BODY CONTROL AND HABIT FORMATION 357 



the body only ends with sleep or unconsciousness. The afternoon 

 nap, snatched by the brain worker, gives him renewed energy for 

 his evening's work. It is not hard application to a task that 

 wearies the brain ; it is continuous work without rest. 



THE SENSES 



Touch. — In animals having a hard outside covering, such as certain 

 worms, insects, and crustaceans, minute hairs, which are sensitive to touch, 

 are found growing oiit from the body covering. At the base of these hairs 

 are found neurones which send axons inward to the central nervous sj^stem. 

 Organs of Touch. — In man, the nervous mechanism which governs 

 touch is located in the folds of the dermis or in the skin. Special nerve 

 endings, called the tactile corpus- 

 cles, are found there, each in- _6 

 closed in a sheath or capsule of 

 connective tissue. Inside is a 

 complicated nerve ending, and 

 axons pass inward to the central 

 nervous system. The number 

 of tactile corpuscles present in a 

 given area of the skin determines 

 the accuracy and ease with which 

 objects may be known by touch. 



If you test the different parts 

 of the body, as the back of the 

 hand, the neck, the skin of the 

 arm, of the back, or the tip of 

 the tongue, with a pair of open 



dividers, a vast difference in the accuracy with which the two points 

 may be distinguished is noticed. On the tip of the tongue, the two points 

 need only be separated by 2V of ^^ iiich to be so distinguished. In the 

 small of the back, a distance of 2 inches may be reached before the dividers 

 feel like two points. 



Temperature, Pressure, Pain. — The feeling of temperature, pressure, 

 and pain is determined by different end organs in the skin. Two kinds 

 of nerve fibers exist in the skin, which give distinct sensations of heat and 

 cold. These nerve endings can be located by careful experimentation. 

 There are also areas of nerve endings which are sensitive to pressure, 

 and still others, most numerous of all, sensitive to pain. 



Nerves in the skin: a, nerve fiber; 6, tactile 

 papillae, containing a tactile corpuscle ; 

 c, papillae containing blood vessels. 

 (After Benda.) 



