Sacrum - Anatomy Pictures and Information Home > Skeletal System > Spine > Sacrum Sacrum The sacrum is a large wedge shaped vertebra at the inferior end of the spine. It forms the solid base of the spinal column where it intersects with the hip bones to form the pelvis. The sacrum is a very strong bone that supports the weight of the upper body as it is spread across the pelvis and into the legs. Developmentally, the sacrum forms from five individual vertebrae that start to join during late adolescence and early adulthood to form a single bone by around the age of thirty.... Move up/down/left/right: Click compass arrows Rotate image: Click and drag in any direction, anywhere in the frame Identify objects: Click on them in the image 2D Interactive 3D Rotate & Zoom Change Anatomical System Change View Angle Full Sacrum Description [Continued from above] . . . A ridge of tubercles along the posterior surface of the sacrum represents the spinous processes of these fused bones. At its wide superior end, the sacrum forms the fibrocartilaginous lumbosacral joint with the fifth lumbar vertebra above it.

The sacrum tapers to a point at its inferior end, where it forms the fibrocartilaginous sacrococcygeal joint with the tiny coccyx (tail bone). On the left and right lateral sides the sacrum forms the sacroiliac joints with the ilium of the hip bones to form the rigid pelvis. Many ligaments bind the sacroiliac joints together tightly to reduce motion and solidify the pelvis. Along its anterior surface the sacrum is concave to provide a larger space within the pelvic cavity. The female sacrum is shorter, wider, and curved more posteriorly than the male sacrum to provide more room for the passage of the fetus through the birth canal during childbirth. Many nerves of the cauda equina at the inferior end of the spinal cord pass through the sacrum. These nerves enter the sacrum from the vertebral foramen of the lumbar vertebrae through the tunnel-like sacral canal. From the sacral canal these nerves branch out and exit the sacrum through four pairs of holes on the sides of the canal called the sacral foramina or through the sacral hiatus at the inferior end of the canal.

The sacrum serves several important functions in the skeletal, muscular, nervous, and female reproductive systems. Acting as the keystone of the pelvis, the sacrum locks the hip bones together on the posterior side and supports the base of the spinal column as it intersects with the pelvis. Several key muscles of the hip joint, including the gluteus maximus, iliacus and piriformis, have their origins on the surface of the sacrum and pull on the sacrum to move the leg. The sacrum also surrounds and protects the spinal nerves of the lower back as they wind their way inferiorly toward the end of the trunk and into the legs. Finally, the sacrum helps to form the pelvic cavity that supports and protects the delicate organs of the abdominopelvic cavity and provides space for a fetus to pass through during childbirth. Prepared by Tim Taylor, Anatomy and Physiology Instructor The Sacrum and Coccyx The Sacrum and Coccyx Print The Human Skeleton is the body's framework or scaffolding system. Skeletal bones are classified as long, short, flat, or irregular and vary in length, width, and depth. The bones in the spine are irregular in shape and provide places to connect to other bones.

The function of the skeletal system is to support the body against the force of gravity, protect soft body parts, produce red blood cells, store inorganic calcium, and phosphorus salts, and to provide sites for muscle attachment to enable body movement. The Spinal Column is also called the vertebral column. The bones in the spine are called vertebrae (ver-ta-bray). The column starts at the base of the skull and continues to the pelvis. Alternate layers of bone (vertebrae) and cartilage (car-til-ledge, the intervertebral discs) stack vertically one on top of the other in the spinal column. The lattice-like structure of the cancellous bone (cancel-lus, the spongy interior) in a vertebra absorbs external pressure. Five Cervical vertebrae follow below the atlas and axis. The Thoracic spine is located in the chest area and contains 12 vertebrae. The ribs connect to the thoracic spine and protect many vital organs. Next is the Lumbar spine. Most people have five lumbar vertebrae although it is not unusual to have six.

The lumbar vertebrae are larger than the cervical or thoracic as this spinal region carries most of the body's weight. The sacrum and coccyx are uniquely shaped. Medical professionals often abbreviate the levels (vertebrae) of the spinal column. For example, the seven cervical vertebrae are C1, C2, C3, C4, C5, C6 and C7. The thoracic levels are T1, T2, and T3 through T12. Similarly the lumbar levels are L1 through L5 (or L6). The sacrum is simply S1. The coccyx is not abbreviated or numbered. With the exception of the atlas, axis, sacrum and coccyx, each cervical, thoracic, and lumbar vertebra is similarly shaped. Spinal Column Sacrum Anatomy, Area & Definition | Body Maps Your message has been sent. OK We're sorry, an error occurred. We are unable to collect your feedback at this time. However, your feedback is important to us. Please try again later. Close Sacrum The sacrum is the name of the bone located at the base of the spine that consists of five fused vertebrae.

