THE SPECIAL SENSES 



227 



sense, or the sense of touch. The special organs of this 

 sense are usually simple hairs or papillae connecting with a 

 nerve. These tactile hairs or papillae may be distributed 

 pretty evenly over most of the body, or may be mainly con- 

 centrated upon certain parts in crowded groups. Many of 

 the lower animals have projecting parts, like the feeling 

 tentacles of many marine invertebrates, or the antennas 

 (feelers) of crabs and insects, which are the special seat 

 of the tactile organs. Among the vertebrates the tactile 

 organs are either like those of the invertebrates, or are 

 little sac-like bodies of connective tissue in which the 

 end of a nerve is curiously folded and convoluted (Fig. 

 141). These little touch corpuscles simply lie in the cell 

 layer of the skin, covered over thinly by the cuticle. Some- 

 times they are simply free, branched 

 nerve-endings in the skin. These 

 tactile corpuscles or free nerve-end- 

 ings are especially abundant in those 

 parts of the body which can be best 

 used for feeling. In man the fin- 

 ger-tips are thus especially supplied ; 

 in certain tailed monkeys the tip of 

 the tail, and in hogs the end of the 

 snout. The difference in abundance 

 of these tactile corpuscles of the skin 

 can be readily shown by experiment. 

 With a pair of compasses, whose 

 points have been slightly blunted, 

 touch the skin of the forearm of a 



person who has his eyes shut, with the points about three 

 inches apart and in the direction of the length of the arm. 

 The person touched will feel the points as two. Eepeat 

 the touching several times, gradually lessening the dis- 

 tance between the points. When the points are not more 

 than an inch to an inch and a half apart, the person 

 touched will feel but a single touch that is, the touching 



FIG. 141. Tactile papilla of 

 skin of man. , nerve. 

 After KOELLIKER. 



