunk, they normally descend to the thoracic region, where they reenter the upper four or five thoracic spinal cord segments, along with the afferents from the thoracic region of the sympathetic trunk. Visceral afferents associated with the sympathetic system conduct pain sensation from the heart, which is detected at the cellular level as tissue-damaging events (i.e., cardiac ischemia). This pain is often “referred” to cutaneous regions supplied by the same spinal cord levels (see “In the clinic: Referred pain.” p. 46, and “Case 1,” pp. 244–246). | Gray's Anatomy |
The pulmonary trunk is contained within the pericardial sac (Fig. 3.85), is covered by the visceral layer of serous pericardium, and is associated with the ascending aorta in a common sheath. It arises from the conus arteriosus of the right ventricle at the opening of the pulmonary trunk slightly anterior to the aortic orifice and ascends, moving posteriorly and to the left, lying initially anterior and then to the left of the ascending aorta. At approximately the level of the intervertebral disc between vertebrae TV and TVI, opposite the left border of the sternum and posterior to the third left costal cartilage, the pulmonary trunk divides into: the right pulmonary artery, which passes to the right, posterior to the ascending aorta and the superior vena cava, to enter the right lung; and the left pulmonary artery, which passes inferiorly to the arch of the aorta and anteriorly to the descending aorta to enter the left lung.
The ascending aorta is contained within the pericardial sac and is covered by a visceral layer of serous pericardium, which also surrounds the pulmonary trunk in a common sheath (Fig. 3.85A).
The origin of the ascending aorta is the aortic orifice at the base of the left ventricle, which is level with the lower edge of the third left costal cartilage, posterior to the left half of the sternum. Moving superiorly, slightly forward and to the right, the ascending aorta continues to the level of the second right costal cartilage. At this point, it enters the superior mediastinum and is then referred to as the arch of the aorta.
Immediately superior to the point where the ascending aorta arises from the left ventricle are three small outward bulges opposite the semilunar cusps of the aortic valve. These are the posterior, right, and left aortic sinuses. The right and left coronary arteries originate from the right and left aortic sinuses, respectively.
The inferior half of the superior vena cava is located within the pericardial sac (Fig. 3.85B). It passes through the fibrous pericardium at approximately the level of the second costal cartilage and enters the right atrium at the lower level of the third costal cartilage. The portion within the pericardial sac is covered with serous pericardium except for a small area on its posterior surface.
After passing through the diaphragm, at approximately the level of vertebra TVIII, the inferior vena cava enters the fibrous pericardium. A short portion of this vessel is within the pericardial sac before entering the right atrium. While within the pericardial sac, it is covered by serous pericardium except for a small portion of its posterior surface (Fig. 3.85B).
A very short segment of each of the pulmonary veins is also within the pericardial sac. These veins, usually two from each lung, pass through the fibrous pericardium and enter the superior region of the left atrium on its posterior surface. In the pericardial sac, all but a portion of the posterior surface of these veins is covered by serous pericardium. In addition, the oblique pericardial sinus is between the right and left pulmonary veins, within the pericardial sac (Fig. 3.85B).
The superior mediastinum is posterior to the manubrium of the sternum and anterior to the bodies of the first four thoracic vertebrae (see Fig. 3.57).
Its superior boundary is an oblique plane passing from the jugular notch upward and posteriorly to the superior border of vertebra TI.
Inferiorly, a transverse plane passing from the sternal angle to the intervertebral disc between vertebra TIV/V separates it from the inferior mediastinum.
Laterally, it is bordered by the mediastinal part of the parietal pleura on either side.
The superior mediastinum is continuous with the neck above and with the inferior mediastinum below. | Gray's Anatomy |
The major structures found in the superior mediastinum (Figs. 3.86 and 3.87) include the: thymus, right and left brachiocephalic veins, left superior intercostal vein, superior vena cava, arch of the aorta with its three large branches, trachea, esophagus, phrenic nerves, vagus nerves, left recurrent laryngeal branch of the left vagus nerve, thoracic duct, and other small nerves, blood vessels, and lymphatics.
The thymus is the most anterior component of the superior mediastinum, lying immediately posterior to the manubrium of the sternum. It is an asymmetrical, bilobed structure (see Fig. 3.58).
The upper extent of the thymus can reach into the neck as high as the thyroid gland; a lower portion typically extends into the anterior mediastinum over the pericardial sac.
Involved in the early development of the immune system, the thymus is a large structure in the child, begins to atrophy after puberty, and shows considerable size variation in the adult. In the elderly adult, it is barely identifiable as an organ, consisting mostly of fatty tissue that is sometimes arranged as two lobulated fatty structures.
Arteries to the thymus consist of small branches originating from the internal thoracic arteries. Venous drainage is usually into the left brachiocephalic vein and possibly into the internal thoracic veins.
Lymphatic drainage returns to multiple groups of nodes at one or more of the following locations: along the internal thoracic arteries (parasternal); at the tracheal bifurcation (tracheobronchial); and in the root of the neck.
The left and right brachiocephalic veins are located immediately posterior to the thymus. They form on each side at the junction between the internal jugular and subclavian veins (see Fig. 3.86). The left brachiocephalic vein crosses the midline and joins with the right brachiocephalic vein to form the superior vena cava (Fig. 3.88).
The right brachiocephalic vein begins posterior to the medial end of the right clavicle and passes vertically downward, forming the superior vena cava when it is joined by the left brachiocephalic vein. Venous tributaries include the vertebral, first posterior intercostal, and internal thoracic veins. The inferior thyroid and thymic veins may also drain into it.
