THE COCCYGEAL VERTEBRA 43 
laris), 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 rough- 
ened 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 nearly twice its height. Traced 
backward it is seen to diminish in size rapidly, and the posterior opening is small 
and triangular. 
The term lateral part (Pars lateralis) designates the portion lateral to the 
foramina, which results from the fusion of the transverse processes. 
Development.—The several sacral vertebre 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 
till 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. 
THE COCCYGEAL VERTEBRAE 
The coceygeal vertebrae (Vertebree coccygex) vary considerably in number, but 
eighteen may be taken as an average. From first to last they become reduced 
in size and, with the exception of a few at the beginning of the series, consist of 
; V9) S pi 5 Doe 
s Spinous process Spinous process 
i w4 Vertebral foramen Vertebral foramen 
Arch 
Arch 
Transverse process 
Transverse proc- 
ess 
Groove 
Body Body 
Fic. 20.—Firsr Coccygpat VERTEBRA OF Hors5; Fic. 21.—Srconp CoccyGEAL VERTEBRA OF HORSE; 
PosTeRIOR VIEW. Posterior VIEW. 
Leader to arch points to rudimentary articular process. 
bodies only. The first three have bodies which are somewhat flattened dorso- 
ventrally, constricted in the middle, and have at the ends convex, elliptical, artic- 
ular surfaces. The ventral surface has a median groove (Sulcus vasculosus) for 
the coceygeal artery. The arch is small and triangular; it is formed of two flat 
plates which are prolonged to form a short spinous process with a thickened and 
often double summit. The anterior notches are absent. Functional articular 
processes are not present, but small rudiments of the anterior pair commonly occur. 
The transverse processes are relatively large ‘plates which project horizontally 
outward. Further back the arch becomes incomplete dorsally, and soon disap- 
pears; the transverse processes gradually fade out, and the vertebre are reduced 
to cylindrical rods of diminishing size. The last one has a pointed end. 
1 Tt is not rare to find fusion of some of the bodies incomplete even in adult subjects. 
