552 HANDBOOK OF PHYSIOLOGY. 



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

 one sensation. Experiments have been made to determine the tactile 

 properties of different parts of the skin, as measured by this power of 

 distinguishing distances. These consist of 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. 



Table of variations in the tactile sensibility of different parts. 



The 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, . . . , -j 



Palmar surface of third phalanx of forefinger, . iV '* 

 Palmar surface of second phalanges of fingers, 



Eed surface of under-lip, . . . . i " 



Tip of the nose, . . . . . ^ " 



Middle of dorsum of tongue, . . . (C 



Palm of hand, . . . . . . X 



Centre of hard palate, . . . . \ " 



Dorsal surface of first phalanges of fingers, . . T \ " 



Back of hand, . . . . . 1-i- " 



Dorsum of foot near toes, . . . lj " 



Gluteal region, . . . . . 1^ ' 



Sacral region, . . . . . 1 " 



Upper and lower parts of forearm, . . . 1 " 



Back of neck near occiput, . . . 2 " 



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



Middle part of forearm, . . . 2-J- " 



Middle of thigh, ..... 24 " 



Mid-cervical region, . . . . 2J " 



Mid-dorsal region, . . . . . 2$ " 



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. 



