HANDBOOK OF PHYSIOLOGY. 



the whole body be immersed. So, too, water appears much hotter to 

 the hand than to a single finger. 



The delicacy of the sense of temperature coincides in the main with 

 that of touch, and appears to depend largely on the thickness of the 

 skin; hence, in the elbow, where the skin is thin, the sense of tempera- 

 ture is delicate, though that of touch is not remarkably so. Weber has 

 further ascertained the following facts : two compass points so near to- 

 gether on the skin that they produce but a single impression, at once 

 give rise to two sensations, when one is hotter than the other. More- 

 over, of two bodies of equal weight, that which is the colder feels heavier 

 than the other. 



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 com- 

 pare 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 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 propor- 

 tion to the time which has elapsed between them. 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 excit- 

 ing cause should have long ceased to act. Both painful and pleasurable 

 sensations afford many examples of this fact. 



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 fa- 

 tigue, etc., may be produced by internal causes. Neuralgic pains, the 

 sensation 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 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 sensation of pain in a part predisposed to 

 it; numerous examples of this influence might be quoted. 



Pain. As regards painful sensations, three views can be taken: 1, 

 that it is a special sensation provided with a special conducting apparatus 

 in each part of the body; 2, that it is produced by an over-stimulation 

 of the special nerves concerned with touch or temperature, or of the 

 other nerves of special sense ; or 3, that it is an over-stimulation of the 

 nerves of common sensation, which tell us of the condition of our own 

 bodies, both of the surface and also of the internal organs. There 



