The Sacral vertebrae are all fused together into one bone, called the sacrum.

The Sacrum is a triangular bone formed of the five sacral vertebrae, whose bodies and transverse processes have become fused together, distinct ridges being seen at the line of fusion on the anterior surface.

The anterior surface is concave from above downwards, and has four foramina on each side of the middle line for the passage of nerves. These are homologous with the intervertebral foramina, and, through the fusion of the transverse processes with one another, appear both anteriorly and posteriorly. Above, the margin is projecting, forming the promon-tory of the sacrum. This is more marked in the male than the female.

The posterior surface is convex from above downwards, and has the four foramina on each side, similar to the anterior surface. In the middle line the spines of the vertebrae still persist as four tubercles.

Fig. 26. - Sacrum (Anterior Surface).

On each side of the upper part of the bone are the lateral masses, large stout masses of bone, on the external aspects of which are the surfaces for articulation with the innominate bone.

Fig. 27. - Sacrum (Posterior Surface).