THE NERVOUS SYSTEM OF MAN I49 



duction. This minute bit of protoplasm is 

 adapted to its environment and responds to light, 

 chemical and mechanical stimuli. Here are found 

 the elementary characteristics of the law of sen- 

 sation. No specific structures are differentiated 

 for special stimuli, or for conducting the effect of 

 such stimuli or for reflex action. But rather the 

 entire protoplasm participates in these processes 

 just as it does in reproduction or metabolism. 



But just as soon as one comes to examine the 

 more complex animals, like the worms or crabs, 

 different kinds of tissues and organs are the first 

 things to be noted. Extending throughout the 

 length of the body of such animals is found a chain 

 of ganglia connected by strands of nerve fibers. 

 (Figs. 48-49.) These ganglia are composed of 

 nerve cells that either send their fibers to the mus- 

 cles or from ganglia to ganglia. In the skin and 

 special sense organs of these same animals are 

 found several kinds of cells that are differentiated 

 to respond to environmental stimuli. Through 

 these nerve cells, the receptors, the animal is able 

 to appreciate certain changes. All of the tissues 

 and organs of such animals are regulated by these 

 nerve cells. When the nerve cells are thus or- 

 ganized and occupy such a definite position in the 

 body, one is able to say that the worm has a 

 central nervous system. 



In all of the vertebrates, the central nervous 