These vertebrae are unfused in children, but by the early to mid twenties, they will be fused together. It is triangular in shape and connects the final lumbar vertebra with the coccyx, which is commonly referred to as the tailbone. The sacrum is curved, which allows more room in the pelvic cavity for various organs. The sacrum is a little unusual in that it is differently shaped in males and females, which is known as sexual dimorphism. In males, it is narrower and longer. The lower half is at a smaller angle than in females. As a result, the male pelvic cavity is generally narrower. Females have a wider pelvic cavity in order to allow for pregnancy and childbirth, as well as to house the reproductive organs. Occasionally, the sacrum may consist of four pieces, rather than the usual five. This is due to the lack of fusion of the first and second sacral vertebrae. In addition, the angle at which the sacrum curves can vary widely from person to person (even of the same sex). Sacrum Sacrum Science Main Index The sacrum is a large, triangular bone at the base of the spine. It is part of the pelvic girdle connecting the two hip bones.

Read on to see pictures of the sacrum and to learn more about why you need a sacrum. On this page: Top of Page Sacrum - Anterior or Front View The name sacrum is based on the Latin word sacer or "sacred." The sacrum was consider a sacred bone because it protected the reproductive organs. As can be seen in the figures on the right, the sacrum is somewhat triangular in shape. The upper part of the sacrum connects with the last lumbar vertebra at the base of the spine . On the left and right, the sacrum connects with the two hip bones of the pelvis . At the bottom of the sacrum is the coccyx or tailbone. A primary role of the sacrum is in forming the back or posterior wall of the pelvis where it strengthens and stabilizes the pelvis. Close Up of Sacrum - Anterior or Front View The central part of the sacrum is curved outward towards the posterior or rear. This allows for more room in the pelvic cavity. The sacrum is a slightly different shape in males and females. The female sacrum is shorter and wider than the male sacrum. Your Bones Your Bones Tus huesos Think back to last Halloween for a minute.

Wherever you looked, there were vampires, ghosts, or bony skeletons grinning back at you. Vampires and ghosts don't really exist, but skeletons sure do! Every single person has a skeleton made up of many bones. These bones give your body structure, let you move in many ways, protect your internal organs, and more. It's time to look at all your bones — the adult human body has 206 of them! What Are Bones Made Of? If you've ever seen a real skeleton or fossil in a museum, you might think that all bones are dead. Although bones in museums are dry, hard, or crumbly, the bones in your body are different. The bones that make up your skeleton are all very much alive, growing and changing all the time like other parts of your body. Almost every bone in your body is made of the same materials: The outer surface of bone is called the periosteum (say: pare-ee-OSS-tee-um). It's a thin, dense membrane that contains nerves and blood vessels that nourish the bone. The next layer is made up of compact bone. This part is smooth and very hard. It's the part you see when you look at a skeleton.

Within the compact bone are many layers of cancellous (say: KAN-sell-us) bone, which looks a bit like a sponge. Cancellous bone is not quite as hard as compact bone, but it is still very strong. In many bones, the cancellous bone protects the innermost part of the bone, the bone marrow (say: MAIR-oh). Bone marrow is sort of like a thick jelly, and its job is to make blood cells. continue How Bones Grow When you were a baby, you had tiny hands, tiny feet, and tiny everything! Slowly, as you grew older, everything became a bit bigger, including your bones. A baby's body has about 300 bones at birth. These eventually fuse (grow together) to form the 206 bones that adults have. Some of a baby's bones are made entirely of a special material called cartilage (say: KAR-tel-ij). Other bones in a baby are partly made of cartilage. This cartilage is soft and flexible. During childhood, as you are growing, the cartilage grows and is slowly replaced by bone, with help from calcium. By the time you are about 25, this process will be complete.

After this happens, there can be no more growth — the bones are as big as they will ever be. All of these bones make up a skeleton that is both very strong and very light. Your Spine Your spine is one part of the skeleton that's easy to check out: Reach around to the center of your back and you'll feel its bumps under your fingers. The spine lets you twist and bend, and it holds your body upright. It also protects the spinal cord, a large bundle of nerves that sends information from your brain to the rest of your body. The spine is special because it isn't made of one or even two bones: It's made of 26 bones in all! These bones are called vertebrae (say: VER-tuh-bray) and each one is shaped like a ring. There are different types of vertebrae in the spine and each does a different kind of job: The first seven vertebrae at the top are called the cervical (say: SIR-vih-kul) vertebrae. These bones are in the back of your neck, just below your brain, and they support your head and neck.

Your head is pretty heavy, so it's lucky to have help from the cervical vertebrae! Below the cervical vertebrae are the thoracic (say: thuh-RAS-ik) vertebrae, and there are 12 in all. These guys anchor your ribs in place. Below the thoracic vertebrae are five lumbar (say: LUM-bar) vertebrae. Beneath the lumbar vertebrae is the sacrum (say: SAY-krum), which is made up of five vertebrae that are fused together to form one single bone. Finally, all the way at the bottom of the spine is the coccyx (say: COK-siks), which is one bone made of four fused vertebrae. The bottom sections of the spine are important when it comes to bearing weight and giving you a good center of gravity. So when you pick up a heavy backpack, the lumbar vertebrae, sacrum, and coccyx give you the power. When you dance, skip, and even walk, these parts help keep you balanced. In between each vertebra (the name for just one of the vertebrae) are small disks made of cartilage.

