CHAPTER XI 

 THE SPECIAL SENSES 



336. Sensation. Everybody knows that the tongue is 

 sensitive to taste, the nose to smell, the ear to sound, and 

 the eye to light. In other words, each one of our sense 

 organs has its own peculiar structure and is sensitive to 

 some special agency called a stimulus. These stimuli give 

 rise to nerve impulses or sensations which are transmitted 

 by nerve fibers to the central nervous system. 



Exactly how feeling, or sensation, leaves its imprint on the 

 cells of the brain and rises to consciousness is not known. 



337. General Sensations. Some sensations, or feelings, 

 are of a very general character. Thus, we have a feeling of 

 hunger or thirst, indicating a need of food or drink. To 

 these may be added the sensations of pain, tickling, and 

 itching. Other general sensations, such as those of fatigue, 

 restlessness, and faintness, spring up within us in some 

 mysterious way, sometimes without any obvious cause. 



338. Sensations resulting from an Outward Agency. The 

 great majority of sensations, however, result from some out- 

 ward stimulus or agency. Thus, if we hear a child cry or a 

 bird sing, we have a sensation of sound. If we put a piece 

 of sugar on the tongue, hold a rose to the nostrils, or prick 

 the skin with a needle, certain sense organs receive the 

 impressions. The sensory nerves carry these impressions 

 to the brain, and we become conscious of a sensation. 



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