THE SACRUM 41 
THE SACRUM 
The sacrum (Os sacrum) is formed by the fusion of five vertebrae, and is con- 
veniently 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 (Facies dorsalis) presents centrally the five sacral spines 
(Processus spinosi), which are directed upward and backward, and have (with the 
exception of the first) tuberous summits which are sometimes bifid. 
Wing 
Se 
Auricular surface 
Lateral part 
Lateral border 
Tra nsverse proce ss 
Fie. 18.—Sacrum or Horse; Dorso-LaTeraL VIEW. 
I-V, Spinous processes; 1-4, dorsal sacral foramina; 5, 5’, articular processes; 6, surfaces of wings for articulation 
with transverse processes of last lumbar vertebra; 7, body of first sacral vertebra. Arrows point into sacral 
canal. 
_. _ The first spine is relatively thin and narrow, and is not so high as the sacral angle of the 
ilium. The second is the longest and highest, and the length and height diminish to the last. 
The bases of the spines are often fused in old subjects. 
On either side of the spines there 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 pelvic surface (Facies pelvina) is concave in its length, wide in front, 
narrow behind. The curvature is variable and is more pronounced in the mare 
than in the stallion. It is marked by four more or less distinct transverse lines 
(Line transverse), which indicate the demarcation of the bodies of the vertebre. 
At the ends of these lines are the ventral sacral foramina (Foramina sacralia ven- 
