406 THE NERVOUS SYSTEM AND ORGANS OF SENSE 



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 out from the body covering. At the base of these hairs 

 are found nerve cells which send a nerve fiber inward to the central nerv- 

 ous system. 



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 cor- 

 puscles, are found there, each 

 inclosed in a sheath, or capsule, 

 of connective tissue. Inside is 

 a comphcated nerve ending, and 

 nerve fibers pass inward to the 

 central nervous system. The 

 number of tactile corpuscles pres- 

 ent 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 acom-acy with which the two points 

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

 need only be separated by ^\ of an inch to be so 

 distinguished. In the small of the back, a dis- 

 tance of 2 inches may be reached before tlie 

 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 Idnds 

 of nerve fibers exist in the skin, which give distinct 

 sensations of heat and cold. These areas 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. 



^Taste Organs. — The surface of the tongue is 

 folded into a number of little projections known 

 as papUlas. These may be more easily found on your own tongue if a 

 drop of vinegar is placed on its broad surface. In the folds, between 



JSI 



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

 papillas, containing a tactile corpuscle ; i , 

 papillae containing blood vessels. (After 

 Benda.) 



C 



A, isolated taste bud, 

 from whose upper free 

 end project the ends of 

 the taste cells; B, sup- 

 porting or protecting 

 cell ; C, sensory cell. 



