712 THE SENSE OF TOUCH. 



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 pain- 

 ful and pleasurable sensations afford many examples of 

 this fact. 



The law of contrast, which we have shown modifies the 

 sensations 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, rela- 

 tive 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, etc., may be 

 produced by internal causes. Neuralgic pains, the sensa- 

 tion of rigor, formication or the creeping of ants, and the 

 states of the sexual organs occurring during sleep, afford 

 striking examples of subjective sensations. 



The mind, also, has a remarkable power of exciting 

 sensations in the nerves of common sensibility; just as the 

 thought of the nauseous excites sometimes the sensation of 

 nausea, so the idea of pain gives rise to the actual sensa- 

 tion of pain in a part predisposed to it. The thought of 

 anything horrid excites ^the sensation of shuddering ; the 

 feelings of eager expectation, of pathetic emotion, of 

 enthusiasm, excite in some persons a sensation of " con- 

 centration" at the top of the head, and of cold trickling 



