Fig. 6.--1, 2, 3, the three sacral vertebræ. _a_, _a′_, spinous
processes; _b_, cranial articular process of first sacral vertebra;
_c_, expanded transverse process of first sacral vertebra; _d_, caudal
articular processes of third sacral vertebra; _e_, transverse
processes of third sacral vertebra; _f_, tubercles formed by fused
articular processes of the vertebræ; _g_, dorsal (or posterior) sacral
foramina.
Fig. 7.--1, 2, 3, the three sacral vertebræ. _a_, the transverse
ridges formed by the union of the centra; _b_, cranial articular
processes of first vertebra; _c_, transverse process of first
vertebra; _d_, caudal articular processes of third vertebra; _e_,
transverse processes of third sacral vertebra; _f_, _f′_, ventral (or
anterior) sacral foramina; _g_, notch which helps to form third
ventral sacral foramen.]
The ventral or pelvic surface (Fig. 7) is smooth, concave craniad,
convex caudad, and crossed by two transverse ridges (_a_) along which
are seen the ossified remains of the intervertebral fibro-cartilages. At
the ends of the first ridge is a pair of nearly circular =ventral= (or
anterior) =sacral foramina= (_f_) for the passage of sacral nerves. At
the end of the second ridge is a pair of ventral sacral foramina (_f′_),
smaller than the first pair and continued laterocaudad into shallow
grooves for the ventral rami of the sacral nerves. That portion of the
bone lying laterad of a line joining the medial borders of these two
pairs of foramina is known as the =lateral mass= of the sacrum and is
composed of the fused transverse processes of the sacral vertebræ. At
the caudal margin of the ventral surface there is a notch between the
lateral mass and the centrum (_g_). When the caudal vertebræ are
articulated, this notch helps to form a foramen for the third sacral
nerve.
The dorsal surface (Fig. 6) is narrower at its cranial end than is the
ventral surface. Its cranial border bears laterally a pair of cranial
articular processes (_b_) with their medially directed facets and
between them it is concave, so that a large dorsal opening is left into
the vertebral canal between the last lumbar vertebra and the sacrum.
Caudad of the articular processes are two pairs of tubercles (_f_).
These are the fused cranial and caudal articular processes of the sacral
vertebræ. Caudad of them are the caudal articular processes of the last
sacral vertebra (_d_). Craniolaterad of the middle and cranial tubercles
are =dorsal= (posterior) =sacral foramina= (_g_) for the transmission of
the dorsal rami of the sacral nerves. Three spinous processes (_a_)
appear between these rows of tubercles. They decrease in height caudad.
That part of the surface included between the spinous process and the
tubercles is made up of the fused laminæ of the sacral vertebræ. That
part between the tubercles and a line joining the lateral margins of the
dorsal (posterior) sacral foramina is formed by the fused radices of the
sacral vertebræ.
The lateral surface may be divided into two parts. Craniad is a large
rough triangular area with equal sides and with one of its angles
directed ventrocraniad. It is the lateral face of the pseudo-transverse
process of the first sacral vertebra (Fig. 6, _c_). A smooth curved
surface (the auricular facet) along its ventral edge articulates with
the ilium, while the dorsal portion is rough for attachment of
ligaments. Caudad is the narrow longitudinal triangular area of the
lateral faces of the fused transverse processes of the second and third
sacral vertebræ.
=Caudal Vertebræ.= =Vertebræ caudales= (Fig. 1, _y_, and Figs. 8 and
9).--The caudal vertebræ (21-23 in number) decrease gradually in size to
the last one. Caudad they become longer and more slender and lose the
character of vertebræ. They become finally reduced to mere
centra,--slender rods of bone knobbed or enlarged at their two ends
(Fig. 8). The last one is more pointed than the others and bears at its
caudal end a small separate conical piece, the rudiment of an additional
vertebra.
[Illustration: FIG. 8.