Special Senses: Thermal Receptors

an en­ entirely new view of the peripheral afferent part of the thermoregulatory system, as well as of many sensory phenomena hitherto the subject of much debate. For that reason it was considered justified to limit this review en­ tirely to thermal receptors. The temperature movements in the skin have been subject to theoretical as well as experimental investigations by Hensel (2, 3), who has developed effective methods for recording the intracutaneous temperature gradient in stationary conditions as well as a precision-flow-ca lorimeter for measuring the steady heat flow given off from small skin areas, the mean error of the method being only ± 0.0001 cal. cm.2 sec.-1 The new Stromungskalorime ter constructed by Hensel (4) can be used clinically also for the determination of the blood flow through the skin. For details, see Hensel's monograph der Thermoreception in Ergebnisse der Physiologie (5). The depths of cold receptors in the tongue of the cat have been recorded in aIi objective way by Hensel, Strom, and Zotterman (6). This method is based on the recording of the spike potential of specific cold fibres in the lingual nerve set up by applying rapid cold jumps to the tongue and by simul­ taneous recording of the temperature changes. The depth of the cold recep­ tors was calculated from determinations of (a) the threshold, (b) the tempera­ ture change of the thermode, (c) the latency of the specific cold spikes, (d) the thermal diffusion coefficient of the tongue. From a great number of meas­ urements on the tongue of the cat, a mean value of 0.18 mm. ±0.04 was ob­ tained. The cold receptors are thus situated closely beneath the epithelium. At this depth an abundance of afferent nerve endings of various configura­ tions have been described by histologists (ef. Sanders (7)). The investigations of the receptive field for thermoceptors has often led to much debate. The question is, as Jenkins (8) formulates it, whether it should be compared with an unsensitive surface with spread circumscript sensitive spots or rather as a continuum with peaks, valleys, and plateaus. The peaks should then cor­ respond to the spots according to the classical view. Hensel (5) maintains 1 The survey of the literature pertaining to this review was concluded in June,