CHAP, vi.] SOME OTHER SENSATIONS. 291 



rod narrowed to a point, it will be found that some points of the 

 skin are very sensitive to changes of temperature, while other 

 points are insensitive to temperature, the application of heat or 

 cold giving rise to pain only and not to specific sensations of heat 

 or cold. Further, the points of the skin which ^are sensitive to 

 pressure are those which are not sensitive to heat or cold, and 

 vice versa. Such results as these are only intelligible on the 

 supposition that the terminal organs for pressure are different 

 from those for heat and cold and differently distributed over the 

 surface of the skin. 



888. The punctiform method of exploring the sensitiveness 

 of the skin has further led to a result which is unexpected and 

 indeed presents difficulties. Heat and cold in themselves differ 

 only in degree ; they are positive and negative phases of the same 

 thing. We should therefore naturally expect that the same 

 terminal organs would be employed for sensations both of heat 

 and of cold, and that the same points of the skin would be alike 

 sensitive both to heat and to cold. But the results of experi- 

 mentation by the method in question contradict this expectation. 

 It is found that some points are sensitive to heat, that is to say 

 a sensation is developed when the temperature of the point of the 

 skin is raised above what it happens to be at the time of experi- 

 menting, but are not sensitive to cold, that is to say no sensations 

 are developed when the temperature of the point of the skin is 

 lowered below what it happens to be at the time of experiment- 

 ing ; and other points may similarly be found to be sensitive to 

 cold but not to heat. Moreover this result is in accord with 

 results gained otherwise. If the arm or leg be " sent to sleep " by 

 pressure on the brachial or sciatic nerves the skin will be found 

 at a certain stage to be little sensitive to warmth though dis- 

 tinctly sensitive to cold. So also the whole surface of the glans 

 penis, in contrast to the prepuce, is very slightly sensitive to cold, 

 but distinctly sensitive to warmth. Moreover cases of disease of 

 the central nervous system have been recorded in which the skin 

 of a limb was sensitive to warmth, that is to degrees of temperature 

 above that of the limb, but insensitive to cold. It may be re- 

 marked that in these cases, as in that of the limb "gone to sleep," 

 the sensations of touch proper and of cold seem to run together 

 and sensations of pain and of heat also to run together. 



It seems probable then from these considerations that we 

 possess three sets of terminal organs and three sets of fibres, one 

 for pressure, a second for heat and a third for cold. It must be 

 borne in mind however that the three sensations are not wholly 

 independent, since sensations of pressure are modified if changes 

 in temperature be taking place at the same time in the same spot 

 of skin. Thus a penny cooled down nearly to zero and placed on 

 the forehead will be judged by most people to be as heavy or even 

 heavier than two pennies of the temperature of the forehead itself, 