These disks keep the vertebrae from rubbing against one another, and they also act as your spine's natural shock absorbers. When you jump in the air, or twist while slamming a dunk, the disks give your vertebrae the cushioning they need. previous continue Your Ribs Your heart, lungs, and liver are all very important, and luckily you've got ribs to keep them safe. Ribs act like a cage of bones around your chest. It's easy to feel the bottom of this cage by running your fingers along the sides and front of your body, a few inches below your heart . If you breathe in deeply, you can easily feel your ribs right in the front of your body, too. Some thin kids can even see a few of their ribs right through their skin. Your ribs come in pairs, and the left and right sides of each pair are exactly the same. Most people have 12 pairs of ribs, but some people are born with one or more extra ribs, and some people might have one pair less. All 12 pairs of ribs attach in the back to the spine, where they are held in place by the thoracic vertebrae.

The first seven pairs of ribs attach in the front to the sternum (say: STUR-num), a strong bone in the center of your chest that holds those ribs in place. The remaining sets of ribs don't attach to the sternum directly. The next three pairs are held on with cartilage to the ribs above them. The very last two sets of ribs are called floating ribs because they aren't connected to the sternum or the ribs above them. But don't worry, these ribs can't ever float away. Like the rest of the ribs, they are securely attached to the spine in the back. Your Skull Your skull protects the most important part of all, the brain. You can feel your skull by pushing on your head, especially in the back a few inches above your neck. The skull is actually made up of different bones. Some of these bones protect your brain, whereas others make up the structure of your face. If you touch beneath your eyes, you can feel the ridge of the bone that forms the hole where your eye sits. And although you can't see it, the smallest bone in your whole body is in your head, too.

The stirrup bone behind your eardrum is only .1 to .13 inches (2.5 to 3.3 millimeters) long! Want to know something else? Your lower jawbone is the only bone in your head you can move. It opens and closes to let you talk and chew food. Your skull is pretty cool, but it's changed since you were a baby. All babies are born with spaces between the bones in their skulls. This allows the bones to move, close up, and even overlap as the baby goes through the birth canal. As the baby grows, the space between the bones slowly closes up and disappears, and special joints called sutures (say: SOO-churs) connect the bones. previous continue Your Hands As you sit and type at the keyboard, while you swing on a swing, even when you pick up your lunch, you're using the bones in your fingers, hand, wrist, and arm. Each arm is attached to a shoulder blade or scapula (say: SKA-pyuh-luh), a large triangular bone on the upper back corner of each side of the ribcage.

The arm is made up of three bones: the humerus (say: HYOO-muh-rus), which is above your elbow, and the radius (say: RAY-dee-us) and ulna (say: UL-nuh), which are below the elbow. Each of these bones is wider at the ends and skinnier in the middle, to help give it strength where it meets another bone. At the end of the radius and ulna are eight smaller bones that make up your wrist. Although these bones are small, they can really move! Twist your wrist around or wave and you'll see how the wrist can move. The center part of your hand is made up of five separate bones. Each finger on your hand has three bones, except for your thumb, which has two. So between your wrists, hands, and all your fingers, you've got a grand total of 54 bones — all ready to help you grasp things, write your name, pick up the phone, or throw a softball! Your Legs Sure, your arm, wrist, hand, and finger bones are great for picking up the phone, but how are you supposed to run to answer it? Well, with the bones of the legs and feet!

Your legs are attached to a circular group of bones called your pelvis. The pelvis is a bowl-shaped structure that supports the spine. It is made up of the two large hip bones in front, and behind are the sacrum and the coccyx. The pelvis acts as a tough ring of protection around parts of the digestive system, parts of the urinary system, and parts of the reproductive system. Your leg bones are very large and strong to help support the weight of your body. The bone that goes from your pelvis to your knee is called the femur (say: FEE-mur), and it's the longest bone in your body. At the knee, there's a triangular-shaped bone called the patella (say: puh-TEL-luh), or kneecap, that protects the knee joint. Below the knee are two other leg bones: the tibia (say: TIH-bee-uh) and the fibula (say: FIH-byuh-luh). Just like the three bones in the arm, the three bones in the leg are wider at the ends than in the middle to give them strength.

The ankle is a bit different from the wrist; it is where the lower leg bones connect to a large bone in the foot called the talus (say: TAL-iss). Next to the talus are six other bones. But the main part of the foot is similar to the hand, with five bones. Each toe has three tiny bones, except for your big toe, which has just two. This brings the bone total in both feet and ankles to 52! Most people don't use their toes and feet for grabbing stuff or writing, but they do use them for two very important things: standing and walking. Without all the bones of the foot working together, it would be impossible to balance properly. The bones in the feet are arranged so the foot is almost flat and a bit wide, to help you stay upright. So the next time you're walking, be sure to look down and thank those toes! Your Joints The place where two bones meet is called a joint. Some joints move and others don't. Fixed joints are fixed in place and don't move at all. Your skull has some of these joints (called sutures, remember?), which close up the bones of the skull in a young person's head.

