410 THE NERVES AND THE SENSE ORGANS 



minds, must interpret them: we must know by our judg- 

 ment and our "common sense" what the impulses mean. 

 We say that the messages that come to the brain produce 

 sensations in the mind. The ability to interpret the 

 messages correctly comes to the mind through experience. 

 When the neurons in the finger carry to the brain an im- 

 pulse that is started by a needle point, the mind has the 

 sensation of touch, or of pain. The usual impulses sent 

 to the brain from the stomach produce in the mind the 

 sensation of hunger; the usual impulses sent by the 

 throat produce the sensation of thirst. We call the sensa- 

 tions that come from impulses within the body, general 

 sensations; those coming from im- 

 pulses outside of the body, we call 

 special sensations. 



410. Touch. In the skin, and in 

 the mucous membrane of the tongue, 

 are little elevations ("papillae"; cf. 

 392) that contain the touch corpus- 

 cles (Fig. 296). Inside the touch cor- 

 puscles are the ends of the nerve 

 Fig. 296. fibers. When we touch an object, the 



A Touch corpuscle. c h anges { n fa pressure against the 



touch corpuscles set up impulses in the nerve fibers. We 

 feel the object as smooth or rough, large or small, sharp 

 or blunt, hot or cold, hard or soft, according to the 

 account which the afferent neurons send to the brain. 



The sense of touch is not equally good everywhere, as we well know. 

 Its acuteness depends on the number of touch corpuscles in a given 

 area. We can measure its acuteness by means of the tips of a pair of 



