724 



CUTANEOUS SENSATIONS 



[CH. LI1. 



taining how close the points may be brought to each other, and still 

 be felt as two points. (Weber). A few results are as follow : 



Tip of tongue 



Palmar surface of third phalanx of forefinger 

 Palmar surface of second phalanges of fingers 



Palm of hand 



Dorsal surface of first phalanges of fingers. 



Back of hand 



Upper and lower parts of forearm 



Middle of thigh and back .... 



A 



1 mm 

 2 



4 



10 

 14 

 25 

 37 

 62 



Moreover, in the case of the limbs, it was found that before they 

 were recognised as two, the points of the compasses had to be further 

 separated when the line joining them was in the long axis of the 

 limb, than when in the transverse direction. 



According to Weber the mind estimates the distance between two 

 points by the number of unexcited nerve-endings which intervene 

 between the two points touched. But the number of nerve-endings 

 is not the only factor in the case. An important role is played by 

 " local signature." Minute areas of the body surface have each their 

 " local sign," i.e., the sensation arising from stimulation of one area 

 differs in some obscure quality from the sensations arising from 

 stimulation of neighbouring areas, thereby acquiring its own spatial 

 colouring which enables us to identify the area when stimulated. 

 The difference of local sign between two near points may be imper- 

 ceptible in one region of the body, but fully recognisable in another. 

 Again, the delicacy of the sense of touch may be very much increased 

 by practice. A familiar illustration occurs in the case of the blind, 

 who, by constant practice, can acquire the power of reading raised 

 letters, the forms of which are almost if not quite undistinguishable 

 by the sense of touch to an ordinary person. 



The power of correctly localising sensations of touch is gradually 

 derived from experience. Thus, infants when in pain simply cry 

 but make no effort to remove the cause of irritation, as an older 

 child or adult would, on account of their imperfect knowledge 

 of its exact situation. As education proceeds the brain gets to 

 know more and more accurately the surface of the body, and the 

 map of the surface in the brain is most accurately known where 

 there is most practice of the sense of touch. The great delicacy of 

 the tongue as a touch organ in judging the form and size of objects 

 can be explained by the fact that this organ has to rely upon the 

 sense of touch alone. Usually, in ascertaining the shape of an object 

 or the part of the skin it touches, we use our eyes as well. In the 

 case of the interior of the mouth this is impossible. 



The different degrees of sensitiveness possessed by different parts 

 may give rise to errors of judgment in estimating the distance 

 between two points where the skin is touched. Thus, if the blunted 



