SACRUM AXD COCCYX. 



131 



[ 257. The SACRUM (fig. 118) is of a triangular shape, its 

 base (1) facing upwards and forwards ; its apex, which is 

 Fig. 117. Fig. 118 



truncated (2), also facing forwards. It is concave before (b), 

 from above downwards, and irregularly convex behind (fig. 1 20, 

 a) in the same direction. 

 In the young subject it 

 consists of five verte- 

 brae, which in the adult 

 become soldered into 

 a single bone. In 

 mammals it is much 

 narrower than in man, 

 and forms in them a 

 straight line with the 

 spine ; the separate 

 pieces thereof remaining permanently united by ligaments. In 

 animals which sometimes hold themselves erect, as monkeys, 

 bears, sloths, and many rodents, it is proportionally larger than 

 in other mammals. On the concave anterior surface of the sacrum 

 we observe holes (4) for the passage of the nerves ; and on its 

 posterior surface (fig. 120), similar apertures (11, 11, 11) for 

 the same purpose are seen. Fig. 119 is a profile of this bone. 



[ 258. The COCCYX consists of four small bones, which re- 

 tain only a rudimentary centrum, and are soldered together in 

 man (fig. 1 1 9, 2.) These bones are, in fact, the rudiment of an 

 organ, the tail, which attains great importance and dimensions 

 in some animals, as shown in the comparative table ( 260). 



[ 259. The VERTEBRAE are firmly united together by pro- 

 cesses of bone (fig. 1 1 4 1 1 6, 2 and 3) that lock into each other. 

 Between every two vertebrae, an elastic fibro-cartilaginous cushion 



K 2 



