i CUTANEOUS SENSIBILITY 33 



temperature, according as they are exposed or covered. Tin- 

 temperature that produces no thermal sensation is not at any 

 definite point of the thermometric scale, but, according to 

 Leegaard, seldom ranges over more than 0-5 C. This indifferent 

 temperature alters not only in the different regions of the skin, 

 but also in the same region at various times. For instance, on 

 passing from a room in which no thermal sensation is felt into 

 one that is hotter or colder there is an immediate sensation of 

 heat or cold. But if the difference in the temperature of the 

 two rooms is not very great a new equilibrium will soon be set 

 up so that no thermal sensation is perceptible. The surrounding 

 temperature may therefore vary between considerable limits 

 without producing any persistent thermal sensation. It might 

 be supposed that this adaptability depends upon variations in 

 the blood-supply to the skin, which to a certain extent protects 

 the peripheral thermal end -organs from the oscillations of 

 temperature in the environment. Thunberg, however, pointed 

 out that it can be observed on a hand previously rendered 

 bloodless. The adaptation therefore depends on an alteration 

 of the excitability of the peripheral thermal end-organs, which 

 causes a displacement of the level of the indifferent temperature 

 or physiological zero-point (Hering) of thermal sensibility. 



Starting from this fact Hering maintains that any intrinsic 

 temperature of the thermal organs above the physiological zero- 

 point is perceived as heat, and any temperature below the 

 zero-point as cold. The intensity of the sensation of heat or 

 cold increases with the variation of the intrinsic temperature 

 of the end -organ from the physiological zero. Any intrinsic 

 temperature of the end -organ appreciated as heat causes an 

 upward displacement of the zero-point : any temperature appreci- 

 ated as cold, a downward displacement. All sensation of heat 

 and cold ceases when, owing to the displacement of the zero- 

 point, the latter coincides with the intrinsic temperature of the 

 end-organ. 



The existence of two distinct senses for heat and cold is not 

 fundamentally irreconcilable with this theory of Hering. It may 

 be assumed that a rise in the cutaneous temperature acts only 

 upon the organs of heat, and a fall upon the organs of cold. 

 But the so-called paradoxical sensation of cold cannot be explained 

 either by Weber's or by Bering's theory. If a metal point 

 warmed to 45-50 C. is applied to a cold spot a sensation of cold 

 is felt (Lehmann and v. Frey). The accuracy of this observation 

 has been confirmed by many authors (Alrutz, Kiesow, Thunberg, 

 Veress, Bader). All cold spots react by a sensation of cold when 

 brought into contact with a warm point. When a thermo- 

 aesthesiometer is applied over an extensive surface, cold spots 

 are stimulated as well as heat spots, hut the sensation of warmth 



VOL. IV D 



