XVI THE NERVOUS SYSTEM 289 



matter on both sides of the cord. The ventral roots of the 

 spinal nerves are broader and consist of several isolated 

 strands. 



The Spinal Nerves. — The frog possesses but ten pairs of 

 spinal nerves. The tadpole has a much larger number 

 (twenty-two in R. fused), but the posterior ones disappear 

 with the degeneration of the tail. There is also a pair of 

 nerves which appears in the embryo in front of what is the 

 first pair of spinal nerves of the adult, but we shall continue 

 to speak of the latter as the first pair. Each spinal nerve 

 arises from the cord by a dorsal and a venb'al root which 

 unite just outside the inter-vertebral foramina through which 

 they emerge. Near its junction with the ventral root each 

 dorsal root bears a ganglion whose cells give rise to most of 

 the fibers of which that root is composed as well as the 

 sensory fibers of the peripheral portion of the nerve. At the 

 outer end of the ganglion each nerve divides into a dorsal 

 and a ventral branch. Each of these contains both sensory 

 and motor fibers. The dorsal branches divide into several 

 nerves which supply the skin and muscles of the dorsal 

 side of the body ; the ventral branches supply the ventral 

 musculature and limbs ; a short communicating nerve con- 

 nects each ventral branch with the trunk of the sympathetic 

 system. The distribution of the spinal nerves, exclusive 

 of their dorsal rami, is as follows : — 



The first nerve emerges between the first and second 

 vertebrae ; its principal branch, the hypoglossal, innervates 

 the tongue and sev^eral of the muscles attached to the hyoid ; 

 a short communicating branch joins the second nerve. 



The seeond pair of nerves emerges between the second 



and third vertebrae. This pair, which is of large size, 



forms with branches received from the first and third 



pairs the braehial plexus, from which the nerves arise 



u 