The left brachiocephalic vein begins posterior to the medial end of the left clavicle. It crosses to the right, moving in a slightly inferior direction, and joins with the right brachiocephalic vein to form the superior vena cava posterior to the lower edge of the right first costal cartilage close to the right sternal border. Venous tributaries include the vertebral, first posterior intercostal, left superior intercostal, inferior thyroid, and internal thoracic veins. It may also receive thymic and pericardial veins. The left brachiocephalic vein crosses the midline posterior to the manubrium in the adult. In infants and children the left brachiocephalic vein rises above the superior border of the manubrium and therefore is less protected.
The left superior intercostal vein receives the second, third, and sometimes the fourth posterior intercostal veins, usually the left bronchial veins, and sometimes the left pericardiacophrenic vein. It passes over the left side of the aortic arch, lateral to the left vagus nerve and medial to the left phrenic nerve, before entering the left brachiocephalic vein (Fig. 3.89). Inferiorly, it may connect with the accessory hemiazygos vein (superior hemiazygos vein).
The vertically oriented superior vena cava begins posterior to the lower edge of the right first costal cartilage, where the right and left brachiocephalic veins join, and terminates at the lower edge of the right third costal cartilage, where it joins the right atrium (see Fig. 3.86). | Gray's Anatomy |
The lower half of the superior vena cava is within the pericardial sac and is therefore contained in the middle mediastinum.
The superior vena cava receives the azygos vein immediately before entering the pericardial sac and may also receive pericardial and mediastinal veins.
The superior vena cava can be easily visualized forming part of the right superolateral border of the mediastinum on a chest radiograph (see Fig. 3.67A).
Arch of aorta and its branches
The thoracic portion of the aorta can be divided into ascending aorta, arch of the aorta, and thoracic (descending) aorta. Only the arch of the aorta is in the superior mediastinum. It begins when the ascending aorta emerges from the pericardial sac and courses upward, backward, and to the left as it passes through the superior mediastinum, ending on the left side at vertebral level TIV/V (see Fig. 3.86). Extending as high as the midlevel of the manubrium of the sternum, the arch is initially anterior and finally lateral to the trachea.
Three branches arise from the superior border of the arch of the aorta; at their origins, all three are crossed anteriorly by the left brachiocephalic vein.
The first branch
Beginning on the right, the first branch of the arch of the aorta is the brachiocephalic trunk (Fig. 3.90). It is the largest of the three branches and, at its point of origin behind the manubrium of the sternum, is slightly anterior to the other two branches. It ascends slightly posteriorly and to the right. At the level of the upper edge of the right sternoclavicular joint, the brachiocephalic trunk divides into: the right common carotid artery, and the right subclavian artery (see Fig. 3.86).
The arteries mainly supply the right side of the head and neck and the right upper limb, respectively.
Occasionally, the brachiocephalic trunk has a small branch, the thyroid ima artery, which contributes to the vascular supply of the thyroid gland.
The second branch
The second branch of the arch of the aorta is the left common carotid artery (Fig. 3.90). It arises from the arch immediately to the left and slightly posterior to the brachiocephalic trunk and ascends through the superior mediastinum along the left side of the trachea.
The left common carotid artery supplies the left side of the head and neck.
The third branch
The third branch of the arch of the aorta is the left subclavian artery (Fig. 3.90). It arises from the arch of the aorta immediately to the left of, and slightly posterior to, the left common carotid artery and ascends through the superior mediastinum along the left side of the trachea.
The left subclavian artery is the major blood supply to the left upper limb.
The ligamentum arteriosum is also in the superior mediastinum and is important in embryonic circulation, when it is a patent vessel (the ductus arteriosus). It connects the pulmonary trunk with the arch of the aorta and allows blood to bypass the lungs during development (Fig. 3.90). The vessel closes soon after birth and forms the ligamentous connection observed in the adult.
The trachea is a midline structure that is palpable in the jugular notch as it enters the superior mediastinum.
Posterior to it is the esophagus, which is immediately anterior to the vertebral column (Fig. 3.92, and see Figs. 3.86 and 3.87). Significant mobility exists in the vertical positioning of these structures as they pass through the superior mediastinum. Swallowing and breathing cause positional shifts, as may disease and the use of specialized instrumentation.
As the trachea and esophagus pass through the superior mediastinum, they are crossed laterally by the azygos vein on the right side and the arch of the aorta on the left side. | Gray's Anatomy |
The trachea divides into the right and left main bronchi at, or just inferior to, the transverse plane between the sternal angle and vertebral level TIV/V (Fig. 3.93), whereas the esophagus continues into the posterior mediastinum.
Nerves of the superior mediastinum
The vagus nerves [X] pass through the superior and posterior divisions of the mediastinum on their way to the abdominal cavity. As they pass through the thorax, they provide parasympathetic innervation to the thoracic viscera and carry visceral afferents from the thoracic viscera.
Visceral afferents in the vagus nerves relay information to the central nervous system about normal physiological processes and reflex activities. They do not transmit pain sensation.
The right vagus nerve enters the superior mediastinum and lies between the right brachiocephalic vein and the brachiocephalic trunk. It descends in a posterior direction toward the trachea (Fig. 3.94), crosses the lateral surface of the trachea, and passes posteriorly to the root of the right lung to reach the esophagus. Just before the esophagus, it is crossed by the arch of the azygos vein.
As the right vagus nerve passes through the superior mediastinum, it gives branches to the esophagus, cardiac plexus, and pulmonary plexus.
The left vagus nerve enters the superior mediastinum posterior to the left brachiocephalic vein and between the left common carotid and left subclavian arteries (Fig. 3.95). As it passes into the superior mediastinum, it lies just deep to the mediastinal part of the parietal pleura and crosses the left side of the arch of the aorta. It continues to descend in a posterior direction and passes posterior to the root of the left lung to reach the esophagus in the posterior mediastinum.
