CHAP, vi.] SOME OTHER SENSATIONS. 1041 



being composed of visual areas or units ; but all that was there 

 said concerning the subjective nature of the limits of visual areas, 

 applies equally well, mutatis mutandis, to tactile areas. When 

 two points of the compasses are felt as two distinct sensations, 

 it is not necessary that two, and only two, nerve-fibres should be 

 stimulated, or, putting the matter more generally, that two or 

 only two discrete sets of sensory impulses, should travel along 

 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 develop- 

 ment 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 develop- 

 ment of a resistance which limits the radiation taking place 

 from the centres of the several areas. 



Sensations of Heat and Cold. 



647. 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 temperature and the hand another, though the two tem- 

 peratures may differ widely. It appears then that we are only 

 conscious of a cutaneous sensation of heat when the tempera* 

 ture of a region of the skin which has previously been fairly 

 constant is raised ; we may add suddenly raised, for in sensa- 

 tions 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 



66 



