THE FROG 



it passes around the side of the throat, turns forward below 

 and behind the angle of the jaw and ends in muscles of the 

 floor of the mouth. 



(&) The second spinal or brachial is the largest spinal 

 nerve. It leaves the vertebral canal between the second and 

 third vertebrae, and passes outward to supply branches to 

 the shoulder region and arm. Why is it so large? The 

 second spinal is connected with the third by one or more 

 delicate cross-branches; usually it is also connected with the 

 first by other very slender twigs. The nerves of this region 

 where the cross-connections occur are sometimes said to 

 constitute a brachial plexus. There is considerable varia- 

 tion in the cross-branching in different specimens of the 

 frog, but the brachial plexus is never so well developed as 

 in the mammals where it takes the form of a complicated 

 network. 



(c) The third spinal nerve is small and emerges from 

 between the third and fourth vertebras; for some distance 

 it runs along the posterior edge of the brachial nerve with 

 which it is connected by one or more slender branches, and 

 finally supplies some muscles of the body wall. 



(d) The fourth, fifth and sixth spinal nerves are very 

 slender and run obliquely backward. Determine their dis- 

 tribution. 



(e) The seventh, eighth and ninth spinal nerves pass 

 almost directly backward and anastomose with each other 

 to form the sciatic plexus. The eighth and ninth unite to 

 form the large sciatic nerve which may be followed by 

 separating the great muscles of the dorsal surface of the leg. 



(/) The tenth pair of spinal nerves are very small and 

 pass almost directly backward. They innervate the bladder 

 and the cloaca. 



