SPltfAL CORD CRANIAL NERVES. 489 



Spinal cord. After the formation of the brain vesicles, 

 the remainder of the medullary canal forms the spinal 

 cord. 



The canal is for a time continuous posteriorly with the 

 food canal beneath, so that a ID -shaped tube results. The 

 connection between them is called the neurenteric canal 

 (Fig. 254, ne.c.\ and though it is only temporary, its frequent 

 occurrence is of much interest. 



The wall of the medullary canal becomes very much 

 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. 



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: 



[TABLE. 



