THERMAL SENSATIONS 443 



This opinion is quite incompatible with the electro- 

 physiological findings of Zotterman (96), Hensel & 

 Zotterman (52, 54) and Hensel (45, 46) that cold 

 and warmth are subserved by specific peripheral 

 neurons which are relatively inexcitable by mechani- 

 cal stimuli. Hensel's thermoelectrical recordings show- 

 that a metal thermode through which water flows 

 quickly dominates entirely the temperature condi- 

 tions in the superior layers of the skin. The blood 

 temperature has little or no effect. At a constant 

 thermode temperature at 3o°C or above, no appreci- 

 able temperature change was observed in the layer of 

 the cold receptors in the tongue when the blood flow- 

 was arrested or released. The release of the blood flow 

 to the tongue which previously had been ischemic for 

 some minutes gave rise to an immediate return and 

 enhancement of the steady discharge from the cold 

 receptors previously paralyzed by the ischemia. The 

 Ebbecke phenomenon can thus not be explained by 

 thermal changes but by chemical changes induced by 

 the ischemia. The effects of ischemia occur equally at 

 all temperatures between 20 and 32 °C and afso when 

 there is no thermal effect of the blood flow. They 

 must all be due to oxygen lack (47). 



Paradoxical Sensations 



Striimpell (86) described patients with neurological 

 diseases displaying specific anesthesia to cold and 

 reported a very distinct heat sensation when the skin 

 was touched by pieces of ice. The reverse was less 

 often found, i.e. that heating the skin produced a 

 sensation of cold. In 1895 von Frey (91) definitely 

 established that the stimulation of single cold spots 

 with heat above 45°C caused a sensation of cold which 

 he named 'paradoxical cold sensation.' The existence 

 of a paradoxical cold sensation has been generally 

 accepted, while the corresponding paradoxical warm 

 sensation still is under debate. Lehmann (64), Al- 

 rutz (i) and later Rein (72) failed to produce any 

 paradoxical sensation of warmth. Thunberg (8g) 

 suggested in 1905 that this most likely is caused by 

 the fact that the intensive cooling evokes a very in- 

 tense cold sensation which masks the paradoxical 

 sensation of warmth which in Striimpell's case of cold 

 anesthesia was obtained unmasked. Recent electro- 

 phy.siological studies (22) reveal that warm fibers 

 actually respond to rapid cooling of 8 to i5°C but 

 this has more the character of an off discharge of a 

 phasic nature since it soon fades away. This behavior 

 of the warm receptors or the peripheral parts of the 

 warm fiber endings explains why this paradoxical 



discharge of warmth is more difficult to detect (cf. 

 page 44B). 



7 liermal After-Sensations 



Weber (94) had great difficult\- in interpreting the 

 phenomenon of the 'persisting cold sensation' experi- 

 enced for instance when a cold metal object which has 

 been pressed for about half a minute against the skin 

 of the forehead is removed. In this famous experiment 

 a cold sensation is thus experienced while the tempera- 

 ture of the receptor layer of the skin is gradually 

 warming which according to Weber's theory should 

 lead to a sensation of warmth. Weber himself sug- 

 gested that this cold .sensation was due to a further 

 spread of the cooling to surrounding parts of the skin, 

 a view which had been already rejected by Hering 

 (58) because of the inadequate spread of the cooling 

 compared to the marked rewarming of the cooled 

 area. Alrutz (2) and Holm (60) suggested that the 

 persisting cold sensation was due to paradoxical 

 stimulation of the cold receptors by their sudden 

 rewarming by the blood. The interpretation of 

 Weber was again refuted by Holm (60) who anes- 

 thetized the cooled area of the skin leaving the sur- 

 rounding area intact. In spite of normal thermal 

 sensibility in the surrounding zone, no sensation of 

 cold appeared. Further, Bazett & McGlone (4) re- 

 corded the .skin temperature below the cooled area 

 and proved that the cold after-sensation coincided with 

 an actual rewarming of the skin although they believed 

 as Weber that in their case the sensation could be 

 attributed to a spread of the cooling to the surround- 

 ing skin. 



More recently Hensel (42) has recorded the actual 

 course of the intracutaneous temperature movement 

 below as well as outside the thermode. He demon- 

 strated that the spread of cooling to adjacent parts of 

 the skin is very slight, the quantitative relation be- 

 tween the rewarming of the cooled area and the cool- 

 ing of the surrounding being 18:1 at the time of the 

 most intensive cold after-sensation. 



Thus the cold after-sensation cannot be explained 

 by a subsequent spread of cooling. At low skin tem- 

 peratures a cold sensation can be present even when 

 the temperature of the .skin is gradually rising. This 

 cold sensation is just a normal cold sensation due to 

 the low temperature of the receptor layer of the skin. 

 Electrophysiological studies of the activity of the cold 

 fibers in the cat (cf. page 446) very substantially 

 supports the view that the cold receptors at low tem- 

 peratures are displaying a steady discharge which 



