464 THE SENSORY FUNCTIONS OF THE SKIN 



points are stimulated than when the needles are applied, as in Weber's experi- 

 ments, quite at random. Under suitable conditions of the experiment the 

 smallest distance at which any two stimuli applied to the skin can be recog- 

 nized as distinct, corresponds closely with the distance, as determined by 

 successive stimulation, of neighboring tactile points from each other (v. Frey 

 and Metzner). 



The differences already observed between different points of the skin, obtains 

 also for the stimulation of isolated pressure points, as the following summary 

 will show. The smallest perceptible distance for the nail joints, volar side is 

 0.1 mm.; for the balls of the fingers, 0.1-0.2 mm.; palm of the hand, 0.1-0.5; 

 ball of the thumb, 0.2-0.4 ; nose and chin, 0.3 ; back of the hand, 0.3-0.8 ; cheek, 

 arm, brow, 0.4-1.0; foreleg, abdomen, 1.0-2.0; thigh, 3.0; back, 4.0-6.0 mm. 



It has been found also that the smallest perceptible distances are shorter 

 when the points applied simultaneously are placed in the transverse direction 

 from each other than when the line joining them lies in the longitudinal 

 direction of the part, and that they decrease with the distance of the points 

 tested from the axis of rotation of the members; thus, on the arm above, 

 53.8 mm.; below, 44.6; on the forearm above, 41.2; below, 22.5; on the hand 

 above, 20.4; below, 7.8; on the third finger above, 7.5; below, 2.5 (Vierordt). 



The power of localization is reduced by fatigue, anemia, low temperature, 

 etc., and is intensified by hypersemia of the skin. Children have a more precise 

 power of localization than adults. 



"It is really very remarkable that we have the power to distinguish two 

 points as two when they are applied to the skin simultaneously. For the 

 mere excitation from the one must be just like that aroused from the other. 

 But the fact that we have the power to feel them as two must mean that the 

 two sensations of pressure differ in some definite property. Since now we 

 can distinguish simultaneously stimulated points better the farther they are 

 apart, it follows further that this difference between the sensations produced 

 from different points is greater, the farther they are apart. 



This difference between the resulting sensations which enables us to locate 

 the place of stimulation is known, since Lotze, as the local sign. Since every 

 sensation arises in the last analysis through cerebral processes, we may con- 

 ceive of the local sign as a difference in some property of the different sections 

 of the brain, excited by stimulation of the different pressure nerves. In a 

 crudely schematic way we may imagine that every pressure nerve is connected 

 in some way with a special nerve cell and that excitation of this nerve cell 

 produces a specific shade of sensation which differs from all other sensations 

 of pressure. 



The temperature nerves of the skin likewise possess this power of localiza- 

 tion, but it is not so highly developed for temperature as it is for pressure. 

 The cold spots appear to have a more precise power of localization than the 

 heat spots. 



The power of localization of the retina, especially of the fovea centralis, will 

 be taken up in Chapter XXI. 



