SPINAL COLUMN, or SPINE, THE, is the most important and characteristic part of the skeleton of the highest animal sub-kingdom, which includes manfrnals, birds, rep tiles, amphibians, and fishes. In each of these classes it is composed of a series of bones placed one above, or in front, of another, and called vertebra (q.v.); and hence, these animals, having this distinguishing characteristic in common, are all included in the term vertebrates. The vertebra vary greatly in number in different animals, and even in members of the same class, and the number have no apparent relation to the other organs of the animal. Moreover, in their shape, they differ extremely, even in different parts of the same spine, in accordance with their special functions. In man, the number of vertebra winch collectively form the spinal column is 7 in the neck (cervical vertebra), 12 in the hack (dorsal vertebra), 5 in the loins (lumbar vertebra), all of which are capa ble of being detached from one another, and are termed true vertebra; and 5 vertebra ossified together, and forming the sacrum, and 4 or 5 similarly united forming the termi nation of the column, and constituting the bone called the coccyx, which are known as false vertebra. However long or short the neck may be, every mammal has 7 cervical ver tebra, excepting the three-toed sloth, which has 9, and the sea cow, which has 6. In the other regions of the spine no such law exists. Each vertebra is attached to the two between which it lies by numerous strong and more or less elastic ligaments, and between each pair of vertebra there is interposed a lenticular disk of fibro-cartilage, which acts as a buffer. By these arrangements the spinal column is rendered highly elastic, the communication of jars or shocks is prevented, and a very considerable general range of movement permitted, although the motion between any two adjacent vertebra is slight. The elasticity of the column is further increased by the component vertebra, being arranged in curves, instead of being placed perpendicularly. The curves should be exactly in the cntero-postcrior di:Tetion, any well-marked lateral deviation from the perpendicular being abnormal; but a very slight lateral curvature with the convexity to the right may often be detected in the upper and middle parts of the back, and is supposed to be de pendent on the more frequent use and greater strength of the right arm as compared with the left. From their position they are termed the cervical, dorsal, lumbar, and pelvic
curves. The dorsal and pelvic curves have their concavities in front, and thus enlarge the spaces in which the thoracic and pelvic viscera arc contained; the two other curves are convex anteriorly, and thus afford support to the parts above them. The upper three curves are so arranged that their chords are in the same vertical dine in the erect position of the body, and this vertical line corresponds with the line of gravity of the head. The cause of these curves is to be sought for partly in the shape of the vertebral bodies, and partly in that of the intervertebral substance. Among the uses of these curves it may be mentioned (1) that they enable the spine to bear a greater vertical weight than it could otherwise maintain; it is calculated that, nine times as great a vertical force is required to bend it as if it had been straight; (2) that they facilitate the movements of the body, especially in the net of running; and (3) that they are so disposed as to protect the chord In movements cf the spine. Similar curves are seen in the spine of other mammals, though the degree of flexure is liable to great deviations. The lumbar curve, which has especial reference to the erect position, is always much less marked than in man.
The vertebral canal formed by the apposition of the spinal foramina, or neural arches (see SKELETON), and containing and protecting the spinal chord, varies in its size at different parts of the column. It is largest in its anteni-posterior diameter in the neck and loins (measuring at the last lumbar vertebra of an inch), where the antcro-posterior movements of the spine are greatest, and where the cord is least closely attached to the vertebra; while in its lateral diameter it is greatest at the atlas, where it measures nearly an inch and a half. A transverse section of the canal is nearly circular through tho greater part of the back. The intervertebral foramina through which the nerves emerge vary in shape and position in different parts, but are always of sufficient size to prevent injurious pressure on the nerves during movement of the spine; and in the dorsal region, which is the ordinary seat of angular curvature, the nerves are so protected by bony arches that they may escape injury, even when the bodies of several dorsal vertebra have been destroyed by ulceration.