CH. LI.] LOCALISATION OF TACTILE SENSATIONS. 



701 



closed, with the points of a pair of compasses, and in ascertaining 

 how close the points of the compasses may be brought to each 

 other, and still be felt as two points. 



Table of variations in the tactile sensibility of different parts. 

 TJie in <'ii an rein i- nt indicates the lenxt dixtnnce at which the two 

 point* of a pair of compasses could be separately distinguished. 

 (E. H. Weber.) 



Tip of tongue 



Palmar surface of third phalanx of forefinger 

 Palmar surface of second phalanges of fingers . 

 Red surface of under-lip .... 



Tip of the nose 



Middle of dorsum of tongue . . -' 



Palm of hand 



Centre of hard palate . . . . 



Dorsal surface of first phalanges of fingers 



Back of hand 



Dorsum of foot near toes . . 



Gluteal region . . . ." . " . 



Sacral region 



Upper and lower parts of forearm . . . 



Back of neck near occiput 



Upper dorsal and mid-lumbar regions . 



Middle j>art of forearm 



Middle of thigh 



Mid-cervical region 



Mid-dorsal region 



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 uuexcited nerve-endings which inter- 

 vene between the two points touched. It would appear that 

 a certain number of intervening unexcited nerve-endings are 

 necessary before the points touched can be recognised as separate, 

 and the greater this number the more clearly are the points of 

 contact distinguished as separate. But the number of nerve- 

 endings is not the only factor in the case, for by practice the 

 delicacy of a sense of touch may be very much increased. A 

 familiar illustration occurs in the case of the blind, who, by con- 

 stant 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, doubtless on account of their imperfect 



