ACUTENESS OF THE SENSE 633 



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 corpus- 

 cles 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 distributed, and which may be in 

 some way connected with the sensation, 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, 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 everyone, 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 required 

 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 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 primi- 

 tive fibers which an organ receives, the more likely is it that several impres- 

 sions on different contiguous points will act on only one nervous 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 compasses might be brought to each other and still be felt as 

 two bodies. 



TABLE OF 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 mm. 



Palmar surface of second phalanges of fingers 4 mm. 



Red surface of under-lip 4 mm. 



