ACUTENESS OF THE SENSE 599 



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

 ever, be here again mentioned. They are of two kinds, viz., i, Touch cor- 

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

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

 where, but not in the skin proper. As regards the Pacinian corpuscles and 

 similar end-organs, which are so widely distributed, and which may be in some 

 way connected with the sensation, when they are found in the skin they are situ- 

 ated 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, 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 very largely on the cutaneous 

 circulation, which is of course greatly influenced by external temperature. 

 Hence the numbness, familiar to every one, produced 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 proportioned 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 isolat- 

 ing the sensations produced by two points placed close together. This latter 

 is a measure of the power of localization in a degree. 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 nervous fiber, 

 and hence be confounded, and perhaps produce but one sensation. Experi- 

 ments 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 compasses might be brought to each other and still 

 be felt as two bodies. 



TABLE or VARIATIONS IN THE TACTILE SENSIBILITY OF THE DIFFERENT PARTS OF 



THE SKIN. 



The measurement indicates the least distance at which the two blunted points of a pair 

 of compasses could be separately distinguished. (E. H. Weber.) 



Tip of tongue, ........... i mm. 



Palmar surface of third phalanx of forefinger, 2 



Palmar surface of second phalanges of fingers, ..... 4 



Red surface of under-lip, ......... 4 



Tip of nose, 6 



Middle of dorsum of tongue, ........ 