As the left vagus nerve passes through the superior mediastinum, it gives branches to the esophagus, the cardiac plexus, and the pulmonary plexus.
The left vagus nerve also gives rise to the left recurrent laryngeal nerve, which arises from it at the inferior margin of the arch of the aorta just lateral to the ligamentum arteriosum. The left recurrent laryngeal nerve passes inferior to the arch of the aorta before ascending on its medial surface. Entering a groove between the trachea and esophagus, the left recurrent laryngeal nerve continues superiorly to enter the neck and terminate in the larynx (Fig. 3.96).
The phrenic nerves arise in the cervical region mainly from the fourth, but also from the third and fifth, cervical spinal cord segments.
The phrenic nerves descend through the thorax to supply motor and sensory innervation to the diaphragm and its associated membranes. As they pass through the thorax, they provide innervation through somatic afferent fibers to the mediastinal pleura, fibrous pericardium, and parietal layer of serous pericardium.
The right phrenic nerve enters the superior mediastinum lateral to the right vagus nerve and lateral and slightly posterior to the beginning of the right brachiocephalic vein (see Fig. 3.94). It continues inferiorly along the right side of this vein and the right side of the superior vena cava.
On entering the middle mediastinum, the right phrenic nerve descends along the right side of the pericardial sac, within the fibrous pericardium, anterior to the root of the right lung. The pericardiacophrenic vessels accompany it through most of its course in the thorax (see Fig. 3.60). It leaves the thorax by passing through the diaphragm with the inferior vena cava. | Gray's Anatomy |
The left phrenic nerve enters the superior mediastinum in a position similar to the path taken by the right phrenic nerve. It lies lateral to the left vagus nerve and lateral and slightly posterior to the beginning of the left brachiocephalic vein (see Fig. 3.89), and continues to descend across the left lateral surface of the arch of the aorta, passing superficially to the left vagus nerve and the left superior intercostal vein.
On entering the middle mediastinum, the left phrenic nerve follows the left side of the pericardial sac, within the fibrous pericardium, anterior to the root of the left lung, and is accompanied by the pericardiacophrenic vessels (see Fig. 3.60). It leaves the thorax by piercing the diaphragm near the apex of the heart.
Thoracic duct in the superior mediastinum
The thoracic duct, which is the major lymphatic vessel in the body, passes through the posterior portion of the superior mediastinum (see Figs. 3.87 and 3.92). It: enters the superior mediastinum inferiorly, slightly to the left of the midline, having moved to this position just before leaving the posterior mediastinum opposite vertebral level TIV/V; and continues through the superior mediastinum, posterior to the arch of the aorta, and the initial portion of the left subclavian artery, between the esophagus and the left mediastinal part of the parietal pleura.
The posterior mediastinum is posterior to the pericardial sac and diaphragm and anterior to the bodies of the mid and lower thoracic vertebrae (see Fig. 3.57).
Its superior boundary is a transverse plane passing from the sternal angle to the intervertebral disc between vertebrae TIV and TV.
Its inferior boundary is the diaphragm.
Laterally, it is bordered by the mediastinal part of parietal pleura on either side.
Superiorly, it is continuous with the superior mediastinum.
Major structures in the posterior mediastinum include the: esophagus and its associated nerve plexus, thoracic aorta and its branches, azygos system of veins, thoracic duct and associated lymph nodes, sympathetic trunks, and thoracic splanchnic nerves.
The esophagus is a muscular tube passing between the pharynx in the neck and the stomach in the abdomen. It begins at the inferior border of the cricoid cartilage, opposite vertebra CVI, and ends at the cardiac opening of the stomach, opposite vertebra TXI.
The esophagus descends on the anterior aspect of the bodies of the vertebrae, generally in a midline position as it moves through the thorax (Fig. 3.97). As it approaches the diaphragm, it moves anteriorly and to the left, crossing from the right side of the thoracic aorta to eventually assume a position anterior to it. It then passes through the esophageal hiatus, an opening in the muscular part of the diaphragm, at vertebral level TX.
The esophagus has a slight anterior-to-posterior curvature that parallels the thoracic portion of the vertebral column, and is secured superiorly by its attachment to the pharynx and inferiorly by its attachment to the diaphragm.
Relationships to important structures in the posterior mediastinum
In the posterior mediastinum, the esophagus is related to a number of important structures. The right side is covered by the mediastinal part of the parietal pleura.
Posterior to the esophagus, the thoracic duct is on the right side inferiorly, but crosses to the left more superiorly. Also on the left side of the esophagus is the thoracic aorta.
Anterior to the esophagus, below the level of the tracheal bifurcation, are the right pulmonary artery and the left main bronchus. The esophagus then passes immediately posteriorly to the left atrium, separated from it only by pericardium. Inferior to the left atrium, the esophagus is related to the diaphragm. | Gray's Anatomy |
Structures other than the thoracic duct posterior to the esophagus include portions of the hemiazygos veins, the right posterior intercostal vessels, and, near the diaphragm, the thoracic aorta.
The esophagus is a flexible, muscular tube that can be compressed or narrowed by surrounding structures at four locations (Fig. 3.98): the junction of the esophagus with the pharynx in the neck; in the superior mediastinum where the esophagus is crossed by the arch of the aorta; in the posterior mediastinum where the esophagus is compressed by the left main bronchus; in the posterior mediastinum at the esophageal hiatus in the diaphragm.
