472 



STRUCTURE OF VERTEBRATA. 



In (lie cord it is usually easy to distinguish an external region of while 

 matter, composed of medullated nerve-fibres, and an internal region of 

 grey matter, containing ganglionic cells, and non-medullated fibres. 



The arrangement of the grey matter, together with the longitudinal 

 fissures, give the cord a distinct bilateral symmetry, which is sometimes 

 obvious at a very early stage. 



The brain substance is also composed of grey and white matter, 

 but there, at any rate in higher forms, the arrangement is very 

 complicated. 



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 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, i.e. transmitting impulse 

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

 i.e. transmitting impulses from the centre to the body. 