One of these joints is called the parieto-temporal (say: par-EYE-ih-toh TEM-puh-rul) suture — it's the one that runs along the side of the skull. Moving joints are the ones that let you ride your bike, eat cereal, and play a video game — the ones that allow you to twist, bend, and move different parts of your body. Some moving joints, like the ones in your spine, move only a little. Other joints move a lot. One of the main types of moving joints is called a hinge joint. Your elbows and knees each have hinge joints, which let you bend and then straighten your arms and legs. These joints are like the hinges on a door. Just as most doors can only open one way, you can only bend your arms and legs in one direction. You also have many smaller hinge joints in your fingers and toes. Another important type of moving joint is the ball and socket joint. You can find these joints at your shoulders and hips. They are made up of the round end of one bone fitting into a small cup-like area of another bone. Ball and socket joints allow for lots of movement in every direction.

Make sure you've got lots of room, and try swinging your arms all over the place. Have you ever seen someone put oil on a hinge to make it work easier or stop squeaking? Well, your joints come with their own special fluid called synovial fluid (say: SIH-no-vee-ul) that helps them move freely. Bones are held together at the joints by ligaments (say: LIH-guh-mints), which are like very strong rubber bands. Taking Care of Bones Your bones help you out every day so make sure you take care of them. Here are some tips: Protect those skull bones (and your brain inside!) by wearing a helmet for bike riding and other sports. When you use a skateboard, in-line skates, or a scooter, be sure to add wrist supports and elbow and knee pads. Your bones in these places will thank you if you have a fall! If you play sports like football, soccer, lacrosse, or ice hockey, always wear all the right equipment. And never play on a trampoline. Many kids end up with broken bones from jumping on them. Broken bones can eventually heal, but it takes a long time and isn't much fun while you wait.

Strengthen your skeleton by drinking milk and eating other dairy products (like low-fat cheese or frozen yogurt). They all contain calcium, which helps bones harden and become strong. Be active! Another way to strengthen your bones is through exercise like running, jumping, dancing, and playing sports. Take these steps to be good to your bones, and they will treat you right! II. Osteology. 3a. 4. The Sacral and Coccygeal Vertebræ. Gray, Henry. 1918. Anatomy of the Human Body. Henry Gray (18211865). Anatomy of the Human Body. 1918. 3a. 4. The Sacral and Coccygeal Vertebræ The sacral and coccygeal vertebræ consist at an early period of life of nine separate segments which are united in the adult, so as to form two bones, five entering into the formation of the sacrum, four into that of the coccyx. Sometimes the coccyx consists of five bones; occasionally the number is reduced to three.

1 The Sacrum (os sacrum).The sacrum is a large, triangular bone, situated in the lower part of the vertebral column and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones; its upper part or base articulates with the last lumbar vertebra, its apex with the coccyx. It is curved upon itself and placed very obliquely, its base projecting forward and forming the prominent sacrovertebral angle when articulated with the last lumbar vertebra; its central part is projected backward, so as to give increased capacity to the pelvic cavity. 2 Pelvic Surface (facies pelvina).The pelvic surface (Fig. 95) is concave from above downward, and slightly so from side to side. Its middle part is crossed by four transverse ridges, the positions of which correspond with the original planes of separation between the five segments of the bone. The portions of bone intervening between the ridges are the bodies of the sacral vertebræ.

The body of the first segment is of large size, and in form resembles that of a lumbar vertebra; the succeeding ones diminish from above downward, are flattened from before backward, and curved so as to accommodate themselves to the form of the sacrum, being concave in front, convex behind. At the ends of the ridges are seen the anterior sacral foramina, four in number on either side, somewhat rounded in form, diminishing in size from above downward, and directed lateralward and forward; they give exit to the anterior divisions of the sacral nerves and entrance to the lateral sacral arteries. Lateral to these foramina are the lateral parts of the sacrum, each consisting of five separate segments at an early period of life; in the adult, these are blended with the bodies and with each other. Each lateral part is traversed by four broad, shallow grooves, which lodge the anterior divisions of the sacral nerves, and are separated by prominent ridges of bone which give origin to the Piriformis muscle. 3 If a sagittal section be made through the center of the sacrum (Fig.

99), the bodies are seen to be united at their circumferences by bone, wide intervals being left centrally, which, in the fresh state, are filled by the intervertebral fibrocartilages. In some bones this union is more complete between the lower than the upper segments. FIG. 95 Sacrum, pelvic surface. ( See enlarged image ) Dorsal Surface (facies dorsalis).The dorsal surface (Fig. 96) is convex and narrower than the pelvic. In the middle line it displays a crest, the middle sacral crest, surmounted by three or four tubercles, the rudimentary spinous processes of the upper three or four sacral vertebræ. On either side of the middle sacral crest is a shallow groove, the sacral groove, which gives origin to the Multifidus, the floor of the groove being formed by the united laminæ of the corresponding vertebræ. The laminæ of the fifth sacral vertebra, and sometimes those of the fourth, fail to meet behind, and thus a hiatus or deficiency occurs in the posterior wall of the sacral canal.

