THERMAL SENSATIONS 



437 



CONDITIONS FOR THERMAL SENSATIONS 



The variety of opinion concerning the conditions 

 for thermal stimulation, which until lately has charac- 

 terized the discussion of the temperature senses ever 

 since Weber presented his famous theory in 1 846, was 

 to a great extent dependent upon imperfections in the 

 physical methods used in studying the thermal move- 

 ments in the skin as well as upon the use of subjective 

 reports as an indicator of the stimulating eflTcct. The 

 main problems in the physiology of the thermal senses 

 have been the question of whether temporal tempera- 

 ture changes or the absolute temperature levels were 

 the adequate stimulus and the intimately connected 

 question of the physical or physiological interpreta- 

 tion of adaptation. 



Conductiiin of Heal in Skin 



While the majority of writers have on the whole 

 accepted Weber's opinion that the temporal diflfer- 

 ential quotient of the temperature change represents 

 the adequate stimulus, there are others who like 

 Hering (58) have given attention to the influence of 

 the prevailing temperature in itself (37). Thunberg 

 stated in 1905 in Nagel's Handhucli that this question 

 cannot be settled until the physical constants of the 

 external layers of the skin are so well known that the 

 thermal exchange in the skin can be computed quanti- 

 tatively. Following the work of Bazett et al. (5), 

 Hensel (42, 43) succeeded in developing methods 

 for the determination of intracutaneous temperature 

 at exactly localized depths, a very fine thermocouple 

 being introduced through a thin cannula or through 

 an intracutaneous punctured channel. 



Further, Hensel (43) constructed a precision-flow- 

 calorimeter for measuring the steady heat flow given 

 oflf from small skin areas. The Stromungskalorim- 

 eter — a flat cylindrical measuring chamber through 

 which water of constant temperature flows with con- 

 stant velocity — is placed on the skin above the two 

 thermocouples which are situated at different depths. 

 The amount of heat given off is then obtained from 

 the flow velocity and temperature difference between 

 inflowing and outflowing water. The mean error of 

 the method is as low as about ±0.001 cal. per cm- 

 per sec. 



The thermal movement in nonstationary conditions 

 depends not only upon the thermal conductivity of 

 the tissue but also on its specific heat and density. 



The determining constant, the thermal diffusion 

 coefficient, a, is obtained by the following equation : 



where X (calories per cm per sec. per degree) repre- 

 sents the thermal conductivity, C (cal. per gm per 

 degree) the specific heat, and p (gm per cm-*) the 

 density of the substance. As the determination of C 

 and X are problematic in the living skin, Hensel C44) 

 elaborated a method for direct determination of the 

 diffusivity, a. By means of the above described thermo- 

 electrical methods the temperature movements were 

 recorded at diflferent depths of the skin when rectangu- 

 lar temperature pulses were applied to the surface by 

 the application of metal bodies of constant tempera- 

 ture. From the curves obtained the diffusivity, a, 

 could be determined in that the curves for various 

 values of a were constructed and it was found at 

 which value of a the computed curve best fitted the 

 recorded curve. For human skin in depths up to 2 

 mm the values for a varied from 0.0004 to 0.0018 



32 



M 



ss 



a, 

 S 



21 



^com'/iK —^fioiiiiK.-^joae'hK. - 



» 



Time 



-VlvK 



ill''.' 



fimse 



FIG. 3. Recorded intracutaneous temperature change at a 

 depth of 0.6 mm on application of a thermode at I7°C on the 

 skin at 33.5^0. A distinct cold sensation persisted throughout 

 the whole experiment although the rate of change after 3 rnin. 

 fell below the minimum value of o.oo25°C per sec. given by 

 Gertz for the maintenance of a cold sensation. [From Hensel 

 (42)-] 



