CRANIAL NERVES. 



441 



Cranial Nerves. 



The origin and distribution of the cranial nerves may be 

 summarised as follows : — 



The fourth or pathetic nerve is peculiar among motor nerves in that it arises from 

 the extreme dorsal summit of the brain, between the mid and hind brain, from the 

 region known as the "valve of Vieussens." In Fishes, the seventh nerve is mainly 

 a nerve of special sense ; in higher Vertebrates it has lost most of lis sensory branches, 

 and become chiefly motor. 



*■" The letter .r. is a contraction for sensory or afferent, i.e.., transmitting impulses 

 from a sensitive area to the centre; and ;«. is a contraction for motor or eff'erent, i.e., 

 transmitting impulses from the centre to ihe body. 



There is much uncertainty in regard to the morphological value of 

 the various cranial nerves, but the following conclusions are impor- 

 tant :— 



(i) The nerves arise either as outgrowths of the central system, or as 

 specialisations of peripheral cells. Each spinal nerve has two roots — 

 a dorsal and a ventral, but in most cases at least a cranial nerve 

 has primitively a single dorsal root developing from a neural ridge 

 of the dorsal surface of the I^rain. In many cases this root divides 