On the lateral aspect of the sacral groove is a linear series of tubercles produced by the fusion of the articular processes which together form the indistinct sacral articular crests. The articular processes of the first sacral vertebra are large and oval in shape; their facets are concave from side to side, look backward and medialward, and articulate with the facets on the inferior processes of the fifth lumbar vertebra. The tubercles which represent the inferior articular processes of the fifth sacral vertebra are prolonged downward as rounded processes, which are named the sacral cornua, and are connected to the cornua of the coccyx. Lateral to the articular processes are the four posterior sacral foramina; they are smaller in size and less regular in form than the anterior, and transmit the posterior divisions of the sacral nerves. On the lateral side of the posterior sacral foramina is a series of tubercles, which represent the transverse processes of the sacral vertebræ, and form the lateral crests of the sacrum.

The transverse tubercles of the first sacral vertebra are large and very distinct; they, together with the transverse tubercles of the second vertebra, give attachment to the horizontal parts of the posterior sacroiliac ligaments; those of the third vertebra give attachment to the oblique fasciculi of the posterior sacroiliac ligaments; and those of the fourth and fifth to the sacrotuberous ligaments. FIG. 96 Sacrum, dorsal surface. ( See enlarged image ) Lateral Surface.The lateral surface is broad above, but narrowed into a thin edge below. The upper half presents in front an ear-shaped surface, the auricular surface, covered with cartilage in the fresh state, for articulation with the ilium. Behind it is a rough surface, the sacral tuberosity, on which are three deep and uneven impressions, for the attachment of the posterior sacroiliac ligament.

The lower half is thin, and ends in a projection called the inferior lateral angle; medial to this angle is a notch, which is converted into a foramen by the transverse process of the first piece of the coccyx, and transmits the anterior division of the fifth sacral nerve. The thin lower half of the lateral surface gives attachment to the sacrotuberous and sacrospinous ligaments, to some fibers of the Glutæus maximus behind, and to the Coccygeus in front. FIG. 97 Lateral surfaces of sacrum and coccyx. ( See enlarged image ) FIG. 98 Base of sacrum. ( See enlarged image ) Base (basis oss. sacri).The base of the sacrum, which is broad and expanded, is directed upward and forward. In the middle is a large oval articular surface, the upper surface of the body of the first sacral vertebra, which is connected with the under surface of the body of the last lumbar vertebra by an intervertebral fibrocartilage.

Behind this is the large triangular orifice of the sacral canal, which is completed by the laminæ and spinous process of the first sacral vertebra. The superior articular processes project from it on either side; they are oval, concave, directed backward and medialward, like the superior articular processes of a lumbar vertebra. They are attached to the body of the first sacral vertebra and to the alæ by short thick pedicles; on the upper surface of each pedicle is a vertebral notch, which forms the lower part of the foramen between the last lumbar and first sacral vertebræ. On either side of the body is a large triangular surface, which supports the Psoas major and the lumbosacral trunk, and in the articulated pelvis is continuous with the iliac fossa. This is called the ala; it is slightly concave from side to side, convex from before backward, and gives attachment to a few of the fibers of the Iliacus. The posterior fourth of the ala represents the transverse process, and its anterior three-fourths the costal process of the first sacral segment. FIG. 99 Median sagittal section of the sacrum.

( See enlarged image ) FIG. 100 Coccyx. ( See enlarged image ) Apex (apex oss. sacri).The apex is directed downward, and presents an oval facet for articulation with the coccyx. 8 Vertebral Canal (canalis sacralis; sacral canal).The vertebral canal (Fig. 99) runs throughout the greater part of the bone; above, it is triangular in form; below, its posterior wall is incomplete, from the non-development of the laminæ and spinous processes. It lodges the sacral nerves, and its walls are perforated by the anterior and posterior sacral foramina through which these nerves pass out. Articulations.The sacrum articulates with four bones; the last lumbar vertebra above, the coccyx below, and the hip bone on either side. 11 Differences in the Sacrum of the Male and Female.In the female the sacrum is shorter and wider than in the male; the lower half forms a greater angle with the upper; the upper half is nearly straight, the lower half presenting the greatest amount of curvature.

The bone is also directed more obliquely backward; this increases the size of the pelvic cavity and renders the sacrovertebral angle more prominent. In the male the curvature is more evenly distributed over the whole length of the bone, and is altogether greater than in the female. 12 Variations.The sacrum, in some cases, consists of six pieces; occasionally the number is reduced to four. The bodies of the first and second vertebræ may fail to unite. Sometimes the uppermost transverse tubercles are not joined to the rest of the ala on one or both sides, or the sacral canal may be open throughout a considerable part of its length, in consequence of the imperfect development of the laminæ and spinous processes. The sacrum, also, varies considerably with respect to its degree of curvature. 13 The Coccyx (os coccygis).The coccyx (Fig. 100) is usually formed of four rudimentary vertebræ; the number may however be increased to five or diminished to three.

