SPINAL COLUMN (Lat. spinalis, relating to a thorn or to the spine, from spina, thorn, spine), or SPINE. The most important and char acteristic part of the skeleton of the highest ani mal sub-kingdom, which includes mammals, birds, reptiles, 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 the and hence, these animals, having this distinguishing characteristic in common, are all included in the term vertebrates. The vertebrm vary greatly in number in different animals, and in shape they differ extremely, even in differ ent parts of the same spine, in accordance with their special functions. In man the number of vertebra? which collectively form the spinal col umn is 7 in the neck (cervical i2 in the back (dorsal vertebra?), 5 in the loins (lum bar vertebra?), 5 ossified together, forming the sacrum, and 4 similarly united forming the ter mination of the column or coccyx. However long or short the neck may be, every mammal has 7 cervical vertebrm, 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 vertebrae there is interposed a lenticular disk of tibro-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 gen eral range of movement permitted, although the motion between any two adjacent vertebrae is slight. The elasticity of the column is further increased by the component vertebrre being ar ranged in curves, instead of being placed perpen dicularly. These curves enable the spine to bear a greater vertical weight than it could otherwise maintain; they facilitate the movements of the body, especially in the act of running; and they are so disposed as to protect the cord in move ments of the spine.
The vertebral canal formed by the apposition of the spinal foramina, or neural arches, and containing and protecting the spinal cord, varies in its size at different parts of the column. The intervertebral foramina through which the nerves emerge vary in shape and position in different parts, but are always of sufficient size to pre vent injurious pressure on the nerves during movements 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 vertebrae have been destroyed by ulceration.