These constrictions have important clinical consequences. For example, a swallowed object is most likely to lodge at a constricted area. An ingested corrosive substance would move more slowly through a narrowed region, causing more damage at this site than elsewhere along the esophagus. Also, constrictions present problems during the passage of medical instruments.
The arterial supply and venous drainage of the esophagus in the posterior mediastinum involve many vessels. Esophageal arteries arise from the thoracic aorta, bronchial arteries, and ascending branches of the left gastric artery in the abdomen.
Venous drainage involves small vessels returning to the azygos vein, hemiazygos vein, and esophageal branches to the left gastric vein in the abdomen.
Lymphatic drainage of the esophagus in the posterior mediastinum returns to posterior mediastinal and left gastric nodes.
Innervation of the esophagus, in general, is complex. Esophageal branches arise from the vagus nerves and sympathetic trunks.
Striated muscle fibers in the superior portion of the esophagus originate from the branchial arches and are innervated by branchial efferents from the vagus nerves.
Smooth muscle fibers are innervated by cranial components of the parasympathetic part of the autonomic division of the peripheral nervous system, visceral efferents from the vagus nerves. These are preganglionic fibers that synapse in the myenteric and submucosal plexuses of the enteric nervous system in the esophageal wall.
Sensory innervation of the esophagus involves visceral afferent fibers originating in the vagus nerves, sympathetic trunks, and splanchnic nerves.
The visceral afferents from the vagus nerves are involved in relaying information back to the central nervous system about normal physiological processes and reflex activities. They are not involved in the relay of pain recognition.
The visceral afferents that pass through the sympathetic trunks and the splanchnic nerves are the primary participants in detection of esophageal pain and transmission of this information to various levels of the central nervous system.
After passing posteriorly to the root of the lungs, the right and left vagus nerves approach the esophagus. As they reach the esophagus, each nerve divides into several branches that spread over this structure, forming the esophageal plexus (Fig. 3.99). There is some mixing of fibers from the two vagus nerves as the plexus continues inferiorly on the esophagus toward the diaphragm. Just above the diaphragm, fibers of the plexus converge to form two trunks: the anterior vagal trunk on the anterior surface of the esophagus, mainly from fibers originally in the left vagus nerve; the posterior vagal trunk on the posterior surface of the esophagus, mainly from fibers originally in the right vagus nerve.
The vagal trunks continue on the surface of the esophagus as it passes through the diaphragm into the abdomen. | Gray's Anatomy |
The thoracic portion of the descending aorta (thoracic aorta) begins at the lower edge of vertebra TIV, where it is continuous with the arch of the aorta. It ends anterior to the lower edge of vertebra TXII, where it passes through the aortic hiatus posterior to the diaphragm. Situated to the left of the vertebral column superiorly, it approaches the midline inferiorly, lying directly anterior to the lower thoracic vertebral bodies (Fig. 3.101). Throughout its course, it gives off a number of branches, which are summarized in Table 3.3.
Azygos system of veins
The azygos system of veins consists of a series of longitudinal vessels on each side of the body that drain blood from the body wall and move it superiorly to empty into the superior vena cava. Blood from some of the thoracic viscera may also enter the system, and there are anastomotic connections with abdominal veins.
The longitudinal vessels may or may not be continuous and are connected to each other from side to side at various points throughout their course (Fig. 3.102).
The azygos system of veins serves as an important anastomotic pathway capable of returning venous blood from the lower part of the body to the heart if the inferior vena cava is blocked.
The major veins in the system are: the azygos vein, on the right; and the hemiazygos vein and the accessory hemiazygos vein, on the left.
There is significant variation in the origin, course, tributaries, anastomoses, and termination of these vessels.
The azygos vein arises opposite vertebra LI or LII at the junction between the right ascending lumbar vein and the right subcostal vein (Fig. 3.102). It may also arise as a direct branch of the inferior vena cava, which is joined by a common trunk from the junction of the right ascending lumbar vein and the right subcostal vein.
The azygos vein enters the thorax through the aortic hiatus of the diaphragm, or it enters through or posterior to the right crus of the diaphragm. It ascends through the posterior mediastinum, usually to the right of the thoracic duct. At approximately vertebral level TIV, it arches anteriorly, over the root of the right lung, to join the superior vena cava before the superior vena cava enters the pericardial sac.
Tributaries of the azygos vein include: the right superior intercostal vein (a single vessel formed by the junction of the second, third, and fourth intercostal veins), fifth to eleventh right posterior intercostal veins, the hemiazygos vein, the accessory hemiazygos vein, esophageal veins, mediastinal veins, pericardial veins, and right bronchial veins.
The hemiazygos vein (inferior hemiazygos vein) usually arises at the junction between the left ascending lumbar vein and the left subcostal vein (Fig. 3.102). It may also arise from either of these veins alone and often has a connection to the left renal vein.
The hemiazygos vein usually enters the thorax through the left crus of the diaphragm, but may enter through the aortic hiatus. It ascends through the posterior mediastinum, on the left side, to approximately vertebral level TIX. At this point, it crosses the vertebral column, posterior to the thoracic aorta, esophagus, and thoracic duct, to enter the azygos vein.
Tributaries joining the hemiazygos vein include: the lowest four or five left posterior intercostal veins, esophageal veins, and mediastinal veins. | Gray's Anatomy |
The accessory hemiazygos vein (superior hemiazygos vein) descends on the left side from the superior portion of the posterior mediastinum to approximately vertebral level TVIII (Fig. 3.102). At this point, it crosses the vertebral column to join the azygos vein, or ends in the hemiazygos vein, or has a connection to both veins. Usually, it also has a connection superiorly to the left superior intercostal vein.
