c. The Sacrum. In the adult, the sacrum (figure 2-20C) is a single bone formed
by the fusion of the five sacral segments. It is a large wedge-shaped (triangular) bone
situated in the lower part of the vertebral column and at the upper and back part of the
pelvic cavity where it is wedged between the two hipbones. Its base is directed upward
and its apex is directed downward. In the center, the base presents the kidney-shaped
body; behind this is the superior opening of the sacral canal, which is bounded laterally
by the articular processes. The sacral promontory is a prominent ridge at the upper
anterior margin of the body. The body articulates with the body of L-5 to form the
lumbosacral articulation. On either side of the body is a wing-like surface called the ala
(wing) of the sacrum.
(1) The dorsal surface of the sacrum is rough and convex. In the middle
line, the dorsal surface displays a ridge, the median sacral crest, made up of three or
four rudimentary spinous processes that are more or less fused to form the crest. There
are four posterior sacral foramina, which transmit several sacral nerves.
(2) The ventral or pelvic surface of the sacrum is smooth and concave. The
anterior sacral foramina transmit some of the sacral nerves.
(3) The lateral surface or margin of the sacrum presents in front an ear
shaped surface for articulation with the articular surface of the ilium.
d. The Coccyx. The coccyx usually consists of three to five rudimentary
coccygeal segments that fuse in adult life (figure 2-20D). From its base downward to its
apex, the coccyx diminishes in size. It curves downward and forward from its
articulation with the sacrum, often deviating from the median plane of the body.
2-21. VERTEBRAL LANDMARKS
The important landmarks of the vertebral column are shown in figure 2-22. Study
these carefully. Note other structures at similar levels.