CHAPTER XIV 
INSTINCT AND REASON 
125. Irrvitability—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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