THERMAL SENSATIONS 



441 



side this temperature region the required slope 

 dd/dt sinks further until it finally attains a zero value, 

 i.e. where the temperature level itself is sufficient for 

 elicitation of the sensation. 



When the temperature change starts from different 

 temperature levels (adaptation temperature), the 

 thresholds for the warm .sensation will reach different 

 values (see fig. 9). With a constant rate of change of 

 0.0017° P^i" sec. it is thus found that the threshold for 

 warmth will depend upon the initial temperature to 

 which the receptors have been adapted. The lower 

 this initial temperature, the greater the heating has 

 to be in order to elicit a sensation of warmth. For the 

 cold sensation it is the other way when initial tem- 

 perature is lowered. Here the cooling necessary 

 becomes less and less intense until the temperature 

 region is reached where a steady cold sensation ensues. 

 For higher initial temperatures, the opposite holds. 

 Here the cold receptors for equal cold steps become 

 less sensitive the higher the initial temperature is 

 taken (23, 37, 42). 



It has long been recognized that the stimulated 

 area and thus the number of stimulated thermal re- 

 ceptors must be of great importance in the production 



t-i 



U 

 (U 



£ 



H 



37 



"C 

 3f 



35 



3V 



33 



32 



3! 



30 



29 



26 



2S 

 25 



W* 



w* 



W+ warm threshold 

 W++ distinct warmth 



Time t 



J L 



J 



8 min 70 



FIG. 9. Warm thresholds on forearm exposed to a rectilinear 

 increase of temperature of o.oi7°C per sec. from initial tempera- 

 tures of 25°, 30° and 35°C. (Thermode area, 20 cm^.) [From 

 Hensel (45).] 



of thermal sensations, although for long periods the 

 use of more or less punctiform stimuli has been preva- 

 lent. The temperature sense in life situations is affected 

 o\er much of the body surface as Hensel (45) em- 

 phasizes. This is inter alia seen from the fact that the 

 cold and warm spots were not discovered until modern 

 times (9), although they can be detected by the most 

 simple devices. 



Investigations on the temperature sensations when 

 the whole body surface was exposed have been made 

 by Marechaux & Schafcr (67). In a climate chamber 

 of the type used by Wetzler & Thauer the subjects 

 were exposed to approximately linear increases of the 

 temperature with a slope of o.ooi to o.oi°C per sec. 

 As the temperature of the chamber rose, the skin 

 temperature of the different parts of the body rose 

 relatively linearly. The average rate of the skin tem- 

 perature rise during the most rapid rises amounted 

 to 0.0015 to 0.003° per sec. and during the slowest 

 rise to less than o.ooi ° per sec. Starting from a general 

 coolish sensation the sensation of warmth appeared 

 regularly in the following order: forehead-abdomen- 

 hand-foot, in agreement with the investigations on 

 more limited areas carried out i)y Gilsbach (31) and 

 Hensel (42). The sen.sations produced appeared in this 

 order: cold-indifTerent-faintly warm-distinctly warm. 

 Table 2 shows the warmth threshold temperature 

 with slow rise of the chamber temperature. As will 

 be seen it is not possible even with an extremely slow 

 rate of temperature rise at less than 0.001° per sec. 

 to avoid the production of a sensation of warmth when 

 the temperature of the skin is aijove 35°C. The region 

 of thermal indifference when the whole body is con- 

 cerned is thus limited to a small region between about 

 32 to 35°C. Previous investigations of Rein & Strug- 

 hold (73, 74), Stein & von Weizsacker (82), Bohnen- 

 kamp & Pasquai (11), Hardy & Oppel (38) and 

 Herget et al. (57) have all shown that there is a marked 

 decline in the threshold when the number of stimu- 

 lated sen.sory spots is increased. 



Besides the temporal temperature gradient (dd/dt) 

 a spatial temperature gradient (d^/d*) has been 

 widely discussed. Ebbecke (26) observed that the 

 release of blood flow into a previously clamped and 

 cooled limb elicited an intense and unexpected .sensa- 

 tion of cold. This led him to suggest that a cold sensa- 

 tion is produced by a temperature difference in the 

 skin at the border line of the epidermis and the cutis, 

 while a warmth sensation is produced by a tempera- 

 ture difference at the border line between the cutis 

 and the subcutis, the direction of the temperature 

 gradient being immaterial. This idea was confuted 



