IRRITABILITY 115 



upon the structure and arrangement of the muscles. 

 But it was always found that the reaction was very, 

 very slow in following the stimulation. This means 

 that if the animal had to depend on the direct stimula- 

 tion of the muscle its life would be in danger because 

 the reaction could not come quickly enough. When 

 animals as highly developed as dogs are deprived of the 

 use of the nerves, they are kept alive with a great deal 

 of difficulty because of the slowness with which the body 

 responds to changes in temperature. 



These experiments show that human beings are utterly 

 dependent upon the nervous system, for without the 

 nerves the muscles would respond to stimulation too 

 slowly to maintain life. But while we recognize the 

 importance of the nerves we must realize wherein it lies. 

 They are in no sense responsible for movements. This 

 is the work of the muscles. They cannot initiate a move- 

 ment. This is the result of a stimulus. But they are 

 extremely sensitive and they conduct a stimulus with 

 wonderful rapidity. They are therefore effective as con- 

 ductors of stimuli. 



Sense Organs. — All nerves are sensitive to chemical 

 and physical stimuli, but nerves connected with the 

 special senses are especially adapted to particular 

 stimuli. The most widely distributed of the senses is 

 that of touch, or sensitiveness to physical contact with 

 external objects. It is found in all animals and it is 

 generally distributed over the whole body. The other 

 senses are restricted to special areas on the body and, 

 in their complete development, to higher animals. The 

 taste nerves are stimulated by substances in solution, 



