Base of the sacrum

The base of the sacrum, which is broad and expanded, is directed upward and forward.

In the middle is a large oval articular surface, the upper surface of the body of the first sacral vertebra, which is connected with the under surface of the body of the last lumbar vertebra by an intervertebral fibrocartilage.

Behind this is the large triangular orifice of the sacral canal, which is completed by the laminae and spinous process of the first sacral vertebra.

The superior articular processes project from it on either side; they are oval, concave, directed backward and medialward, like the superior articular processes of a lumbar vertebra.

They are attached to the body of the first sacral vertebra and to the alae by short thick pedicles; on the upper surface of each pedicle is a vertebral notch, which forms the lower part of the foramen between the last lumbar and first sacral vertebrae.

On either side of the body is the ala of sacrum.