ACUTENESS OF THE SENSE 683 



cutis laid bare, the sensation of contact is replaced by one of pain. Further, 

 in all highly sensitive parts, the papillae are numerous and highly vascular, 

 and the sensory nerves are connected with special end-organs which have 

 been described on page 74 et seq. 



The special endings of the nerves which have to do with touch may, 

 however, be again mentioned here. They are of two kinds, viz.: i. Touch 

 corpuscles, which are found chiefly in the hands and feet, particularly on 

 the palmar surface of the hands and fingers, but also on the under sXirface 

 of the forearm, on the nipple, eyelids, lips, and the genital organs. Touch 

 corpuscles are situated in the cutis vera. 2. End bulbs are found in the 

 conjunctiva and other mucous membranes, the lips, genital organs, tongue, 

 rectum, and elsewhere, but not in the skin proper. As regards the 

 Pacinian corpuscles and similar end-organs^ which are so widely distrib- 

 uted, and which may be in some way connected with the development of 

 the sensation of touch, when they are found in the skin they are situated 

 very deeply in the cutis vera or in the subcutaneous tissue. They are 

 extremely numerous on the nerves of the palmar surface of the fingers. 

 In addition to these special nerve endings in sense organs, nerve fibers 

 terminate everywhere in the skin between the cells of the Malpighian 

 stratum of the epidermis. 



The acuteness of the sense of touch depends in no small degree on the 

 cutaneous circulation and is of course greatly influenced by external tem- 

 perature. This explains the numbness familiar to everyone that is pro- 

 duced by the application of cold to the skin. 



Acuteness of the Sense. The perfection of the sense of touch on 

 different parts of the surface is proportional to the minimal pressure re- 

 quired to stimulate the point, i.e., the threshold stimulus. Or it can be 

 measured by the power which such parts possess of distinguishing and 

 isolating the sensations produced by two stimulating points placed close 

 together. This latter is in a degree a measure of the power of localiza- 

 tion. This power depends, at least in part, on the number of primitive 

 nerve fibers; for the fewer the primitive fibers which an organ receives, the 

 more likely is it that several impressions on different contiguous points will 

 act on only one nerve fiber, and hence be confounded, and perhaps produce 

 but one sensation. Experiments have been made to determine the 

 tactile properties of different parts of the skin, as measured by this power 

 of distinguishing distances between points of simultaneous contact. 

 These consist in touching the skin with the points of a pair of compasses 

 sheathed with cork, and in ascertaining how close the points of the com- 

 passes may be brought to each other and still be felt as two bodies. 



