42 



THE SKELETON 



articulates by means of an intervertebral disc with the coccyx. In advanced 

 life the apex of the sacrum becomes united to the coccyx by bone. 



The sacral canal is the continuation of the spinal canal through the sacrum. 

 Like the bone, it is curved and triangular in form at the base and flattened toward 

 the apex. It terminates at the hiatus sacralis between the sacral cornua, where 

 the laminae of the fourth and fifth sacral vertebrse are incomplete. The canal 

 opens on the surface by the anterior and posterior sacral foramina and lodges the 

 lower branches of the cauda equina, the filum terminale, and the lower extremity 

 of the dura and arachnoid. The sub-dural and sub-arachnoid spaces extend 

 downward within the canal as far as the body of the third sacral vertebra. 



Differences in the two sexes. — The sacrum of the female is usually broader in proportion 

 to its length, much less curved, and directed more obliquely backward than in the male. The 

 curvature of the female sacrum belongs chiefly to the lower part of the bone, whereas in the 

 male it is equally distributed over its whole length; but the curvature is subject to considerable 

 variation in different skeletons. 



Fig. 46. — Base of Sacrum. 



Articular process 



Lamina 



Sacral canal 



Racial differences. — The human sacrum is characterised by its great breadth in comparison 

 with its length, though in the lower races it is relatively longer than in the higher. The propor- 



tion is expressed by the sacral index 



' The average sacral index in the British 



length 



male is 112, in the female 116. Sacra in which the index is above 100 are plalyhieric, as in 

 Europeans; those under 100 are dolichohieric, as in most of the black races (Sir W. Turner). 



Fig. 47. — The Coccyx. A. Posterior view; B. Anterior view. 

 A. B. 



Transverse 

 process 



Gluteus 

 mazimus 



External 



Sphincter^ 



ani 



THE COCXJYGEAL VERTERR/E 



The four coccygeal vertebrae are united in the adult to form the coccyx [os 

 coccygisj (fig. 47). While four is the usual number of these rudimentary vertebrae, 

 occasionally there are five, and rarely three. In middle life the first piece is 

 usually sej)arate, and the original division of the remaining portion of the coccyx 