In each of the first three segments may be traced a rudimentary body and articular and transverse processes; the last piece (sometimes the third) is a mere nodule of bone. All the segments are destitute of pedicles, laminæ, and spinous processes. The first is the largest; it resembles the lowest sacral vertebra, and often exists as a separate piece; the last three diminish in size from above downward, and are usually fused with one another. 14 Surfaces.The anterior surface is slightly concave, and marked with three transverse grooves which indicate the junctions of the different segments. It gives attachment to the anterior sacrococcygeal ligament and the Levatores ani, and supports part of the rectum. The posterior surface is convex, marked by transverse grooves similar to those on the anterior surface, and presents on either side a linear row of tubercles, the rudimentary articular processes of the coccygeal vertebræ.

Of these, the superior pair are large, and are called the coccygeal cornua; they project upward, and articulate with the cornua of the sacrum, and on either side complete the foramen for the transmission of the posterior division of the fifth sacral nerve. 15 Borders.The lateral borders are thin, and exhibit a series of small eminences, which represent the transverse processes of the coccygeal vertebræ. Of these, the first is the largest; it is flattened from before backward, and often ascends to join the lower part of the thin lateral edge of the sacrum, thus completing the foramen for the transmission of the anterior division of the fifth sacral nerve; the others diminish in size from above downward, and are often wanting. The borders of the coccyx are narrow, and give attachment on either side to the sacrotuberous and sacrospinous ligaments, to the Coccygeus in front of the ligaments, and to the Glutæus maximus behind them. 17 Apex.The apex is rounded, and has attached to it the tendon of the Sphincter ani externus.

It may be bifid, and is sometimes deflected to one or other side. 18 Ossification of the Vertebral Column.Each cartilaginous vertebra is ossified from three primary centers (Fig. 101), two for the vertebral arch and one for the body. 16 Ossification of the vertebral arches begins in the upper cervical vertebræ about the seventh or eighth week of fetal life, and gradually extends down the column. The ossific granules first appear in the situations where the transverse processes afterward project, and spread backward to the spinous process forward into the pedicles, and lateralward into the transverse and articular processes. Ossification of the bodies begins about the eighth week in the lower thoracic region, and subsequently extends upward and downward along the column. The center for the body does not give rise to the whole of the body of the adult vertebra, the postero-lateral portions of which are ossified by extensions from the vertebral arch centers.

The body of the vertebra during the first few years of life shows, therefore, two synchondroses, neurocentral synchondroses, traversing it along the planes of junction of the three centers (Fig. 102 ). In the thoracic region, the facets for the heads of the ribs lie behind the neurocentral synchondroses and are ossified from the centers for the vertebral arch. At birth the vertebra consists of three pieces, the body and the halves of the vertebral arch. During the first year the halves of the arch unite behind, union taking place first in the lumbar region and then extending upward through the thoracic and cervical regions. About the third year the bodies of the upper cervical vertebræ are joined to the arches on either side; in the lower lumbar vertebræ the union is not completed until the sixth year. Before puberty, no other changes occur, excepting a gradual increase of these primary centers, the upper and under surfaces of the bodies and the ends of the transverse and spinous processes being cartilaginous. About the sixteenth year (Fig.

102), five secondary centers appear, one for the tip of each transverse process, one for the extremity of the spinous process, one for the upper and one for the lower surface of the body (Fig. 103 ). These fuse with the rest of the bone about the age of twenty-five. FIG. 101 Ossification of a vertebra ( See enlarged image ) FIG. 102 No caption. ( See enlarged image ) FIG. 103 No caption. ( See enlarged image ) FIG. 104 Atlas. ( See enlarged image ) FIG. 105 Axis. ( See enlarged image ) FIG. 106 Lumbar vertebra. ( See enlarged image ) FIG. 107 No caption. ( See enlarged image ) FIG. 108 No caption. ( See enlarged image ) FIG. 109 Ossification of the sacrum. ( See enlarged image ) Exceptions to this mode of development occur in the first, second, and seventh cervical vertebræ, and in the lumbar vertebræ. 20 Atlas.The atlas is usually ossified from three centers (Fig.

104 ). Of these, one appears in each lateral mass about the seventh week of fetal life, and extends backward; at birth, these portions of bone are separated from one another behind by a narrow interval filled with cartilage. Between the third and fourth years they unite either directly or through the medium of a separate center developed in the cartilage. At birth, the anterior arch consists of cartilage; in this a separate center appears about the end of the first year after birth, and joins the lateral masses from the sixth to the eighth yearthe lines of union extending across the anterior portions of the superior articular facets. Occasionally there is no separate center, the anterior arch being formed by the forward extension and ultimate junction of the two lateral masses; sometimes this arch is ossified from two centers, one on either side of the middle line. 21 Epistropheus or Axis.The axis is ossified from five primary and two secondary centers (Fig. 105 ). The body and vertebral arch are ossified in the same manner as the corresponding parts in the other vertebræ, viz., one center for the body, and two for the vertebral arch.

