THE FROG AND VERTEBRATES IN GENERAL 213 



which are distributed to the muscles of the shoulder, neck, and 

 tongue. 



B. Spinal Nerves. There are ten pairs of spinal nerves in the 

 Frog, all of which arise in the spinal cord and are distributed to 

 various regions of the body. In the higher Vertebrates there is a 

 considerable increase in the number of pairs of spinal nerves. 

 Thus, in Man, there are 31 pairs. In all the Vertebrates, however, 

 the structure of the spinal nerves is essentially the same. Each 

 spinal nerve arises in the spinal cord by a dorsal and a ventral 

 root. The two roots unite a short distance from the cord to form a 

 single nerve composed, therefore, of both dorsal and ventral ele- 

 ments. On the dorsal root, just before its union with the ventral 

 root, there is a swelling known as the dorsal ganglion. The 

 dorsal roots are known as the sensory, or afferent, roots. Ac- 

 cordingly the nerve fibers of which they are composed are con- 

 nected with sensory nerve cells in the skin or other regions, which 

 receive the stimuli. The stimuli incite nerve impulses which are 

 conveyed to the spinal cord by the dorsal root. The ventral roots 

 of the spinal nerves are known as the motor, or efferent, roots. 

 They run from the spinal cord to the peripheral regions where 

 they connect with muscle tissue. The impulse, which has come 

 through the sensory dorsal root, is relayed by the spinal cord to the 

 proper motor ventral root, through which it reaches and stimulates 

 the muscles necessary to bring about the appropriate response. 

 The spinal nerves of the Frog are as follows. (W. fs. 142, 143.) 



1. The first pair of spinal nerves arises from the anterior end 

 of the spinal cord just posterior to the medulla and emerges 

 from between the first and second vertebrae. Each of these nerves 

 usually gives off a small branch to the second spinal nerve and 

 then continues in a lateral direction into the muscles of the body 

 wall. (W. f. 142, Spn. 1.) 



2. This is a large pair of spinal nerves. Each one usually re- 

 ceives a branch from the first nerve and also a branch from the 

 third. The second nerve with these branches forms the brachial 

 plexus. The important nerves which arise from this plexus sup- 

 ply the muscles of the shoulders and fore limbs. (W. f. 142, Br.) 



3. The third pair of spinal nerves is small, and each, after giving 

 off a branch to the brachial plexus as noted, can be traced laterally 

 into the musculature of the body wall. (W. f. 142, Br.) 



4. 5, and 6. These three pairs of small nerves, which arise from the 



