NERVES 269 



and so the feelings are called special sensations. Unlike 

 common sensations, the meaning of the sensations must 

 be learned. 



483. Common sensations. Hunger, thirst, and fatigue 

 are the usual common sensations felt by the mind. 

 Hunger seems to be located in the stomach. If a sub- 

 stance swallowed is not nutritious, hunger soon returns, 

 even if the organ is filled full. On the other hand, if 

 nutritious food is introduced into the body through the 

 intestine, the feeling of hunger will pass away, even 

 though the stomach remains empty. Some persons suffer- 

 ing from indigestion are always hungry, though they eat 

 enormously. But the food is not digested, and does not 

 reach the cells, and there is always a feeling of hunger. 



Thirst seems to be located in the mouth. Moistening 

 the mouth allays it but for a moment only, while if water 

 is introduced into the intestine or veins, the thirst disap- 

 pears, even though the mouth receives no water. 



The amount of common sensations is small compared 

 with similar impulses which we do not feel. Every cell 

 is continually sending tiny messages of its needs, and the 

 central nerve cells promptly respond. 



484. Special sensations. Knowledge of the outside 

 world is gained by means of the touch, sight, hearing, 

 smell, and taste. Of these, touch is located in all parts 

 of the body, while special organs are needed to enable the 

 nerves to catch the delicate impressions of sight, sound, 

 smell, and taste. 



485. Sensations of touch. When an object touches 

 the epithelium of the skin, it causes an impulse to travel 

 to the central nerve cells as a sensation either of touch, 

 temperature, pain, or weight. All these sensations are 

 included under the general term of touch. Touch proper 



