440 STRUCTURE OF VERTEBRATA. 



thickened, the roof and floor grow less rapidly, and thus the 

 cord is marked by ventral and dorsal longitudinal furrows. 

 At the same time, the canal itself is constricted, and persists 

 in the fully formed structure only as a minute canal lined by 

 ciliated epithelium, and continuous with the cavity of the 

 brain. It can hardly be said to have any function ; it may 

 be simply the result of a developmental necessity. But 

 Sutton and Gaskell have independently suggested that the 

 central canal of the nervous system represents a disused 

 alimentary passage, which has been replaced by surrounding 

 nervous material, and which ceased to be functional when 

 the permanent gut became a tube open at each end. This 

 suggestion, however, if indeed it was serious, has not been 

 accepted by any morphologist. 



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. 



Concerning the development of the peripheral nervous 

 system there is far less certainty than with regard to the 

 central. 



The motor nerves, even the motor parts of the cranial 

 nerves, appear to arise as fibrillar or as cellular outgrowths 

 of the central nervous system. All the sensory nerves take 

 their origin from peripheral ganglia, and the root fibres 

 grow thence into the central system. According to some 

 embryologists, certain sensory peripheral nerves, e.g., the 

 lateralis of the tenth, arise in situ from the sensory 

 epithelium. 



[TABLE. 



