THE NERVOUS SYSTEM 289 



animal as by a man, and unlike the special sensations, the 

 meaning of general sensations does not have to be learned, 

 as the memory of them owing to inherited habit seems to 

 reside in the nervous system. General sensations are 

 sometimes called instincts. 



528. Hunger and Thirst Thirst seems to be located in 

 the mouth, yet moistening the mouth satisfies it for a short 

 time only. On the other hand, if water is introduced into 

 the body through the intestine or veins, the thirst ceases, 

 even if the mouth has not received water. Hunger appears 

 to be located in the stomach, yet it cannot be satisfied by 

 filling the stomach if the food is innutritious or remains 

 undigested. 1 



529. Touch. The four special senses of sight, sound, 

 smell, and taste are located in special organs. Touch is 

 located in all parts of the body. When an object touches 

 the skin, an impulse is taken to the brain (Figs. 196, 

 197, 3). There it gives rise to a sensation of touch, tem- 

 perature, pain, or weight. Four kinds of sensations are thus 

 included in the general name of touch. Touch strictly 

 gives only such ideas as those of size, shape, location, 

 smoothness, hardness, and dampness. The ends of the 

 fingers can distinguish two points as separate points if 

 they are only -^ of an inch apart, while if the two points 

 are applied to the back, they feel as if they were one point 

 until they are separated two inches. 



530. The covering of epidermis not only protects the 

 nerves and the other delicate organs in the true skin, from 

 injury, but also modifies any impression, so that in pro- 

 ducing a sensation it is spread over several nerve endings 

 instead of one, and is thus turned into a gentle instead of 

 a painful sensation (Fig. 3). 



1 If nutritious food is introduced into the body through the intestine in a 

 form ready to be absorbed, the feeling of hunger will soon pass away, although 

 the stomach is still empty. 



U 



