The vertebral column (columna vertebralis, rachis, spina,) is situated along the median line, at the posterior part of the trunk, the length of which it determines.
Anteriorly it presents the form of an irregular pyramid — posteriorly, a series of elongated processes (spinas), disposed regularly one beneath the other, from which circumstance the term "spine" is derived. Viewed as a whole, it resembles at first sight the shape of a long bone, but it is very differently constructed. As it receives the weight of the head and trunk, and transmits it to the base on which it rests, it requires to be firm and resisting, its power of re- sistance increasing gradually from above downwards. Being the centre of all the movements of the body, it must be as pliant and flexible as a bow, but yet firm, in order to give adequate protection to the spinal cord which it encloses. All these conditions are attained by its being made up of several small pieces united by an elastic substance, the motion permitted between each pair being slight, while the aggregate of all is considerable.