712 THE SENSE OF TOUCH. 



is very slight ; and persons who do not know that the 

 action of particular muscles is necessary for the production 

 of given movements, do not suspect that the movement of 

 the fingers, for example, depends on an action in the fore- 

 arm. The mind has, nevertheless, a very definite know- 

 ledge of the changes of position produced by movements ; 

 and it is on this that the ideas which it conceives of 

 the extension and form of a body are in great measure 

 founded. 



In order that an impression made on a sensitive surface 

 may be perceived, it is necessary that there should exist 

 a reciprocal influence between the mind and the sense of 

 touch; for, if the mind does not thus co-operate, the 

 organic conditions for the sensation may be fulfilled, but 

 it remains unperceived. Moreover, the distinctness and 

 intensity of a sensation in the nerves of touch depend, 

 in great measure, on the degree in which the mind co- 

 operates for its perception. A painful sensation becomes 

 more intolerable the more the attention is directed to it : 

 thus, a sensation in itself inconsiderable, as an itching in 

 a very small spot of the skin, may be rendered very 

 troublesome and enduring. 



As every sensation is attended with an idea, and leaves 

 behind it an idea in the mind which can be reproduced at 

 will, we are enabled to compare the idea of a past sensation 

 with another sensation really present. Thus we can com- 

 pare the weight of one body with another which we had 

 previously felt, of which the idea is retained in our mind. 

 Weber was indeed able to distinguish in this manner 

 between temperatures, experienced one after the other, 

 better than between temperatures to which the two hands 

 were simultaneously subjected. This power of comparing 

 present with past sensations diminishes, however, in pro- 

 portion to the time which has elapsed between them. 



The after-sensations left by impressions on nerves of com- 

 mon sensibility or touch are very vivid and durable. As 



