_C_, Body of first sacral vertebra; _A_, arch of first vertebra; _Cs_,
sacral canal; _P.s._ 1–5, sacral spines; _P.t._, wings of sacrum; 1,
1′, surfaces for articulation with transverse processes of last
lumbar vertebra; _F.a._, auricular surface; 2, 2′, articular
processes; 3–6, dorsal sacral foramina; 7, interarcuate space; 8,
lateral border; _h_, apex. (Struska, Anat. d. Haustiere.)
]
The =lateral borders= are rough, thick in front, thin behind.
The =base= (Basis ossis sacri) is directed forward, and is relatively
very wide. It presents centrally the =body= of the first sacral segment,
which is wide transversely, flattened dorso-ventrally, and has a rounded
surface which articulates with the last lumbar vertebra through the
medium of an intervertebral fibro-cartilage. The ventral margin projects
slightly, forming the =promontory= (Promontorium). Above the body is the
entrance to the sacral canal, flanked by a pair of =articular
processes=, which project upward and forward from the arch, and have
concave surfaces internally for articulation with those of the last
lumbar vertebra. On each side of these is a smooth notch which is
converted into a foramen by apposition with the last lumbar. The lateral
parts of the base, the =wings= or =alæ= (Alæ sacrales), are strong
prismatic masses with pointed ends, which result from the fusion of the
first with part of the second transverse process. Each has in front a
large, oval, slightly convex surface for articulation with the
transverse process of the last lumbar. Posteriorly there is an elongated
oval area which faces upward, backward, and outward. This is the
=auricular surface= (Facies auricularis), which articulates with the
ilium; it is slightly concave in its length, and somewhat rough and
irregular. The rest of the dorsal surface of the wing is roughened for
ligamentous attachment, while the ventral surface is smooth.
The =apex= (Apex ossis sacri) is the posterior aspect of the last sacral
vertebra and is quite small. It presents the elliptical flattened
surface of the body, above which is the triangular posterior opening of
the sacral canal, surmounted by the last sacral spine. There is a pair
of narrow notches between the arch and body, above which rudiments of
articular processes may occur.
The name =sacral canal= (Canalis sacralis) is applied to that part of
the vertebral canal which traverses the sacrum. Its anterior part is
large and has the form of a triangle with the angles rounded off; its
width is about twice its height. Traced backward it is seen to diminish
in size rapidly, and the posterior opening is quite small and
triangular.
The term =lateral part= (Pars lateralis) designates the portion external
to the foramina, which results from the fusion of the transverse
processes.
[Illustration:
FIG. 20.—FIRST COCCYGEAL VERTEBRA OF HORSE, LEFT VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]
[Illustration:
FIG. 21.—FIRST COCCYGEAL VERTEBRA OF HORSE, DORSAL VIEW. (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]
=Development.=—The several sacral vertebræ ossify in the typical manner.
Separate centers for costal elements in the lateral parts have not yet
been found in the domesticated animals. Fusion begins in front, and is
usually not complete behind till near adult age. The lateral parts unite
before the bodies. It is rather curious that the epiphyseal plates of
adjacent segments unite with each other before they fuse with the main
portion of the bodies.