THE SKELETON 95 



forwards, i.e. towards the head. Both these alterations in position 

 are most clearly reflected in the variations of the nerve plexuses 

 of the limbs, the origin of which will be discussed later. We 

 must, however, first ascertain what these variations are. 



The lumbo- sacral plexus, as compared with the brachial, 

 is the more subject to variation, and the less definitive. Even 

 if the brachial plexus does show slight inconstancy, no such 

 marked differences in the origin of its component nerve trunks 

 occur as in the lumbo -sacral. In most cases, these varia- 

 tions in the limb plexuses are accompanied by variations in the 

 vertebral column. For example, when the lumbo-sacral plexus has 

 a markedly caudal origin, a supernumerary prsesacral vertebra 

 usually occurs ; here we have an atavism, i.e. an indication of the 

 primitive arrangement under which, as above described (ante, 

 p. 33), the pelvis lay farther back. But we know that, during 

 ontogeny, the pelvis undergoes a forward translocation. Cor- 

 relatively, the lumbar plexus assimilates nerves lying farther 

 forward than those which primarily formed it (the ileo-hypo- 

 gastric, ileo-inguinal, and the genito-crural), while the posterior 

 sacral nerves of the adult show signs of instability and degenera- 

 tion, and may gradually altogether disappear. 



The forward gathering of the nerves for the hind-limb is 

 naturally accompanied by modification in the innervation of 

 those parts of the urino-genital and alimentary systems which 

 lie in the pelvis. These are obviously dependent on the pelvic 

 girdle, and compelled to follow when it shifts along the verte- 

 bral column. The ischiadic and the pudendal plexuses are so 

 closely connected that they could not in any case be separated ; 

 but the relationship between the pudendal and caudal plexuses is 

 less intimate, and if the former shifts forwards with the crural 

 plexus, its distal elements separate from it. These retrogressive 

 nerves of the caudal region would necessarily increase in number 

 in proportion to the forward translocation of the hind-limb, if 

 the caudal region itself did not at the same time shorten 

 (Eisler). 



We thus have transition zones ; and this becomes the more 

 clear the farther the lumbo-sacral plexus shifts in a proximal 

 direction. In extreme cases variation may extend as far 

 forwards as the eleventh thoracic nerve, which then sends a 

 loop to the twelfth. 



Similar phenomena accompany the backward displacement of 

 the fore-limb, but this, as already mentioned, appears to have 