The centers for the arch appear about the seventh or eighth week of fetal life, that for the body about the fourth or fifth month. The dens or odontoid process consists originally of a continuation upward of the cartilaginous mass, in which the lower part of the body is formed. About the sixth month of fetal life, two centers make their appearance in the base of this process: they are placed laterally, and join before birth to form a conical bilobed mass deeply cleft above; the interval between the sides of the cleft and the summit of the process is formed by a wedge-shaped piece of cartilage. The base of the process is separated from the body by a cartilaginous disk, which gradually becomes ossified at its circumference, but remains cartilaginous in its center until advanced age. In this cartilage, rudiments of the lower epiphysial lamella of the atlas and the upper epiphysial lamella of the axis may sometimes be found. The apex of the odontoid process has a separate center which appears in the second and joins about the twelfth year; this is the upper epiphysial lamella of the atlas.

In addition to these there is a secondary center for a thin epiphysial plate on the under surface of the body of the bone. 22 The Seventh Cervical Vertebra.The anterior or costal part of the transverse process of this vertebra is sometimes ossified from a separate center which appears about the sixth month of fetal life, and joins the body and posterior part of the transverse process between the fifth and sixth years. Occasionally the costal part persists as a separate piece, and, becoming lengthened lateralward and forward, constitutes what is known as a cervical rib. Separate ossific centers have also been found in the costal processes of the fourth, fifth, and sixth cervical vertebræ. 23 Lumbar Vertebræ.The lumbar vertebræ (Fig. 106) have each two additional centers, for the mammillary processes. The transverse process of the first lumbar is sometimes developed as a separate piece, which may remain permanently ununited with the rest of the bone, thus forming a lumbar riba peculiarity, however, rarely met with. FIG. 110 Base of young sacrum.

( See enlarged image ) Sacrum (Figs. 107 to 110) .The body of each sacral vertebra is ossified from a primary center and two epiphysial plates, one for its upper and another for its under surface, while each vertebral arch is ossified from two centers. 27 PERIODS OF OSSIFICATION.About the eighth or ninth week of fetal life, ossification of the central part of the body of the first sacral vertebra commences, and is rapidly followed by deposit of ossific matter in the second and third; ossification does not commence in the bodies of the lower two segments until between the fifth and eighth months of fetal life. Between the sixth and eighth months ossification of the vertebral arches takes place; and about the same time the costal centers for the lateral parts make their appearance. The junctions of the vertebral arches with the bodies take place in the lower vertebræ as early as the second year, but are not effected in the uppermost until the fifth or sixth year.

About the sixteenth year the epiphysial plates for the upper and under surfaces of the bodies are formed; and between the eighteenth and twentieth years, those for the lateral surfaces make their appearance. The bodies of the sacral vertebræ are, during early life, separated from each other by intervertebral fibrocartilages, but about the eighteenth year the two lowest segments become united by bone, and the process of bony union gradually extends upward, with the result that between the twenty-fifth and thirtieth years of life all the segments are united. On examining a sagittal section of the sacrum, the situations of the intervertebral fibrocartilages are indicated by a series of oval cavities (Fig. 99 ). 28 Coccyx.The coccyx is ossified from four centers, one for each segment. The ossific nuclei make their appearance in the following order: in the first segment between the first and fourth years; in the second between the fifth and tenth years; in the third between the tenth and fifteenth years; in the fourth between the fourteenth and twentieth years.

As age advances, the segments unite with one another, the union between the first and second segments being frequently delayed until after the age of twenty-five or thirty. At a late period of life, especially in females, the coccyx often fuses with the sacrum. What is the function of the sacrum? | Reference.com What is the function of the sacrum? A: Quick Answer The sacrum performs few active roles in the human body, primarily supporting the rest of the human spine. The sacrum exists at the end of the spinal column, appearing as a triangular bone. It rests in the c-shape portion of the lower back, connecting the lumbar vertebrae with the pelvis. Full Answer In addition to providing support, the sacrum works in coordination with the rest of the human spine, which provides movement and flexibility. According to the Mayfield Clinic, the primary role of the sacrum is one of a bridge, as it connects the hipbones, or the iliac crest, with the rest of the spine. The sacrum forms a long triangular shape, which derives from five connecting vertebrae. These vertebrae combine with the iliac bones along the sides, which form the pelvic girdle.

Just above the sacrum is the lumbar spine, also called the lower spine. This section of the spinal cord forms from five discs, named L1 through L5. These discs, although small, support the weight of virtually the entire human body. Just above the lumbar spine is the thoracic spine. This portion of the spine surrounds and protects the heart and lungs. It also forms the rib cage, and offers little movement and flexibility. Pelvis Pelvis Home > Skeletal System > Bones of the Pelvis and Lower Back > Pelvis Pelvis Located in the lower torso, the pelvis is a sturdy ring of bones that protects the delicate organs of the abdominopelvic cavity while anchoring the powerful muscles of the hip, thigh, and abdomen. Several bones unite to form the pelvis, including the sacrum, coccyx (tail bone), and the left and right coxal (hip) bones. Throughout childhood, the pelvis is made of many smaller bones that eventually fuse during adulthood to form a more rigid pelvis. Each of the coxal bones begins as three separate bones: the ilium, ischium, and pubis....

