32 



ANATOMY OF THE CENTRAL NERVOUS SYSTEM. 



of the sensory nerves are irritated, and especially upon the relationship 

 between the cells which form the motor apparatus excited. There is much 

 evidence in favor of the view that such relationships, when once established in 

 the course of evolution, are afterward inherited; so that the structure of a 

 single nerve-center is practically the same for each individual, and that, 

 through this inherited apparatus, numerous apparently complicated actions 

 are made possible once for all. But there are experiences which teach that 



Fig. 11. — Schema of a very simply constructed nervous apparatus, com- 

 prising motor and sensory nerve and center; adapted for explaining the simplest 

 reflexes. S, Sensory cell whose dendrite brings to it impressions from the skin, 

 while its neurite, or neuraxon, passes to the nerve-center, where it influences M 

 and A. M, Motor cell for the upper muscle. A, Association-cell which transmits 

 stimuli received by 8 on to M', where it takes effect either simultaneously or 

 subsequently. M', Motor cell for the lower muscle. 



in certain portions of the nervous system constantly new associations are 

 being formed by exercise. The central nervous system would, then, consist 

 of one part, which is congenital and arises from the primordial racial exercise 



