PRODUCTION OF SENSATIONS 



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In some way, not understood, the mind associates the 

 sensation with the part of the body from which the im- 

 pulses come. Pain, for example, is not felt at the brain 

 where the sensation is produced, but at the place where 

 the injury occurs. This association, by the mind, of the 

 sensations with different parts of the body, is known as 

 " localizing the sensation." 



Sensation Stimuli. While the sensations are dependent 

 upon the afferent impulses, the afferent impulses are in 

 turn dependent upon causes outside of the nervous system. 

 If these are removed, the sensations cease and they do not 

 start up again unless the exciting influences are again 

 applied. Any agency, such as heat or pressure, which, 

 by acting on the neurons of the body, is able to produce a 

 sensation, may be called a sensation stimulus. It has per- 

 haps already been observed that the stimuli that lead to 

 voluntary action, as well as those that produce reflex action 

 of the muscles, cause sensations at the same time. From 

 this we may conclude that sensation stimuli are the same 

 in- character as those that excite motion. On the other 

 hand, it should be noted that sensations are constantly 

 resulting from stimuli that are of too mild a nature to 

 cause motion. 



Classes of Sensations. Perhaps as many as twenty dis- 

 tinct sensations, such as pain, hunger, touch, etc., are 

 recognized. If these are studied with reference to their 

 origin, it will be seen that some of them result from the 

 action of definite forms of stimuli upon the neurons ter- 

 minating in sense organs ; while the others, as a rule, arise 

 from the action of indefinite stimuli upon neurons in parts 

 of the body that do not possess sense organs. The mem- 

 bers of the first class and these include the sensations of 

 touch, temperature, taste, smell, hearing, and sight are 



