The curves of the vertebral column are, in quadrupeds, slightly different from those which characterize the human spine. First, instead of their being, as in the latter, curves in the antero-posterior aspect, because of the general attitude of the body, they are turned in the supero-inferior direction.
The cervical region is not a single curve, as in the human being. It presents two: one superior, with its convexity looking upwards; the other inferior, the convexity of which is turned downwards. This arrangement reminds one of that of a console.
The dorsal and lumbar regions are placed in a single curved line, more or less concave downwards; so that in the lumbar region there is no curve analogous to that which exists in man; a form which, in the latter, is due to the biped attitude—that is to say, the vertical position of the trunk. Briefly, there is in quadrupeds one dorso-lumbar curve; and not both a dorsal and a lumbar, with convexities in opposite directions.
At the extremity of the dorso-lumbar region is the sacrum and the caudal appendix, which describe a curve of which the concavity is directed downwards and forwards.
It is necessary to point out that it is not the curves of the three anterior portions of the spinal column which determine the form of the superior border of the neck and shoulders, and of the same part of the trunk. For the first portion, there is a ligament which surmounts the cervical region, and substitutes its modelling influence for that of the vertebræ. It is the superior cervical ligament, which arises from the spinous process of the first cervical vertebræ, and is inserted into the external occipital protuberance on the upper part of the posterior surface of the skull. The summits of the spinous processes of the vertebræ alone give form to the superior median border of the trunk. In this connection we here repeat that it is not the general curvature of the vertebral column which produces the withers, but the great length of the spinous process of the first vertebræ of the dorsal region.
The Thorax
The dorsal vertebræ form the posterior limit in man, and superior in quadrupeds, of the region of the trunk known as the thorax. A single bone, the sternum, is situated at the aspect opposite; the ribs bound the thorax on its sides.
In its general outlines the thorax in quadrupeds resembles that of man—that is to say, that, as in the latter, the anterior portion—superior in the human being—is narrower than the part opposite. But the progressive widening takes place in a more regular and continuous fashion, so that it presents a more definitely conical outline. This purely conical form is nevertheless found in the human species, but only during infancy; the inferior portion of the thoracic cage being then widely expanded, because of the development of the abdominal viscera, which at that period are relatively large.
