CHAPTER XXV. 

 THE NERVOUS SYSTEM. 



The General Functions and Structure of the Nervous System. 

 When a unicellular organism, such as the amoeba, is stimulated 

 it responds by a movement because its protoplasm possesses 

 among its other properties those of excitability, conductivity and 

 contractility. In the case of multicellular organisms, some cells 

 are set aside for the assimilation of food, others for movement, 

 others to receive stimuli from the outside, others to compose 

 tougher protective tissues on the surface, and still others, in 

 many animals, to compose definite organs of offense. This loca- 

 tion of specific functions in certain group of cells makes it neces- 

 sary, for the welfare of the organism as a whole, that some means 

 of communication be provided between the different parts of the 

 animal, for otherwise the cells which are occupied, say, in ab- 

 sorbing food, would be unable to move away when some destruc- 

 tive agency approached them, and indeed the moving (muscle) 

 cells could never know when they ought to become active. Tn 

 some of the lower organisms these messages are carried by chemi- 

 cal substances present in the fluids that bathe the cells. These 

 belong to the group of hormones which we have already studied 

 in connection with the ductless glands (see p. 124). The re- 

 sponses mediated in this way are, however, too slow for the quick 

 adaptation which it is necessary that the organism should un- 

 dergo in its battle for life. If it had to depend on such a mech- 

 anism alone, the organism would already be within the clutches 

 of its enemy before it could make any attempt to defend itself. 



Some more sensitive mechanism, both for receiving and 

 for transmitting impulses throughout the organism, becomes nec- 

 essary. This is furnished by the nervous system, which, in its 

 simpler form, consists of a cell on the surface of the animal so 

 specialized that it responds to changes in the environment. This 



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