556 THE SENSE OF TOUCH. 



in the surface traversed ; and by repeating this process with 

 regard to the different dimensions of a solid body, acquires a 

 notion of its cubical extent. 



The perfection of the sense of touch on different parts of 

 the surface is proportioned to the power which such parts pos- 

 sess of distinguishing and isolating the sensations produced by 

 two points placed closed 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 differ- 

 ent contiguous points will act on only one nervous fibre, and 

 hence be confounded, and perhaps produce but one sensation. 

 Experiments to determine the tactile properties of different 

 parts of the skin, as measured by this power of distinguishing 

 distances, were made by E. H. Weber. One experiment con- 

 sisted in touching the skin, while the eyes were closed, with 

 the points of a pair of compasses sheathed with cork, and in 

 ascertaining how close the points of the compasses might be 

 brought to each other, and still be felt as two bodies. He ex- 

 amined in this manner nearly every part of the surface of the 

 body, and has given tables showing the relative degrees of sen- 

 sibility of different parts. Experiments of a similar kind 

 have been performed also by Valentin ; and, among the nu- 

 merous results obtained by both these investigators, it appears 

 that the extremity of the third finger, and the point of the 

 tongue are the parts most sensitive : a distance of as little as 

 half a line being here distinguished. Next in sensitiveness to 

 these is the mucous surface of the lips, which can perceive the 

 two points of the compass when separated to the distance of 

 about a line and a half: on the dorsum of the tongue they re- 

 quire to be separated two lines. The parts in which the sense 

 of touch is least acute are the neck, the middle of the back, 

 the middle of the arm, and the middle of the thigh, on which 

 the points of the compass have to be separated to the distance 

 of thirty lines to be perceived as distinct points (Weber). 

 Other parts of the body possess various degrees of sensibility 

 intermediate between the above extremes. 



A sensation in a part endowed with touch appears to the 

 mind to be, cceteris paribus, more intense when it is excited in 

 a large extent of surface than when it is confined to a small 

 space. The temperature of water into which he dipped his 

 whole hand, appeared to Weber to be higher than that of 

 water of really higher temperature, in which he immersed 

 only one finger of the other hand. Similar observations may 

 be made by persons bathing in warm or cold water. 



Part of the ideas which we obtain of the conditions of ex- 



