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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.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). |
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. |
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): |
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 CNSFunctionally, 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 systemThe 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. |
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: |
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 systemThe 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. |