THE BOXES OF THE TRUNK 



31 



foramina at either side, and each is marked anteriorly by four 

 transverse grooves, which prolong outwards the foramina and 

 lodge the transmitted nerves. The pyriformis muscle arises from 

 the front of each lateral mass by three shps, which are inter- 

 posed between, and lie external "^to, the foramina. The lateral 

 masses are formed by the fusion of the pedicles, transverse pro- 

 cesses, and costal elements of the sacral vertebrse. 



Superior Ardcnlar Process 



. .Iliacns 



^ ^-Pyrifonnis 



Inferior Lateral Angle 



— Coccygf us 



4th Anterior Sacral Foramen 



Fig. 22. — The Sacrum (Anterior View.) 



The dorsal surface, which is directed upwards and backwards, 

 is irregularly convex and narrower than the ventral. In the middle 

 line it presents four eminences, which may be distinct, or fused to 

 form a ridge, representing the spinous processes of the upper four 

 sacral vertebrae. The spinous process of the fifth vertebra is absent, 

 the development of its laminae having been arrested, and there is 

 thus left a triangular opening, which is the outlet of the sacral 

 canal, to be presently described. On either side of the median 



