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HANDBOOK OF PHYSIOLOGY --' NEUROPHYSIOLOGY I 



30 



Temperature G 



FIG. 4. 'Adaptation periods' (the time until the temperature sensation disappears) as a function 

 of the stimulus teinperature when using constant temperature for stimulation. The adaptation 

 periods become longer as the stimulus temperature departs from the indifferent temperature 

 (32.5''C). Broken lines indicate periods as function of the stimulus temperature after which the intra- 

 cutaneous temperature change at a depth of i mm has fallen below values of 0.2 "C per sec. and of 

 o.o°C per sec. The adaptation periods do not at all coincide with the subsidence of the intracuta- 

 neous temperature change. [From Hensel (42).] 



cm- per sec. according to depth of the layer and the 

 cutaneous circulation. 



By theoretical as well as by experimental investiga- 

 tion Hensel (44) showed that changes in the blood 

 flow in the skin exerts much less influence upon the 

 diff'usivity (the thermometric conducti\ity) than it 

 does upon the thermal conductivity. 



Relation Between Temperature Change Recorded in 

 Skin and Thermal Sensations 



Using the methods descrilsed above Hensel (43, 44) 

 made thorough investigations of the relation between 

 the thermal sensations reported by the subject and the 

 actual temperature movements in the skin when well 

 defined thermal stimulation was applied to the skin. 



In confirmation of earlier workers, Heilbrun (41) 

 and Hensel (42) demonstrated that thermal sensa- 

 tions still persisted when the temperature of the skin 

 had reached a constant level. With the above de- 

 scriljed method Hensel recorded the temperature 

 movements at a depth of 0.6 mm when a rectangular 



thermal step, t, was applied. As will he seen in figure 

 3, the rate of thermal change had gone down below 

 the value of 0.0025° P^r ^^c. which Gertz (30) had 

 found to be the minimum rate necessary to maintain 

 a thermal sensation, .\fter 20 min., when the tempera- 

 ture had been practically constant for some minutes, 

 there was still reported a diminishing but quite dis- 

 tinct cold sensation. 



In figure 4 the adaptation period (interval from 

 the stimulus application until the disappearance of 

 thermal sensation) and the interval until the tempera- 

 ture change stopped is plotted against the tempera- 

 ture applied to the skin. At temperatures below 20°C 

 and above 40°C constant sensations appear. Hensel 

 (42) found that the adaptation requires a longer time 

 the more the temperature of the stimulus diverges 

 from the temperature of the skin. But the cessation of 

 the thermal sensation and the intracutaneous tempera- 

 ture changes do not coincide, as the sensation usually 

 considerably outlasts the intracutaneous temperature 

 movement. This is particularly the case at extreme 

 temperatures. 



