444 



STRUCTURE OF VERTEBRATA. 



In the cord it is usually easy to distinguish an external region of white 

 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 j. is a contraction for sensory or afferent, i.e. transmitting impulses 

 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. 



