CEANIAL NERVES 



179 



THE CRANIAL NERVES 



The nerves which arise from the brain and pass out through the 

 foramina in the skull are known as the cranial nerves. While in a 

 general way they resemble the spinal nerves, they have been special- 

 ized and modified in many respects in correspondence with the 

 speciaUzation of the head itself, some consisting of sensory fibres 

 alone, some of motor fibres only, while others are mixed, that is, con- 

 tain both kinds of fibres. One striking difference is that none of the 



vtaceral motor 

 somcxtic motor 



vlaceral sensory 



somatic sensory 



sympathetic 



Fig. 188.— Diagram of the relations of the sympathetic system, based on Huber. 

 The character of the different fibres is shown by conventional lines, bv, blood-vessel; 

 eg, chain ganglion; d, dorsal ramus; dr, dorsal root; g, gland; gr, gray ramus; pc, Pacinian 

 corpuscle; pg, peripheral ganglion; pvg, prevertebral ganglion; sg, ganglion of dorsal root; 

 st, sympathetic trunk; v, ventral ramus; vi, visceral ramus; vr, ventral root; wr, white 

 ramus. 



cranial nerves has two roots, and while those which carry sensory 

 fibres have a ganglion near the root, the purely motor nerves have no 

 such enlargement. In the earUer stages of development one of these 

 nerves (the hypoglossal) has both dorsal and ventral roots, but with 

 later stages the dorsal root and its ganglion are lost. In the ichthy- 

 opsida there are ten cranial nerves; in the amniotes two more are 

 added at the hinder end of the brain, giving a total of twelve in 

 the higher groups. 



