The vertebrae, or separate pieces of which the column is made up, are so named from their mobility (vertere, to turn). They are divided into true and false ; the former term being applied to those which remain separate in the adult, and retain their mobility; the latter to such as become united into one mass (viz. the sacrum), or degenerate as it were, and lose all the ordinary characters of vertebrae (viz. the coccyx).
The size of the vertebras increases from above downwards as far as the first pieces of the sacrum, from which it diminishes towards the end of the coccyx, where it terminates by a point; so that the column may be said to consist of two pyramids applied to one another at their bases. The superior, or movable vertebrae, however, do not taper regularly from above downwards ; they become somewhat narrowed and constricted as it were at the third dorsal vertebra, after which they gradually enlarge towards the lower end.
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