THE SACRUM 



41 



THE SACRUM 

 _ The sacrum (Os sacrum) is formed by the fusion of five vertebra3, 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 



y Auricular surface 



Lateral part 

 Lateral border 



Transverse process 



Fig. is. — Sacrum of Horse; Dorso-lateral View. 

 T-F, Spinous processes; i-4, dorsal sacral foramina; 5, 5', articular processes; 5, 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 

 ihum. 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 

 (Linese transversa), which indicate the demarcation of the bodies of the vertebrae. 

 At the ends of these lines are the ventral sacral foramina (Foramina sacralia ven- 



