196 PHYSIOLOGY FOR BEGINNERS CHAP. 



organ for hearing. The sensory nerves of each sense organ 

 end in a special part of the organ, which is therefore the 

 essential part, and from this the impulses pass along the 

 nerve, while the other parts of the organ protect the essential 

 part, or collect and transmit to it the external agencies which 

 cause the sensation. 



Touch. In some of the papillae of the dermis a sensory 

 nerve ends in an oval structure consisting mainly of nucleated 

 cells, called a tactile corpuscle. In addition to the tactile 

 corpuscles and other structures similar to them, which all lie 

 in the dermis, some exceedingly fine fibrils from the sensory 

 nerves in the dermis can be traced into the deeper layers of 

 the epidermis, ending between, or in connection with, some of 

 the cells of the epidermis. The endings of the sensory nerves, 

 both the tactile corpuscles and the endings in the epidermis, 

 are covered externally by either the whole of the epidermis or, 

 at least, its outer or corneous part, so that there is never any 

 actual contact of the thing we touch with the nerve itself. 

 The proper sense of touch cannot arise by direct contact with 

 the nerve ; there must always be intervening the particular 

 epithelial cells in connection with which the nerve ends. 

 When part of the skin is removed by a blister, touching the 

 blistered spot gives rise to a different sensation, pain, and not 

 to the true sense of touch ; we cannot tell what kind of surface 

 the thing touching the blistered spot has, whether it is smooth, 

 rough, or jagged, and the like. 



The sense of touch is most delicate at the tips of the 

 fingers and on the face and tip of the tongue ; it is less 

 delicate in other parts of the body on account of a thicker 

 epidermis and a less plentiful supply of sensory nerve end- 

 ings. If a pair of blunt-pointed compasses, with the points 

 separated only one -tenth of an inch, be gently applied to 

 the tip of the finger, the two points will be distinctly felt, 

 while if they be applied to the back of the hand or the 

 arm, there will only be a sensation of one point, and indeed 

 the points may be put much farther apart and yet be 

 felt as one only. This shows that touch is far more delicate 

 at the tip of the finger than on the back of the hand or arm. 

 Touch is very delicate at the tip of the tongue ; there the com- 

 passes will be felt as two points, even when they are as close 



