490 



STRUCTURE OF VERTEBRATA. 



In higher Vertebrates there are two others, the spinal accessory (n) and the hypo- 

 glossal (12). 



The fourth or pathetic nerve is peculiar among motor nerves in that it appears to 

 arise 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 its sensory 

 branches, and become chiefly motor. 



* The letter s. is a contraction for sensory or afferent, /.*. transmitting impulse 

 from a sensitive area to the centre ; and m. is a contraction for motor or efferent, i.e. 

 transmitting impulses from the centre to the body. 



There is much uncertainty in regard to the morphological value of 

 the various cranial nerves, but the following conclusions may be 

 stated : 



(1) Like the spinal nerves, the cranial nerves are primarily seg- 

 mental, and there are probably about seven of them, three pro-otic 

 and four metotic. The olfactory and optic nerves are quite by 

 themselves and not segmental. 



(2) Like the spinal nerves, the cranial nerves have primarily two 

 roots, a dorsal and a ventral, but the ventral roots do not join the 



