CH. LIII.] TACTILE LOCALISATION 771 



to each other, and still be felt as two points. (Weber.) A few 

 results are as follow : 



Tip of tongue 7jV in ch 1 mm. 



Palmar surface of third phalanx of forefinger . T V 2 



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 . . 1^ 



Middle of thigh and back 2| 



4 



10 

 14 

 25 

 37 

 62 



In the case of the limbs, it is found that before they are recognised 

 as two, the points of the compasses have to be further separated when 

 the line joining them is in the long axis of the limb, than when in 

 the transverse direction. 



We may thus assume that 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 different delicacy of local signature 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 

 points of a pair of compasses (maintained at a constant distance 

 apart) are slowly drawn over the skin of the cheek towards the lips, 

 it is almost impossible to resist the conclusion that the distance 

 between the points is gradually increasing. When they reach the 

 lips they seem to be considerably further apart than on the cheek. 

 Then, too, our estimate of the size of a cavity in a tooth is usually 

 exaggerated when based upon sensations derived from the tongue 

 alone. Another curious illusion is the following: If we close 

 the eyes, and place a marble between the crossed fore and middle 

 fingers, we seem to be touching two marbles. This illusion is due 

 to an error of judgment. The marble is touched by two surfaces 

 which, under ordinary circumstances, could only be touched by two 

 separate marbles ; hence, regardless of the fact that the fingers are 

 crossed, the judgment is formed that the two sensations are due to 

 two marbles. 



