PRODUCTION OF SENSATIONS 341 



demonstrated by a study of the manner in which we 

 acquire knowledge. "Without sensation there can be no 

 thought." 



Steps in the Production of Sensations. The steps in the 

 production of sensations are not essentially different from 

 those in the production of reflex action/ First of all, 

 external stimuli act upon the fiber terminations in the 

 sense organs, or elsewhere, starting impulses in the 

 neurons. These pass into the central nervous system and 

 there excite neurons which in turn discharge impulses 

 into the cerebrum. The result is to arouse an activity of 

 the mind a sensation. The steps in the production of 

 any special sensation naturally involve the following parts : 



1. A sense organ where the terminations of the neurons 

 are acted upon by the stimulus. 



2. A chain of neurons which connect the sense organ 

 with the brain. 



3. The part of the cerebrum which produces the sen- 

 sation. 



Sense Organs. The sense organs are not parts of the 

 afferent neurons, but are structures of various kinds, in 

 which the neurons terminate. Their function is to enable 

 the sensation stimuli to start the impulses. By directing, 

 concentrating, or controlling the stimuli, the sense organs 

 enable them to act to the best advantage upon the neurons. 

 When it is recognized that such widely different forces as 

 light waves, sound waves, heat, pressure, and odors are 

 enabled by them to stimulate neurons, the importance of 

 these organs becomes apparent. As would naturally be 

 inferred, the construction of any sense organ has particular 

 reference to the nature of the stimulus which it is to re- 

 ceive. This is most apparent in the sense organs of sight 

 and hearing. 