Vessels that drain into the accessory hemiazygos vein include: the fourth to eighth left posterior intercostal veins, and sometimes, the left bronchial veins.
Thoracic duct in the posterior mediastinum
The thoracic duct is the principal channel through which lymph from most of the body is returned to the venous system. It begins as a confluence of lymph trunks in the abdomen, sometimes forming a saccular dilation referred to as the cisterna chyli (chyle cistern), which drains the abdominal viscera and walls, pelvis, perineum, and lower limbs.
The thoracic duct extends from vertebra LII to the root of the neck.
Entering the thorax, posterior to the aorta, through the aortic hiatus of the diaphragm, the thoracic duct ascends through the posterior mediastinum to the right of midline between the thoracic aorta on the left and the azygos vein on the right (Fig. 3.103). It lies posterior to the diaphragm and the esophagus and anterior to the bodies of the vertebrae.
At vertebral level TV, the thoracic duct moves to the left of midline and enters the superior mediastinum. It continues through the superior mediastinum and into the neck.
After being joined, in most cases, by the left jugular trunk, which drains the left side of the head and neck, and the left subclavian trunk, which drains the left upper limb, the thoracic duct empties into the junction of the left subclavian and left internal jugular veins.
The thoracic duct usually receives the contents from: the confluence of lymph trunks in the abdomen, descending thoracic lymph trunks draining the lower six or seven intercostal spaces on both sides, upper intercostal lymph trunks draining the upper left five or six intercostal spaces, ducts from posterior mediastinal nodes, and ducts from posterior diaphragmatic nodes.
The sympathetic trunks are an important component of the sympathetic part of the autonomic division of the peripheral nervous system and are usually considered a component of the posterior mediastinum as they pass through the thorax.
This portion of the sympathetic trunks consists of two parallel cords punctuated by 11 or 12 ganglia (Fig. 3.104). The ganglia are connected to adjacent thoracic spinal nerves by white and gray rami communicantes and are numbered according to the thoracic spinal nerve with which they are associated.
In the superior portion of the posterior mediastinum, the trunks are anterior to the neck of the ribs. Inferiorly, they become more medial in position until they lie on the lateral aspect of the vertebral bodies. The sympathetic trunks leave the thorax by passing posterior to the diaphragm under the medial arcuate ligament or through the crura of the diaphragm. Throughout their course the trunks are covered by parietal pleura.
Branches from the ganglia
Two types of medial branches are given off by the ganglia:
The first type includes branches from the upper five ganglia.
The second type includes branches from the lower seven ganglia.
The first type, which includes branches from the upper five ganglia, consists mainly of postganglionic sympathetic fibers, which supply the various thoracic viscera. These branches are relatively small, and also contain visceral afferent fibers. | Gray's Anatomy |
The second type, which includes branches from the lower seven ganglia, consists mainly of preganglionic sympathetic fibers, which supply the various abdominal and pelvic viscera. These branches are large, also carry visceral afferent fibers, and form the three thoracic splanchnic nerves referred to as the greater, lesser, and least splanchnic nerves (Fig. 3.104).
The greater splanchnic nerve on each side usually arises from the fifth to ninth or tenth thoracic ganglia. It descends across the vertebral bodies moving in a medial direction, passes into the abdomen through the crus of the diaphragm, and ends in the celiac ganglion.
The lesser splanchnic nerve usually arises from the ninth and tenth, or tenth and eleventh thoracic ganglia. It descends across the vertebral bodies moving in a medial direction, and passes into the abdomen through the crus of the diaphragm to end in the aorticorenal ganglion.
The least splanchnic nerve (lowest splanchnic nerve) usually arises from the twelfth thoracic ganglion. It descends and passes into the abdomen through the crus of the diaphragm to end in the renal plexus.
The ability to visualize how anatomical structures in the thorax are related to surface features is fundamental to a physical examination. Landmarks on the body’s surface can be used to locate deep structures and to assess function by auscultation and percussion.
How to count ribs
Knowing how to count ribs is important because different ribs provide palpable landmarks for the positions of deeper structures. To determine the location of specific ribs, palpate the jugular notch at the superior extent of the manubrium of the sternum. Move down the sternum until a ridge is felt. This ridge is the sternal angle, which identifies the articulation between the manubrium of the sternum and the body of the sternum. The costal cartilage of rib II articulates with the sternum at this location. Identify rib II. Then continue counting the ribs, moving in a downward and lateral direction (Fig. 3.105).
Surface anatomy of the breast in women
Although breasts vary in size, they are normally positioned on the thoracic wall between ribs II and VI and overlie the pectoralis major muscles. Each mammary gland extends superolaterally around the lower margin of the pectoralis major muscle and enters the axilla (Fig. 3.106). This portion of the gland is the axillary tail or axillary process. The positions of the nipple and areola vary relative to the chest wall depending on breast size.
Visualizing structures at the TIV/V
The TIV/V vertebral level is a transverse plane that passes through the sternal angle on the anterior chest wall and the intervertebral disc between TIV and TV vertebrae posteriorly. This plane can easily be located, because the joint between the manubrium of the sternum and the body of the sternum forms a distinct bony protuberance that can be palpated. At the TIV/V level (Fig. 3.107):
The costal cartilage of rib II articulates with the sternum.
The superior mediastinum is separated from the inferior mediastinum.
The ascending aorta ends and the arch of the aorta begins.
The arch of the aorta ends and the thoracic aorta begins.
The trachea bifurcates.
Visualizing structures in the superior mediastinum
A number of structures in the superior mediastinum in adults can be visualized based on their positions relative to skeletal landmarks that can be palpated through the skin (Fig. 3.108).