Move up/down/left/right: Click compass arrows Rotate image: Click and drag in any direction, anywhere in the frame Identify objects: Click on them in the image 2D Interactive 3D Rotate & Zoom Change Anatomical System Change View Angle Full Pelvis Description [Continued from above] . . . The ilium is the largest, widest, and most superior of the hip bones. When you place your hands on your hips, you can feel the curved ridge of the ilium known as the iliac crest. The narrow ischium is inferior to the ilium and is the bone, along with the coccyx, that you rest your body weight on while sitting. Anterior to the ischium is the pubis, the smallest of the hip bones. The ilium, ischium, and pubis meet in the center of the hip bone to form the deep, cup-like socket of the hip joint called the acetabulum. The sacrum and coccyx also begin life as multiple bones before fusing. Five short, wide vertebrae fuse to form the wedge-shaped sacrum, while four tiny vertebrae fuse to form the coccyx.

The bones of the adult pelvis join together to form four joints: the left and right sacroiliac joints, the sacrococcygeal joint, and the pubic symphysis. The sacroiliac joints form between the sacrum and the left and right ilium to form a tight junction capable of supporting the body’s weight and resisting the force of strong muscles. Although the sacroiliac joints are synovial joints, many strong ligaments and bony ridges on the sacrum and ilium interlock to prevent movement of the bones and strengthen the joint. The cartilaginous sacrococcygeal joint unites the sacrum with the tiny coccyx and allows some slight movement of the tail bone. On the anterior side of the pelvis the pubic symphysis unites the left and right pubic bones. The pubic symphysis is a slightly flexible band of fibrocartilage that allows the independent movement of the hip bones while walking. Women have a significantly wider and more flexible pubic symphysis compared to men, which allows the female pelvis to stretch during childbirth to accommodate the head of a fetus passing through the birth canal.

Many significant structural differences, or sexual dimorphisms, exist between the male and female pelvis. Most of these differences relate to the female reproductive system and the roles of pregnancy and childbirth in the female. The bones of the male pelvis are larger, thicker, and heavier than those of the female and there is very little hollow space within the ring of the pelvic bones. The female pelvis by comparison is significantly shorter and wider, which provides a greater hollow space within for the head of a fetus to pass through during childbirth. The sacrum and coccyx curve much more but are located more posteriorly in the female pelvis to increase the space of the pelvis. Even the shape and angle of the acetabulum and hip joint differ between males and females. The female acetabulum is rotated more anteriorly than in males, resulting in differences in walking gait and posture between men and women. Prepared by Tim Taylor, Anatomy and Physiology Instructor Pelvis Anatomy & Diagram | Body Maps Written and medically reviewed by the Healthline Editorial Team Co-developed by: In Depth: Pelvis The pelvic region is the area between the trunk — or main body — and the lower extremities, or legs.

The female pelvis is morphologically different (different in form) from a male’s but most of the differences are not apparent until puberty. The pelvic bones are larger and broader as they have evolved to create a larger space for childbirth. The most noticeable differences are the width of the pubic outlet, the circular hole in the middle of the pelvic bones, and the width of the pubic arch, or the space under the base of the pelvis. The bones of the pelvis are the hip bones, sacrum, and coccyx. Each hip bone contains three bones — the ilium, ischium, and pubis — that fuse together as we grow older. The sacrum, five fused vertebral bones, joins the pelvis between the crests of the ilium. Below the sacrum is the coccyx, or tailbone, a section of fused bone that is the end of the vertebral column. The pelvis forms the base of the spine as well as the socket of the hip joint. The hip joint is a ball-and-socket joint created by the femur and a part of the pelvis called the acetabulum.

This joint and its ability to rotate in many angles is one of many pieces of anatomy that allows humans to walk. The external female genitals include the vaginal opening, clitoris, urethra, labia minora, and labia majora. Collectively, these parts are called the vulva. The vaginal opening is also home to the urethra, the tube through which the body expels urine. It is an extension of the ureters, or tubes that deliver urine from the bladder. The bladder is situated below the uterus. The uterus is a pear-shaped, hollow organ where a fetus would develop prior to being born. Eggs, the female reproductive cells, are produced in the ovaries. A tube leads from each ovary to the uterus. These tubes are called the oviducts, or fallopian tubes. The pelvic region also holds several digestive organs. These include the large intestine and small intestine. Both are vital to digesting food and expelling solid waste. The large intestine ends in the rear of the pelvis at the anus, a sphincter muscle that controls the disposal of solid waste. The intestines are supported by a series of muscles known as the pelvic floor.

These muscles also help the anus function and help push a baby through the vaginal opening during childbirth. Debugging Tools

Question: Where in the human body is the sacrum bone?
Answer: Pelvis