CHAP, vi.] SOME OTHER SENSATIONS. 1055 



skin be confined to extremely minute areas, if the pressure, or 

 the change of temperature be brought to bear as much as pos- 

 sible on a mere point of the skin, it is found that some points 

 of the skin are sensitive to pressure but not to change of 

 temperature, while others again are sensitive to change of 

 temperature but not to pressure. If a blunt pointed but other- 

 wise fine needle be used to exert pressure, a little exploration 

 will ascertain that at some points the amount of pressure can 

 readily be recognized, the sense of touch is acute, while at other 

 points, and these may be quite near the others, the amount of 

 pressure cannot be recognized, and indeed no sensation is 

 experienced until the pressure is excessive and then the sensa- 

 tion felt is not one of touch proper but of pain. Similarly if 

 heat or cold be applied by means of a metal tube or rod nar- 

 rowed to a fine 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. 



656. 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 

 experimenting, 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 experimenting ; 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 distinctly sensitive to cold. So also the whole sur- 

 face 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 