On each side, the internal jugular and subclavian veins join to form the brachiocephalic veins behind the sternal ends of the clavicles near the sternoclavicular joints.
The left brachiocephalic vein crosses from left to right behind the manubrium of the sternum.
The brachiocephalic veins unite to form the superior vena cava behind the lower border of the costal cartilage of the right first rib. | Gray's Anatomy |
The arch of the aorta begins and ends at the transverse plane between the sternal angle anteriorly and vertebral level TIV/V posteriorly. The arch may reach as high as the midlevel of the manubrium of the sternum.
Visualizing the margins of the heart
Surface landmarks can be palpated to visualize the outline of the heart (Fig. 3.109).
The upper limit of the heart reaches as high as the third costal cartilage on the right side of the sternum and the second intercostal space on the left side of the sternum.
The right margin of the heart extends from the right third costal cartilage to near the right sixth costal cartilage.
The left margin of the heart descends laterally from the second intercostal space to the apex located near the midclavicular line in the fifth intercostal space.
The lower margin of the heart extends from the sternal end of the right sixth costal cartilage to the apex in the fifth intercostal space near the midclavicular line.
Where to listen for heart sounds
To listen for valve sounds, position the stethoscope downstream from the flow of blood through the valves (Fig. 3.110).
The tricuspid valve is heard just to the left of the lower part of the sternum near the fifth intercostal space.
The mitral valve is heard over the apex of the heart in the left fifth intercostal space at the midclavicular line.
The pulmonary valve is heard over the medial end of the left second intercostal space.
The aortic valve is heard over the medial end of the right second intercostal space.
Visualizing the pleural cavities and lungs, pleural recesses, and lung lobes and fissures
Palpable surface landmarks can be used to visualize the normal outlines of the pleural cavities and the lungs and to determine the positions of the pulmonary lobes and fissures.
Superiorly, the parietal pleura projects above the first costal cartilage. Anteriorly, the costal pleura approaches the midline posterior to the upper portion of the sternum. Posterior to the lower portion of the sternum, the left parietal pleura does not come as close to the midline as it does on the right side. This is because the heart bulges onto the left side (Fig. 3.111A).
Inferiorly, the pleura reflects onto the diaphragm above the costal margin and courses around the thoracic wall following an VIII, X, XII contour (i.e., rib VIII in the midclavicular line, rib X in the midaxillary line, and vertebra TXII posteriorly).
The lungs do not completely fill the area surrounded by the pleural cavities, particularly anteriorly and inferiorly.
Costomediastinal recesses occur anteriorly, particularly on the left side in relationship to the heart bulge.
Costodiaphragmatic recesses occur inferiorly between the lower lung margin and the lower margin of the pleural cavity.
In quiet respiration, the inferior margin of the lungs travels around the thoracic wall following a VI, VIII, X contour (i.e., rib VI in the midclavicular line, rib VIII in the midaxillary line, and vertebra TX posteriorly).
In the posterior view, the oblique fissure on both sides is located in the midline near the spine of vertebra TIV (Figs. 3.111B and 3.112A). It moves laterally in a downward direction, crossing the fourth and fifth intercostal spaces and reaches rib VI laterally.
In the anterior view, the horizontal fissure on the right side follows the contour of rib IV and its costal cartilage and the oblique fissures on both sides follow the contour of rib VI and its costal cartilage (Fig. 3.112B).
Where to listen for lung sounds
The stethoscope placements for listening for lung sounds are shown in Fig. 3.113.
Fig. 3.1 Thoracic wall and cavity. | Gray's Anatomy |
Left pleural cavitySternal angleManubrium of sternumXiphoid processInferior thoracic apertureSuperior thoracic apertureBody of sternumRight pleural cavityDiaphragmVertebral columnRib IRibsMediastinum
Fig. 3.2 Joints between ribs and vertebrae.
Fig. 3.3 Superior thoracic aperture.
Manubriumof sternumEsophagusCommon carotid arteryInternal jugular veinRib ISuperior thoracicapertureApex of right lungSubclavianarteryand veinTracheaVertebra TIRib IITracheaPleauralcavity (lung)VeinsNervesArteriesEsophagus
Fig. 3.4 A. Inferior thoracic aperture. B. Diaphragm.
Rib XIDistal cartilaginousends of ribs VII to X;costal marginsCentraltendonLeft domeRight domeAortichiatusEsophagealhiatusInferior thoracicapertureXiphoid processRib XIIVertebra TXIIAB
Fig. 3.5 Subdivisions of the mediastinum.
Fig. 3.6 Pleural cavities.
TracheaLeft pleural cavitysurrounding left lungApex of right lungMediastinumRight main bronchusRight pleural cavityParietal pleuraVisceral pleuraCostodiaphragmaticrecessDiaphragm
Fig. 3.7 Superior thoracic aperture and axillary inlet.
Fig. 3.8 Major structures passing between abdomen and thorax.
Fig. 3.9 Right breast.
Fourth thoracicintercostal nerveMammary glandsLactiferousductsLactiferoussinusesPectoralis majorDeep (pectoral) fasciaAxillary processAxillarylymph nodesSecond, third, and fourth anteriorperforating branches of internalthoracic arteryInternalthoracic arteryParasternallymph nodesLymphatic vessel
Fig. 3.10 Vertebral level TIV/V.
Fig. 3.11 Left-to-right venous shunts.
Fig. 3.12 A. Segmental neurovascular supply of thoracic wall.
B. Anterior view of thoracic dermatomes associated with thoracic spinal nerves. C. Lateral view of dermatomes associated with thoracic spinal nerves.
