CHAP, vi.] SOME OTHER SENSATIONS. 1057 



naked in still air. We are not however justified in assuming 

 that under the above circumstances nothing whatever is taking 

 place in the sensory nerves of the skin, that when we feel a sensa- 

 tion the change in the sensory apparatus (using that phrase in 

 its widest sense to include both peripheral and central parts) 

 is one from absolute quiescence to activity ; it is not impossible, 

 and some facts indeed seem to suggest, that even when we feel 

 no distinct cutaneous sensations, afferent impulses still continue 

 to stream onwards from the periphery to the central nervous 

 system, supplying as it were a groundwork of nervous events 

 which enter largely in various ways into the conduct of the 

 whole body, but which do not distinctly affect consciousness. 

 If this be so, we may infer that the affection of consciousness which 

 we call a sensation is the immediate effect of an adequately 

 large change in this groundwork, rather than of a set of quite 

 new isolated impulses passing straight up from the peripheral 

 organ to the "seat of consciousness." 



In the second place when we do experience sensations of 

 temperature the sensation is caused not by the mere change of 

 temperature but by the altered condition of the skin which 

 results from that change. When an area of the skin having 

 a normal temperature is brought in contact with a cold body, 

 the skin undergoes a change from a normal to a lower tempera- 

 ture, and we experience a sensation of cold. Now, if it were 

 only the change from a normal to a lower temperature which 

 gave rise to the sensation, though the sensation might and prob- 

 ably would last much longer than the change itself, it could not 

 be prolonged by the mere maintenance of the lower temperature 

 when once the change had been established. But experience 

 shews that it is ; we still feel a sensation of cold, at a time when 

 the contact of the cold body is not producing any further lower- 

 ing of temperature and at most is only maintaining the lower 

 temperature already brought about. Nay, more, the sensation of 

 cold continues after the cooling body has been removed, at the 

 time when the skin is returning to its normal temperature, that 

 is to say is undergoing the very opposite change of temperature, 

 namely, one from cold to heat. And the same considerations 

 apply to sensations of heat. 



658. We may conclude then that when the application of 

 cold or of heat to the skin causes a sensation of cold, the cold 

 or heat produces a condition in the material of the skin, which 

 condition starts nervous impulses in the afferent nerves of cold 

 and heat sensations. Since the application of cold or of heat to 

 the nerve fibres underlying the skin does not produce a sensation 

 of cold or heat, but only a sensation of pain, we may further 

 conclude that the material whose condition starts the sensation 

 is placed in the skin itself, in the epidermis or in the imme- 

 diately underlying dermis. Since we experience sensations of 



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