558 THE SENSE OF TOUCH. 



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 condi- 

 tions for the sensation may be fulfilled, but it remains unper- 

 ceived. Moreover, the distinctness and intensity of a sensa- 

 tion 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 at- 

 tention is directed to it : thus, a sensation in itself inconsider- 

 able, 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 compare 

 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 tempera- 

 tures, experienced one after the other, better than between 

 temperatures to which the two hands were simultaneously 

 subjected. This power of comparing present with past sensa- 

 tions diminishes, however, in proportion to the time which has 

 elapsed between them. 



The after-sensations left by impressions on nerves of common 

 sensibility or touch are very vivid and durable. As long as 

 the condition into which the stimulus has thrown the organ 

 endures, the sensation also remains, though the exciting cause 

 should have long ceased to act. Both painful and pleasurable 

 sensations afford many examples of this fact. 



The law of contrast, which we have shown modifies the sen- 

 sations of vision, prevails here also. After the body has been 

 exposed to a warm atmosphere, a degree of temperature a very 

 little lower, which would under other circumstances appear 

 warm, produces the sensation of cold; and a sudden change to 

 the extent of a few degrees from a cold temperature to one less 

 severe, will produce the sensation of warmth. Heat and cold 

 are, therefore, relative terms ; for a particular state of the 

 sentient organs causes what would otherwise be warmth to 

 appear cold. So, also a diminution in the intensity of a long- 

 continued pain gives pleasure, even though the degree of pain 

 that remains would in the healthy state have seemed intoler- 

 able. 



Subjective sensations, or sensations dependent on internal 

 causes, are in no sense more frequent than in the sense of 

 touch. All the sensations of pleasure and pain, of heat and 

 cold, of lightness and weight, of fatigue, &c., may be produced 



