The =sacrum= (Os sacrum) is formed by the fusion of five vertebræ
usually, and is conveniently described as a single bone. It is
triangular in form and is wedged in between the ilia, with which it
articulates very firmly on each side. Its long axis is gently curved,
and slightly oblique, so that the posterior end is a little higher than
the anterior. It presents two surfaces, two borders, a base, and an
apex.
The =dorsal surface= presents centrally the five sacral spines, which
are directed upward and backward, and have (with the exception of the
first) tuberous summits which are sometimes bifid.
The first spine is relatively thin and narrow, and is not so high as
the internal angle of the ilium. The second is the longest, and the
height diminishes rapidly to the last. The bases of the spines are
often fused in old subjects.
On either side of the spines is a groove, in which are the four =dorsal
sacral foramina= (Foramina sacralia dorsalia); the dorsal branches of
the sacral nerves emerge through them.
The =ventral= or =pelvic surface= (Facies pelvina) is concave in its
length, wide in front, narrow behind. It is marked by four more or less
distinct =transverse lines= (Linæ transversæ), which indicate the
demarcation of the bodies of the vertebræ. At the ends of these lines
are the =ventral sacral foramina= (Foramina sacralia ventralia), which
are larger than the dorsal series and diminish in size from first to
last; they transmit the ventral divisions of the sacral nerves.
The dorsal and ventral foramina communicate with the sacral canal and
are together equivalent to the usual intervertebral foramina.
[Illustration: