THE SKELETON 95 
forwards, 7.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 presacral vertebra 
usually occurs; here we have an atavism, 7.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-lmb, 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 
