164 HAND-BOOK OF PHYSIOLOGY. 



Acuteness of Touch. The perfection of the sense of touch on 

 different parts of the surface is proportioned to the power which such 

 parts possess of distinguishing and isolating the sensations produced by 

 two points placed close together. This power depends, at least in part, 

 on the number of primitive nerve-fibres distributed to the part; for the 

 fewer the primitive fibres which an organ receives, the more likely is it 

 that several impressions on different contiguous points will act on only 

 one nervous fibre, and hence be confounded, and perhaps produce but 

 one sensation. Experiments have been made to determine the tactile prop- 

 erties of different parts of the skin, as measured by this power of distin- 

 guishing distances. These consist in touching the skin, while the eyes 

 are closed, with the points of a pair of compasses sheathed with cork, and 

 in ascertaining how close the points of compasses might be brought to 

 each other, and still be felt as two bodies. (E. H. "Weber, Valentin.) 



Table of variations in the tactile sensibility of different parts. 



TJie measurement indicates the least distance at which the two 

 Hunted points of a pair of compasses could be separately distin- 

 guished. (E. H. Weber.) 



Tip of tongue ^ inch 



Palmar surface of third phalanx of forefinger . . TW " 



Palmar surface of second phalanges of fingers . . -J- " 



Red surface of under-lip i ' ' 



Tip of the nose 



Middle of dor sum of tongue -J- " 



Palm of hand T 5 



Centre of hard palate ...... 



Dorsal surface of first phalanges of fingers . . T \- 



Back of hand . . . . . . . . 1-| 



Dorsum of foot near toes 1 



Gluteal region 1J- 



Sacral region ........ 1* 



Upper and lower parts of forearm . . . . 1J- 



Back of neck near occiput . . . . . 2 " 



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



Middle part of forearm 2J " 



Middle of thigh ....... 2-J- " 



Mid-cervical region 2J " 



Mid-dorsal region 2J " 



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 



