268 APPLIED PHYSIOLOGY 



their whole length without affecting anything in their 

 course. 



480. Kinds of nerves. Each thread of a nerve trans- 

 mits influences in only one direction. Some threads carry 

 influences only from the cells of the body to the central 

 nerve cells. Because they often produce sensation they 

 are called sensory nerves. Other threads carry orders for 

 action from the nerve cells to the cells of the body and 

 are called motor nerves. Most nerves are made up of 

 both sensory and motor threads, but some are wholly sen- 

 sory and others wholly motor. There is no difference in 

 their appearance. 



481. Distribution of sensory nerves. Nearly every cell in 

 the body, except in the epidermis and blood, is probably in connection 

 with a sensory nerve, and, through it, is in touch with the central nerve 

 cells. The endings of the nerves are so abundant in the skin just 

 beneath the epithelium, that the point of a fine needle cannot be thrust 

 in without producing pain. In the ends of the fingers they are more 

 numerous than in any other part of the body. The muscles and internal 

 organs have fewer sensory nerves than the skin, so that a cut may be 

 continued into the deeper parts with but little pain. 



482. Kinds of sensations. The cells are continually 

 sending impulses to the central nerve cells telling of 

 their needs, as of food or rest. These impulses often give 

 rise to feelings which may seem to pervade the whole body. 

 Then they are called common sensations. Some are pleas- 

 ant and some are disagreeable. The natural unreasoning 

 inclinations to gratify desires aroused by the needs of the 

 body are instincts. 



When something outside the body is acting upon the 

 nerves it produces a feeling or impression of which a per- 

 son is usually aware. By means of these sensations the 

 mind forms definite ideas of the surroundings of the body, 



