. Animal physiology. Physiology, Comparative; Physiology, Comparative. STRUCTURE AND CONNEXION OF THE VERTEBRA. 463 ture (Fig. 215), which, when several vertebrae are united toge- ther, forms a continuous tube or canal for the a lodgment of the spinal cord. This character is usually lost, however, in the coccygeal vertebrae ; which are so much contracted and simplified, as to contain no aperture. The purpose of the divi-* sion of the spinal column into so large a number & of separate bones, is obviously to allow of consider- fig. 215—swgle able freedom of motion, by a slight change of plac
. Animal physiology. Physiology, Comparative; Physiology, Comparative. STRUCTURE AND CONNEXION OF THE VERTEBRA. 463 ture (Fig. 215), which, when several vertebrae are united toge- ther, forms a continuous tube or canal for the a lodgment of the spinal cord. This character is usually lost, however, in the coccygeal vertebrae ; which are so much contracted and simplified, as to contain no aperture. The purpose of the divi-* sion of the spinal column into so large a number & of separate bones, is obviously to allow of consider- fig. 215—swgle able freedom of motion, by a slight change of place in the individual parts; whilst any sudden bend, which would be injurious to the spinal cord, is avoided. Each vertebra consists of a solid body («), which is situated in front of the spinal canal in Man, bat below it in animals whose back has a hori- zontal position, and which serves to give solidity to the structure, —and of processes or projections (b and c), that serve to form the spinal canal, and to unite the vertebrae to each other. In Man, and other warm-blooded animals, the two surfaces of the body are nearly flat and are parallel to each other; and ^they are united to the corresponding surfaces of the neighbouring vertebrae by a disc of flbro-cartilage (§. 45), which extends through the whole space that intervenes between them, and which, being firmly adherent to both, prevents them from being far separated from each other. 629. But in Reptiles and Fishes, a different plan is adopted. In the animals of the former class, particularly in Serpents, we find one surface of each vertebra convex or projecting, and the other concave or hollowed out; and the convex surface of each vertebra fits into the concave surface of the next, in such a man- ner that the whole spinal column becomes a series of ball-and- socket joints, and is thus endowed with that flexibility which is essential to the peculiar movements of these animals. In Fishes, both surfaces are concave ; and
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