Its superior surface presents a crest, formed by the fusion of the spinous processes of the vertebræ which form it. In certain species these processes are attached only by their bases, and are separated from each other superiorly. In the pig they are wholly wanting. =The Coccygeal Vertebræ.=--These vertebræ, few in number (and sometimes ankylosed) in the human being, form in the latter a small series, the coccyx; which is inclined forwards, that is to say, towards the interior of the pelvis. In quadrupeds, on the contrary, their number is large; they are not ankylosed, and they form the skeleton of the caudal appendix. The first coccygeal vertebræ--that is, those which are next the sacrum--present characters which are common to those of other regions: they have a body, a foramen, and processes. As we trace them backwards, these characters become gradually effaced; and they become little more than small osseous cylinders simply expanded at their extremities. Direction and Form of the Spinal Column 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.