206 THE ELEMENTAEY NERVOUS SYSTEM 



prising to find that the muscle occupies much of the in- 

 terior of the body and that in the nerve chains the cell 

 bodies are on the side next the skin and the transmitting 

 trunks on that away from this layer (Fig. 49). The same 

 sequence occurs in the vertebrates except that in this in- 

 stance the epithelial layer of the skin instead of being 

 left on the exterior of the animal is infolded and lines the 

 central canal of the nervous system whose surface is 

 derived from the exterior of the animal. Proceeding 



from this surface (Fig. 50) 

 with its epithelial covering 

 toward the other surface of the 

 spinal cord the grey matter or 

 region of the ganglion cells and 

 fibrillar material is first passed 

 through and then the white 

 matter, the region of nerve 

 fibers where transmission is 



, , . ,, ,, , . ,-p-i ,1 



tllC Ciller lUnCtlOn. 1JQUS 1116 



... , , 

 SeqU6nC6 01 USSU6S eStaDilSneCl 



in the receptor-effector systems 

 of ccelenterates impresses itself on the differentiated cen- 

 tral organs of the higher invertebrates and even the 

 vertebrates. 



When an attempt is made to follow out the differen- 

 tiation of the nervous elements in the evolving central 

 nervous organ, it is necessary to begin with some of the 

 processes that show themselves first in the nerve-net. As 

 already pointed out, the nerve-net in its most typical con- 

 dition exhibits diffuse nervous transmission and in this 

 respect is in strong contrast with the central nervous or- 

 gans of the higher animals whose transmission capaci- 

 ties are highly specialized and limited. A kind of polar- 



8e c F t!on 0? ^?cord% t f r rv v e e r r t 8 e! 



brate (salamander); c, central canal; 

 e, epithelium; g, grey substance com- 

 posed of ganglion cells and fibrillar 

 material; u>, whits substance or nerve 



