The sacrum is situated between the two iliac bones; with which it articulates, and contributes to the formation of the pelvis. It is obliquely placed, from before backwards, and from below upwards; immediately behind the lumbar section of the vertebral column; and is continued by the coccygeal vertebræ, which form the skeleton of the tail.
It is triangular in outline, and is generally more narrow in proportion than in the human being. All things considered, it is more large and massive, and of greater density, in species which sometimes assume the upright posture, rather than in those which cannot assume that attitude; for example, the sacrum of the ape, of the bear, of the dog, and of the opossum are proportionately larger than those of the horse.[4]
[4] This is particularly striking only in those portions of the sacrum that are not in relation with the other bones of the pelvis. We think that the general form of this bone depends on the mode of its connexions with the iliac bones and the extent of the articular surfaces by which it is in contact with the latter.
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.
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.