Left common carotid arteryInternal thoracic arteriesRight subclavian arteryArch of aortaAnteriorcutaneous branchAnteriorintercostal arteryAPosteriorintercostal arteryLateralcutaneous branchIntercostal nerve
Fig. 3.13 Sympathetic trunks.
Fig. 3.14 Flexible thoracic wall and inferior thoracic aperture.
Sternum moves forwardin inspiration because ofrib elevationElevation of lateral aspectof ribs in inspirationDiaphragm descends toincrease thoracic capacityin inspiration
Fig. 3.15 Innervation of the diaphragm.
Fig. 3.16 Breasts.
Parasternal nodesSuspensory ligamentsLactiferous sinusesLactiferous ductsRetromammary spaceSecretory lobulesPectoralis major muscleMammary branches ofinternal thoracic arteryAxillary processLateral thoracic arteryLateral axillary nodesCentral axillary nodesApical axillary nodesPectoral branch ofthoracoacromial arteryInternal thoracic arteryAreolaSecretorylobulesLymphatic and venous drainagepasses from lateral and superiorpart of the breast into axillaLymphatic and venousdrainage passes from medial partof the breast parasternallySome lymphatic and venous drainagemay pass from inferior part of thebreast into the abdomenPectoral axillary nodes
Fig. 3.17 Muscles and fascia of the pectoral region.
Fig. 3.18 Typical thoracic vertebra. | Gray's Anatomy |
LaminaSpinous processTransverse processPedicleVertebral bodyPosteriorAnteriorVertebral foramenSuperior demifacetFacet for articulation with tubercle of ribSuperiorPosteriorInferiorAnteriorFacet for articulationwith tubercle of ribDemifacets for articulation with head of ribsSuperior articular processInferior articular processSuperior viewSuperolateral view
Fig. 3.19 Atypical thoracic vertebrae.
Vertebra TIVertebra TXVertebra TXISuperior costal facet for head of rib ISingle complete costal facet for head of rib XNo costal facet on transverse process
Fig. 3.20 Ribs.
Fig. 3.21 A typical rib. A. Anterior view. B. Posterior view of proximal end of rib.
Fig. 3.22 Atypical ribs.
Fig. 3.23 Sternum.
Fig. 3.24 Costovertebral joints.
Joint capsuleVertebraVertebraDiscRibJoint cavitiesCostotransverse jointCostotransverse ligamentSuperior costotransverse ligamentLateral costotransverseligamentJoint with vertebral bodyIntra-articular ligamentSuperolateral viewSuperior view
Fig. 3.25 Sternocostal joints.
Fig. 3.26 Intercostal space. A. Anterolateral view. B. Details of an intercostal space and relationships.
Intercostal space. C. Transverse section.
Posterior ramus of spinal nerveLateral branches ofintercostal nerveand vesselsAnterior cutaneousbranch ofintercostal nerve Anterior perforatingbranches ofintercostal vessels Intercostal nerveCostal grooveAortaInternal thoracic arteryand veinPosterior intercostal artery and veinAnterior intercostal artery and veinCollateral branches ofintercostal nerve and vesselsIntercostal veinIntercostal arteryVisceral pleuraParietal pleuraEndothoracic fasciaPleural cavityLungIntercostal nerveCollateral branchesInternal intercostal muscleExternal intercostal muscleSerratus anterior muscleInnermost intercostal muscleSuperficial fasciaSkinAB
Fig. 3.27 Intercostal muscles.
Fig. 3.28 A. Subcostal muscles. B. Transversus thoracis muscles.
Fig. 3.29 Arteries of the thoracic wall.
Superior epigastric arteryInternal thoracic arteryAortaAnterior perforatingbranchesAnterior intercostal arteryCollateral branch of posteriorintercostal arterySupreme intercostal arteryPosterior intercostal arteryMusculophrenic arterySubclavian arteryCostocervical trunk
Fig. 3.30 Veins of the thoracic wall.
Fig. 3.31 Major lymphatic vessels and nodes of the thoracic wall.
Fig. 3.32 Intercostal nerves.
Fig. 3.33 Right thoracotomy for esophageal cancer with intrathoracic large-bore drain. In this case, a neo-esophagus has been fashioned from the stomach.
Fig. 3.34 Diaphragm.
Fig. 3.35 Movement of thoracic wall during breathing. A. Pump handle movement of ribs and sternum. B. Bucket handle movement of ribs.
BuckethandlemovementElevation of lateralshaft of ribSuperior and anteriormovement of sternumBAPump handle
Fig. 3.36 Chest radiograph showing an elevated right hemidiaphragm in a patient with right-sided diaphragmatic paralysis.
Fig. 3.37 Pleural cavities.
Fig. 3.38 Parietal pleura. | Gray's Anatomy |
Costal partPulmonary ligamentMediastinal partPleura surroundingstructures in root of lungCervical pleuraSuprapleural membraneDiaphragmatic part
Fig. 3.39 Pleural reflections.
Fig. 3.40 Parietal pleural reflections and recesses.
Fig. 3.41 CT image of left pleural effusion.
AortaLeft lungRight lungLeft empyema with air-fluid level
Fig. 3.42 Pneumothorax in a patient with extensive subcutaneous emphysema.
Fig. 3.43 Lungs.
Fig. 3.44 Roots and hila of the lungs.
Fig. 3.45 A. Right lung. B. Major structures related to the right lung.
DiaphragmBronchusBronchus to superior lobePulmonary arteryPulmonary veinsRib ISubclavian arterySubclavian veinEsophagusPosteriorAnteriorInferior vena cavaAzygos veinLeft brachiocephalic veinRight brachiocephalic veinHeartInferior lobeSuperior lobeMiddle lobeSuperior vena cavaOblique fissureHorizontal fissureAB
Fig. 3.46 A. Left lung. B. Major structures related to the left lung.
