The Nervous System 143 
former plexus is the eleventh and the most anterior 
nerve to take part in the latter is the twenty-third, 
so that there are eleven nerves that are doubtless 
distributed to the regions not supplied by the two 
plexuses. 
The crural-ischial plexuses (Fig. 32) are made up 
of branches from five nerves, three presacral (a, b, 
and c), the sacral (s=xxvi), and one postsacral (¢); 
the second postsacral shown in the figure appar- 
ently does not enter into the plexus. 
The first and second presacrals terminate chiefly 
in the abdominal and thigh muscles, though the 
second sends a large branch to fuse with a branch 
from the third to form the large obturator nerve 
(N. obt.) that leads to the muscles of the thigh and 
knee. 
The third presacral sends a branch back to fuse 
with the large sacral (s=xxvi), and these two, 
together with a branch from the first postsacral, 
form a complicated network that sends numerous 
branches to the muscles of the pelvic and femoral 
regions, to the skin, legs, and tail, as shown in 
Figure 32. The large muscles of the tail are 
innervated by the regular, metameric nerves of 
that region, and since there are usually thirty-nine 
caudal vertebre, there are probably about that 
many pairs of caudal nerves, although the last few 
vertebree and the muscles of that region are so 
small it may be that some of the nerves are 
lacking. 
