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 vertebrae, there are probably about that 

 many pairs of caudal nerves, although the last few 

 vertebrae and the muscles of that region are so 

 small it may be that some of the nerves are 

 lacking. ^ $\d 



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