708 THE SENSE OF TOUCH. 



ing, following, or examining odours, does to the sense of 

 smell. Every sensitive part of the body which can, by 

 means of movement, be brought into different relations of 

 contact with external bodies, is an organ of " touch." No 

 one part, consequently, has exclusively this function. The 

 hand, however, is best adapted for it, by reason of its 

 peculiarities of structure, namely, its capability of pro- 

 nation and supination, which enables it, by the movement 

 of rotation, to examine the whole circumference of a body ; 

 the power it possesses of opposing the thumb to the rest 

 of the hand ; and the relative mobility of the fingers. 

 Besides the hand, and especially the fingers, are abun- 

 dantly endowed with papilla and touch-corpuscles (pp. 424, 

 425) which are specially necessary for the perfect employ- 

 ment of this sense. 



In forming a conception of the figure and extent of a 

 surface, the mind multiplies the size of the hand or fingers 

 used in the inquiry by the number of times which it is 

 contained 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 possess of distinguishing and isolating the sensa- 

 tions 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 con- 

 founded, and perhaps produce but one sensation. Ex- 

 periments to determine the tactile properties of different 

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

 guishing distances, were made by E. H. Weber. One 

 experiment consisted in touching the skin, while the eyes 

 were closed, with the points of a pair of compasses sheathed 



