1042 ON CUTANEOUS AND [BOOK m. 



be alike, differing only in degree ; but when we appeal to our 

 consciousness we recognize that they differ in kind. So long 

 as sensations of heat and cold remain sensations of heat and 

 cold, they appear to us not as merely different phases of the 

 same thing but as quite unlike ; when the exciting heat or cold 

 is excessive we perhaps may fail to distinguish between the two, 

 but that is because both are lost in the sensation of pain. It 

 appears then that we are conscious of a specific sensation of 

 cold when the temperature of a region of the skin which has 

 previously been fairly constant is with sufficient rapidity lowered. 

 To how large an extent we are, under ordinary circumstances, 

 unconscious of the actual temperature of the skin and how 

 sensitive we are to even slight changes of temperature may be 

 illustrated by using one region of the skin as a stimulus of heat 

 or cold for another. At a time, for instance, when we are not 

 directly conscious of the hand being either colder or hotter than 

 the forehead, by putting the one up to the other we may experi- 

 ence a distinct sensation telling us that the hand decidedly 

 differs in temperature from the forehead ; we feel at once that 

 one is warmer or colder than the other, though it may take 

 some little time to recognize which is the warmer or the colder. 



648. These sensations of heat and cold behave very much 

 in the same way as sensations of pressure. We have already 

 said that the change of temperature like the change of pressure 

 must be effected with a certain rapidity in order to produce a 

 distinct sensation, and in general the more gradual the change 

 the less intense is the sensation. 



As might be expected from the fact that it takes a longer 

 time to produce a change of temperature than to exert pressure, 

 the sensation of either heat or cold is somewhat slowly devel- 

 oped and lasts some considerable time; hence consecutive sensa- 

 tions readily fuse into one. 



Since it is the changed temperature and not the particular 

 temperature arrived at which is the basis of the sensation, a 

 hot body or a cold body gives rise to a sensation only at the 

 first contact or approach and for some little time afterwards, 

 the effect diminishing from the very moment that the change 

 has been established. Hence a hot body or a cold body ap- 

 plied to the skin, even when kept itself at a constant tempera- 

 ture and not cooled or heated by contact with the cooler or 

 warmer skin, ceases after a while to be felt as hot or cold. For 

 this reason the repeated dipping of the hand into hot or cold 

 water produces a greater sensation than when the hand is 

 allowed to remain all the time in the water, though in the 

 latter case the temperature of the skin is most affected. 



The effects of contrast are obvious in sensations of heat 

 ;in<l cold as in those of pressure; when the hand is dipped in 

 hot water the sensation is most intense at the ring where the 

 hand emerges from the surface of the water. 



