662 HANDBOOK OF PHYSIOLOGY. 



Table of variations in the tactile sensibility of the different parts. The mea- 

 surement indicates the least distance at u'hich the tivo blunted points of a 

 pair of compasses could be separately distinguished. (E. H. Weber. ) 



Tip of tongue A inch 1 mm. 



Palmar surface of third phalanx of forefinger . . y 2 " 2 " 



Palmar surface of second phalanges of fingers . i " 4 " 



Red surface of under- lip " 4 " 



Tip of nose . . . . . . . . i " 6 " 



Middle of dorsuni of tongue 8 " 



Palm of hand < T 2 " 10 " 



Centre of hard palate i " 12 " 



Dorsal surface of first phalanges of fingers yV " 14 " 



Back of hand 1 " 25 " 



Dorsuni of foot near toes . . . . . . . H " 37 " 



Gluteal region H " 37 " 



Sacral region . . H " 37 " 



Upper and lower parts of forearm . . . . li " 37 " 



Back of neck near occiput 2 " 50 u 



Upper dorsal and mid -lumbar regions .... 2 " 50 " 



Middle part of forearm 2-J- " 62 " 



Middle of thigh 24 " 62 " 



Mid-cervical region ........ 24- " 62 " 



Mid -dorsal region . 62 " 



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

 were recognized 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 be- 

 tween the two points touched. It would appear that a certain number 

 of intervening unexcited nerve-endings are necessary before two points 

 touched can be recognized as separate, and the greater this number the 

 more clearly are the points of contact distinguished as separate. 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 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. 



Localization. The power of correctly localizing sensations of touch 

 is gradually derived from experience. Thus infants when in pain sim- 

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

 child or adult would, doubtless on account of their imperfect knowledge 

 of its exact situation. 



Illusions. The different degrees of sensitiveness possessed by differ- 

 ent parts may give rise to errors of judgment in estimating the distance 

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

 of a pair of compasses (maintained at a constant distance apart) be 

 slowly drawn over the skin of the cheek toward the lips, it is almost im- 

 possible to resist the conclusion that the distance between the points is 



