THE ANATOMY OF THE NERVOUS SYSTEM 

 FROM THE STANDPOINT OF DEVELOP- 

 MENT AND FUNCTION 



CHAPTER I 



THE ORIGIN AND FUNCTION OF THE NERVOUS SYSTEM 



IRRITABILITY and conductivity, which, as every biological student knows, 

 are two of the fundamental properties of protoplasm, reach their maximum 

 development in the highly differentiated tissue of the nervous system. Indeed, 

 it is in response to the need for increased sensitiveness to stimuli and for better 

 transmission of the impulses aroused by them that the nervous system has 

 developed and been perfected in the long process of evolution which has cul- 

 minated in man. 



When an ameba is touched with a pointed glass rod it moves away from 

 the source of stimulation. Changes are initiated in the superficial protoplasm 

 which are transmitted through the unicellular organism, resulting in a flowing 

 out of pseudopodia on the opposite side. Through a continuation of this stream- 

 ing motion the entire organism moves forward. Thus the relatively undif- 

 ferentiated living substance of which it is composed receives the stimulus, 

 transmits the resulting disturbance, and carries out the appropriate response. 



When in the place of unicellular organisms we study simple metazoa, the 

 sea-anemones for example, we find that considerable differentiation has occurred 

 among the component cells. A cuticle has formed, designed to protect the 

 subjacent parts from the action of the surrounding objects, while other cells 

 have differentiated in the direction of contractile elements or muscle cells. 

 Because the general body surface has been adapted to cope with the environ- 

 ment it becomes necessary to have certain cells at the surface which are sensi- 

 tive to environmental changes. These sensory elements are able to transmit 

 the waves of activation developed in them directly to the subjacent muscle 

 cells. But in higher animals, because of the large size of the body and the 



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