CHAPTEE XIY 



INSTINCT AND REASON 



125. Irritability. All animals of whatever degree of 

 organization show in life the quality of irritability or re- 

 sponse to external stimulus. Contact with external things 

 produces some effect on each of them, and this effect is 

 something more than the mere mechanical effect on the 

 matter of which the animal is composed. In the one- 

 celled animals the functions of response to external stimu- 

 lus are not localized. They are the property of any part of 

 the protoplasm of the body. Just as breathing or digestion 

 is a function of the whole cell, so are sensation and response 

 in action. In the higher or many-celled animals each of 

 these functions is specialized and localized. A certain set 

 of cells is set apart for each function, and each organ or 

 series of cells is released from all functions save its own. 



126. Nerve cells and fibers. In the development of the 

 individual animal certain cells from the primitive external 

 layer or ectoblast of the embryo are set apart to preside 

 over the relations of the creature to its environment. 

 These cells are highly specialized, and while some of them 

 are highly sensitive, others are adapted for carrying or 

 transmitting the stimuli received by the sensitive cells, and 

 still others have the function of receiving sense-impressions 

 and of translating them into impulses of motion. The 

 nerve cells are receivers of impressions. These are gathered 

 together in nerve masses or ganglia, the largest of these 

 being known as the brain, the ganglia in general being 

 known as nerve centers. The nerves are of two classes. 



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