56 ELEMENTARY ANATOMY. [LESS. 



Thus: (A) it may be looked at with regard to its relation to 

 the nerves which pass out through its foramina in front ; r or, 

 (B) with regard to the skeleton only, and this again from three 

 other points of view : (i) with regard to the skeletal ele- 

 ments which comprise it ; (2) with regard to its connexion 

 with the hip-bones, and (3) witn regard to the bony union of 

 more or fewer vertebrae into one solid and complex mass. 



The sacrum of nervous supply (i.e. as estimated by the 

 destination of the nerves passing out through it) is much more 

 constant than the purely osteological sacrum. It never 

 probably embraces more than seven or less than two, perhaps 

 rarely less than three vertebrae. 



But while in man it is the two or three pre-axial sacral 

 vertebrae which have distinct costal elements, and while a 

 similar coexistence is probably universal in his class, very 

 different conditions may possibly obtain when we descend 

 further in the scale of animal life. 



Thus, in Birds the sacrum of nervous supply appears in 

 part to be made up of vertebrae which have no expanded 

 sacral ribs, while large transverse processes are attached to 

 the most post-axial vertebrae of the nervous sacrum. 



The purely osteological sacrum is a very variable part with, 

 regard to all the three conditions above enumerated, varying 

 even in the same species with the advance of age. 



Man agrees with most if not all Mammals in having the 

 more pre-axial part of his large sacral transverse process a 

 distinct costal element. In Birds, however, the vertebrae of the 

 sacrum which have expanded transverse processes, do not de- 

 velop these from distinct ossifications. In lower forms (e.g. 

 Crocodiles and Tailed-Batrachians) the sacral vertebrae have, 

 on the contrary, a distinct and unmistakable rib attached to 

 each transverse process. 



As regards the extent of connexion between the osteologi- 

 cal sacrum and the hip-bones, union is more extensive in man 

 than in most beasts, or animals below Birds. Often in Mam- 

 mals, and almost always in Tailed-Batrachians, it may be con- 

 fined to a single vertebra. On the other hand, ten vertebra? 

 may be involved in this union in Mammals, and twenty in 

 Birds. 



That part of the haunch-bone which we shall find is named . 

 the ilium, alone unites in Birds (Fig. 66, //) with this great 

 number of vertebrae ; but in man's class it only joins four 



1 As to the nerves, see Lesson VIII. 



