The Senses. 249 



Pressure on the skin affects these nerve endings, and 

 starts impulses that pass along the sensor fibers, through 

 the spinal cord, to the brain, and give us sensations of 

 touch. If a nerve fiber is touched, not at the end, but 

 somewhere along its course, we get a sensation, not of 

 touch, but of pain. 



The Sense of Touch. Of the special senses the most 

 general is that of touch. Seeing and hearing, taste and 

 smell, belong to very limited parts of the outside of the 

 body, but we have the power of feeling all over the surface 

 of the body. Except in the mouth and nose, we get little, 

 if any, sense of touch from any organ but the skin. The 

 lining of the digestive tube and the internal organs gener- 

 ally lack this sense. 



The Pressure Sense. The sense of touch, proper, is 

 strictly a pressure sense. If we test the skin to find what 

 regions are able to detect the least pressure, it is found 

 that the forehead is most sensitive, and nearly equally so 

 are the temples, back of the hand, and forearm. 



Location of Touch Sensations. Each small spot of skin 

 has its own nerve endings and each nerve fiber connects 

 with a particular part of the gray matter of the brain. The 

 brain can therefore tell where each nerve current came 

 from, and thus we locate a sensation. 



Accuracy in locating Touch Sensations. The accuracy 

 varies, and is ordinarily keenest where the nerves are most 

 numerous. Where the sense of locality seems to be im- 

 proved by cultivation, this appears to be due to keener per- 

 ception in the brain cells, and not to changes in the nerves 

 or nerve endings. This is shown in the fact that if the 

 fingers of one hand become more skilled in touch by prac- 



