Sacrum [Sacral vertebrae]

The sacral vertebrae fuse together and with their ossified intervertebral discs, to form the sacrum, a single bone in all domestic species.

The cranial part of the sacrum (mainly first sacral vertebra) articulates cranially with the lumbar spine (lumbosacral joint at the lumbosacral vertebral junction) and with the pelvic girdle (sacroiliac joints). The caudal part of sacrum forms the roof of the pelvic cavity.

The cranial extremity of sacrum is termed the base:

• On the dorsal part of the base of sacrum, are the cranial articular processes that articulate with the caudal articular processes of the last lumbar vertebra.
• The ventral border of the base, the promontory, projects together with the last intervertebral disc into the pelvic inlet.
• The lateral parts of the base, composed by the fused transverse processes of sacral vertebrae (mainly the first), expand laterally to form the wings of sacrum. Each wing carries on his dorsal part an oval articular surface covered with cartilage, termed auricular surface , for articulation with the ilium. The sacral tuberosity is the roughed surface, dorsal to the auricular surface, for attachment of ligaments.

The dorsal surface of sacrum presents the dorsal sacral foramina (openings for the exit of the dorsal branches of the sacral nerves) and different crests:

• The median sacral crest, formed by the totally fused spinous processes, in ox, and sometimes in other ruminants (ov. and cap.). In carnivores and horses, the based of spinous processes are fused but the free end remain separate, so there is no median sacral crest. In pigs, it is an indistinct crest.
• The intermediate sacral crest is formed by the fused articular processes, in ruminants,
• The lateral sacral crest is formed by the fused transverse processes.

The pelvic surface of sacrum is the ventral surface. It presents the ventral pelvic foramina, openings for the exit of the ventral branches of the sacral nerves. The transverse lines, on pelvic surface, are the result of fused sacral bodies and indicates the former limits of the individual vertebrae.

The apex of sacrum is the small caudal extremity of sacrum, it presents a caudal articular process in carnivores and pigs.

The sacrum contains the sacral canal: it is the continuation of the vertebral canal in the sacrum (with a diameter much more narrower in the sacrum than in the lumbar region).