The Skeleton - The axial skeleton



Within the framework of the axial skeleton lie all the most vital organs of the body. People have gone on living with the loss of a hand or a leg—indeed, with the loss of any or all of their limbs. But nobody can live without a brain, a heart, a liver, lungs—all of which are carried within the framework of the axial skeleton.

The Human Skeleton

The Skull

The bones of the skull have as their most important function the protection of the brain and some sense organs. There are also, of course, the jawbones that support the teeth and gums and which enable us to bite and chew our food.

Most of the skull appears to consist of a single bone—a hard, unbroken dome. Actually, the brain cage or cranium consists of eight individual platelike bones which have fused together in the process of growth. At birth, these bones are separated, causing the soft spots or fontanelles we can readily feel on a baby's head. As the baby's brain enlarges, the bones grow along their edges to fill in the fontanelles, finally knitting together in what are called suture lines , somewhat resembling inexpertly mended clothes seams. Along the suture lines, the skull bones continue to grow until the individual's mature skull size is reached.

Teeth

The hardest substance in the human body is the enamel that covers the exposed surface of a tooth. Below the gum, the tooth's outside surface is composed of somewhat softer cementum . Beneath enamel and cementum is a bonelike substance, called dentin , which covers the soft interior of the tooth, called pulp . Pulp is serviced by blood vessels and nerves through the root or roots of the tooth. The passageway of nerves and blood vessels that lead up through the tooth from the gum sockets is called a root canal . Tooth and gum are stuck to each other by a tough, adhesive tissue called periodontal (or peridental—“surrounding the tooth”) membrane . See Ch. 22, The Teeth and Gums , for further information on teeth.

The Backbone

At the base of the skull, the backbone begins. The skull is supported by the topmost cervical (neck) vertebra. The curious thing about a backbone is that the word has come to suggest something solid, straight, and unbending. The backbone, however, just isn't like that: it consists of 26 knobby, hollowed-out bones— vertebrae , rather improbably held together by muscles, ligaments, and tendons. It is not straight when we stand, but has definite backward and forward curvatures; and even some of its most important structures (the disks between the vertebrae) aren't made of bone, but of cartilage.

All in all, however, the backbone is a fairly well designed structure in terms of the several different functions it serves—but with some built-in weaknesses. For a discussion of backache, see Ch. 23, Aches, Pains, Nuisances, Worries .

The Vertebrae

Although they will have features in common, no two of our 26 vertebrae are exactly alike in shape, size, or function. This is hardly surprising if we consider, for example, that the cervical vertebrae do not support ribs, while the thoracic vertebrae (upper trunk, or chest) do support them.

But for a sample vertebra, let us pick a rib-carrying vertebra, if for no other reason than that it lies about midway along the backbone. If viewed from above or below, a thoracic vertebra, like most of the others, would look like a roundish piece of bone with roughly scalloped edges on the side facing inward toward the chest and on the side facing outward toward the surface of the back, and would reveal several bony projections. These knobby portions of a vertebra—some of which you can feel as bumps along your backbone—are called processes . They serve as the vertebra's points of connection to muscles and tendons, to ribs, and to the other vertebrae above and below.

A further conspicuous feature is a hole, more or less in the middle of the typical vertebra, through which passes the master nerve bundle of our bodies, the spinal cord, running from the base of the skull to the top of the pelvis. Thus, one of the important functions of the backbone is to provide flexible, protective tubing for the spinal cord.

Between the bones of one vertebra and the next is a piece of more resilient cartilage that acts as a cushion or shock absorber to prevent two vertebrae from scraping or bumping each other if the backbone gets a sudden jolt, or as the backbone twists and turns and bends. These pieces of cartilage are the intervertebral disks—infamous for pain and misery if they become ruptured or slipped disks.

Regions of the Backbone

The backbone can be divided into five regions, starting with the uppermost, or cervical region, which normally has seven vertebrae. Next down is the thoracic (chest) section, normally with 12 vertebrae. From each vertebra a rib extends to curl protectively around the chest area. Usually, the top ten ribs come all the way around the trunk and attach to the breastbone (or sternum ); but the bottom two ribs do not reach the breastbone and are thus called floating ribs. The thoracic section also must support the shoulder girdle, consisting of the collarbones ( clavicles ) and shoulder blades ( scapulas ). At the end of each shoulder blade is a shoulder joint—actually three distinct joints working together—where the arm connects to the axial skeleton.

Below the thoracic vertebrae come the five vertebrae of the lumbar section. This area gets a good deal of blame for back miseries: lower back pain often occurs around the area where the bottom thoracic vertebra joins the top lumbar vertebra. Furthermore, the lumbar region or small of the back is also a well-known site of back pain; indeed, from the word “lumbar” comes lumbago , medically an imprecise term, but popularly used to describe very real back pain.

Below the lumbar region are two vertebrae so completely different from the 24 above them—and even from each other—that it seems strange they are called vertebrae at all: the sacrum and the coccyx . These two vertebrae are both made up of several distinct vertebrae that are present at birth. The sacrum is a large bone that was once five vertebrae. The coccyx was originally four vertebrae.

The Pelvic Girdle

The sacrum is the more important of these two strange-looking vertebrae. It is the backbone's connection to the pelvic girdle , or pelvis. On each side of the sacrum, connected by the sacroiliac joint, is a very large, curving bone called the ilium , tilting (when we stand) slightly forward and downward from the sacrum toward the front of the groin. We feel the top of the ilium as the top of the hip—a place mothers and fathers often find convenient for toting a toddler.

Fused at each side of the ilium and slanting toward the back is the ischium , the bone we sit on. The two pubis bones, also fused to the ilium, meet in front to complete the pelvic girdle. All the bones of the pelvis—ilium, ischium, and pubis—fuse together so as to form the hip joint ( acetabulum ), a deep socket into which the “ball” or upper end of the thighbone fits.



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