PART II 



THE CENTRAL NERVOUS SYSTEM 



By means of the nervous system an animal reacts to 

 changes in its environment. The physical form which the 

 reaction takes is the expression of the ability of the 

 animal to overcome the alteration in the external circum- 

 stances ; the reaction is purposive and protective. Evolu- 

 tion from lower to higher forms is distinguished by nothing 

 so much as by an increase in the variety both in degree 

 and in kind of the responses which the organism is able 

 to make. 



The earliest formation of cells speciahsed to respond 

 to stimuh is seen in Hydra (Fig. 40, I), where certain 

 epithelial cells are endowed with a high degree of irrita- 

 bihty on their superficial surface and with a high degree of 

 contractihty on their deep surface — this being expanded 

 to form a contractile plate. The next stage is the migra- 

 tion of the contractile element away from the epithelium 

 so that it may be exposed to the environment. Accompany- 

 ing this migration is a separation of the single responsive 

 cell into two, one specially endowed with irritabihty, the 

 other with an exalted contractihty (Fig. 40, II). The con- 

 nection between the two cells is by a strand of the irritable 

 cell — the first appearance of a nerve-fibre. In a third 

 stage this strand acquires a nucleus of its own and becomes 

 an independent cell (Fig. 40, III). We now have a con- 

 tractile cell responding to a stimulus arising in another cell 

 situated at a distance from it. The fourth stage consists 

 in the estabhshment of a means of co-ordination between 



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