g THE FROG. PHYLUM CHORDATA 



SPINAL NERVES 



Tlu' ten pairs of spinal nerves pass out between the vertebra 

 to be distributed over the body. Each nerve is surrounded at its 

 origin by a soft, white, calcareous concretion. Every nerve 

 arises by two roots, a dorsal and a ventral {Fig. 300), and the 

 dorsal root bears a small swelling, the dorsal root ganglion. 

 Just outside the backbone the two roots join, and the nerve 

 thus formed proceeds at once to divide, giving rise to (a) a short 

 dorsal branch to the muscles and skin of the back, {b) a long 

 and conspicuous ventral branch to the muscles and skin of 

 the sides and ventral surface of the trunk, and in some cases to 

 the hmbs, and (c) a small ramus communicans to the sympathetic 

 system. The dorsal root is also called afferent or sensory because 



Fig. 300. — A diagram of the origin of the spinal nerves of the frog. 



cm., Centrum ; d.br., dorsal branch of the nerve ; d.r., dorsal root ; d.r.^., dorsal root ganglion ; n.a., neura 

 arch ; r.c, ramus communicans ; v.br., ventral branch; v.c, vertebral canal ; v.r., ventral root. 



impulses pass inwards along it to the spinal cord and produce, 

 among other effects, sensation ; the ventral is called similarly 

 efferent or motor because impulses pass outward along it and 

 produce, among other effects, contraction of muscles and thus 

 movements. These functions are proved by the fact that cutting 

 the dorsal root deprives the parts supplied by its nerve of the 

 power to send impulses to the central nervous system, while 

 cutting the ventral root paralyses the same parts. Each of the 

 branches contains elements derived from both dorsal and ventral 

 roots. Th(i course of the dorsal branches and rami communicantes 

 is much the same in all cases, but that of the ventral branches 

 differs greatly in different nerves and must now be followed. 



The first spinal nerve is known as the hypoglossal, and corre- 

 sponds to the twelfth cranial nerve of the mammal. It leaves the 

 neural canal between the first and second vertebrae, curves round 

 the throat, turns forward below the mouth (Fig. 302) and proceeds 



