278 ON CUTANEOUS AND [BOOK in. 



separate paths to separate cerebral centres. All that is necessary 

 is that the two cerebral sensation-areas should not be too com- 

 pletely fused together. The improvement by exercise of the sense 

 of touch must be explained not by an increased development of 

 the terminal organs, not by a growth of new nerve-fibres in the 

 skin, but by a more exact limitation of the sensational areas in the 

 brain, as for example by the development of a resistance which 

 limits the radiation taking place from the centres of the several 

 areas. 



Sensations of Heat and Cold. 



879. When we bring into contact with, or even into the 

 immediate neighbourhood of a spot of skin, a body distinctly 

 hotter than is the skin at the spot for the time being, we ex- 

 perience a special sensation ; we feel something in the skin that 

 was not there before, but that something is wholly unlike the 

 effect of pressure, and we call the sensation a sensation of heat. 

 The sensation is obviously due to the rise in the temperature of 

 skin which is the direct effect of the contact with or the nearness 

 of the hot body. Our skin has a certain temperature which varies 

 from time to time, according to circumstances, and is not the same 

 in all regions of the skin at the same time. A given spot of skin 

 at a given time will have a certain temperature ; that temperature 

 does not give rise to a distinct sensation though its effects may 

 enter into what we may call general sensibility ; we may not be 

 directly conscious, for instance, that the forehead has one tempera- 

 ture and the hand another, though the two temperatures may 

 differ widely. It appears then that we are only conscious of a 

 cutaneous sensation of heat when the temperature of a region of 

 the skin which has previously been fairly constant is raised ; we 

 may add suddenly raised, for in sensations of heat as of pressure 

 the stimulus must act with a certain rapidity in order to produce 

 a distinct effect on consciousness. 



If the body brought into contact with or near to the skin, instead 

 of being distinctly hotter is distinctly colder than the skin we also 

 experience a special sensation, a sensation of cold ; and this 

 sensation differs in kind not only from that of pressure, but also 

 from that of heat. We might expect perhaps that since cold 

 only differs from heat in degree, both being degrees of temperature, 

 that the sensations of heat and cold would also 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 