Fig. 3.47 A. Bronchial tree. B. Bronchopulmonary segments.
TracheaLeft main bronchusCarinaRight main bronchusLobar bronchiLobar bronchiSegmental bronchiof middle lobeBranch of pulmonary arteryMedial bronchopulmonary segmentof middle lobe of right lungLateral bronchopulmonary segmentof middle lobe of right lungAB
Fig. 3.48 Bronchopulmonary segments. A. Right lung. B. Left lung. (Bronchopulmonary segments are numbered and named.)
Fig. 3.49 Pulmonary vessels. A. Diagram of an anterior view. B. Axial computed tomography image showing the left pulmonary artery branching from the pulmonary trunk. C. Axial computed tomography image (just inferior to the image in B) showing the right pulmonary artery branching from the pulmonary trunk.
Fig. 3.50 Pulmonary innervation.
Fig. 3.51 Lymphatic drainage of lungs.
Fig. 3.52 HRCT of patient with emphysema.
Fig. 3.53 Bronchoscopic evaluation. A. Of the lower end of the trachea and its main branches. B. Of tracheal bifurcation showing a tumor at the carina.
Fig. 3.54 Imaging of the lungs. A. Standard posteroanterior view of the chest showing tumor in upper right lung. B. Axial CT image of lungs showing tumor in right lung. C. Coronal CT image of lungs showing tumor in left lung extending into mediastinum. D. Radionuclide study using FDG PET showing a tumor in the right lung.
Fig. 3.55 Cross-section of the thorax showing the position of the mediastinum.
Fig. 3.56 Lateral view of the mediastinum.
Fig. 3.57 Subdivisions of the mediastinum.
Fig. 3.58 Thymus.
Fig. 3.59 Sagittal section of the pericardium.
FibrouspericardiumParietal layerof serouspericardiumVisceral layerof serouspericardium(epicardium)PericardialcavityJunction between fibrous pericardium and adventitia of great vessels
Fig. 3.60 Phrenic nerves and pericardiacophrenic vessels.
Fig. 3.61 Posterior portion of pericardial sac showing reflections of serous pericardium. | Gray's Anatomy |
Arch of aortaSuperior vena cavaInferior vena cavaBranch of rightpulmonary arteryAscending aortaThoracic aortaCut edge of pericardiumRight pulmonary veinsLeft pulmonary arteryLeft pulmonary veinsOblique pericardial sinus(formed by reflection onto thepulmonary veins of heart)Transverse pericardial sinus(separates arteries from veins)
Fig. 3.62 Coronal CT showing pericardial effusion.
Fig. 3.63 Schematic illustration of the heart showing orientation, surfaces, and margins.
Fig. 3.64 Base of the heart.
Arch of aortaSuperior vena cavaInferior vena cavaRight pulmonary arterySulcus terminalisRight pulmonary veinsRight atriumRight ventricleApexLeft atriumLeft inferior pulmonary veinLeft superior pulmonary veinLeft ventricleCoronary sinusLeft pulmonary artery
Fig. 3.65 Anterior surface of the heart.
RALABicuspidvalveTricuspid valveAortic valvePulmonary trunkPulmonary valveAortaRVLVArch of aortaSuperior vena cavaInferior vena cavaRight coronaryarteryRight atriumRight ventricleAscending aortaInferior marginApexSmall cardiac veinPulmonary trunkLeft auricleLeft ventricleObtuse marginAnterior interventricular grooveGreat cardiac veinAnterior interventricular branch of left coronary artery
Fig. 3.66 Diaphragmatic surface of the heart.
Arch of aortaSuperior vena cavaInferior vena cavaRight pulmonary arteryRight pulmonary veinsRight atriumRight ventricleMiddle cardiac veinMarginal branch ofright coronary arteryApexLeft atriumLeft pulmonary veinsLeft ventricleCoronary sinusLeft pulmonary arteryPosterior interventricular groovePosterior interventricularbranch of right coronaryartery
Fig. 3.67 Chest radiographs. A. Standard posteroanterior view of the chest. B. Standard lateral view of the heart.
Right ventricleLeft atrium Right atriumArch of aortaInferior vena cavaABPulmonary trunkLeft auricleLeft ventricle Superior vena cava Apex of heart
Fig. 3.68 Sulci of the heart. A. Anterior surface of the heart. B. Diaphragmatic surface and base of the heart.
Fig. 3.69 A. The heart has two pumps. B. Magnetic resonance image of midthorax showing all four chambers and septa.
Fig. 3.70 Internal view of right atrium.
Arch of aortaSuperior vena cavaInferior vena cavaLimbus of fossa ovalisCrista terminalisMusculi pectinatiRight ventricleFossa ovalisValve of coronary sinusOpening of coronary sinusRight auricleValve of inferior vena cava
Fig. 3.71 Internal view of the right ventricle.
Arch of aortaSuperior vena cavaTricuspidvalveRight auricleRight atriumAnterior cuspSeptal cuspPosterior cuspPosterior papillary muscleTrabeculae carneaeAnterior papillary muscleChordae tendineaePulmonary trunkPulmonaryvalveLeft auricleSeptal papillary muscleSeptomarginal trabeculaAnterior semilunar cuspRight semilunar cuspLeft semilunar cuspConus arteriosus
Fig. 3.72 Posterior view of the pulmonary valve.
Fig. 3.73 Left atrium. A. Internal view. B. Axial computed tomography image showing the pulmonary veins entering the left atrium. | Gray's Anatomy |