TEMPERATURE SENSES IN FROG's SKIN 93 



with the web folded between them, a very different position from 

 that later observed in the response to cold. Responses increased 

 in vigor with the increase in heat and were often followed by 

 jerking and twisting of the whole body. 



The interval between stimulation and response to heat (actual 

 reaction time) was relatively long. Frogs which responded 

 unfailingly at 40°C. might not do so for twenty seconds after 

 contact with the stimulant. This also contrasted with the short 

 reaction time of the cold response and indicated that the receptors 

 for warmth in the frog's skin lie deep, and those for cold are 

 superficial as in the human skin. The reaction time showed a 

 fairly regular decrease with the increase of the heat. This was 

 observed in all experiments with increasing temperatures. Thus 

 far the experiments with heat show a characteristic response 

 which occurred with clock-like regularity; with increasing heat 

 the first responses were between 35°C. and 43°C. and the reaction 

 time was comparatively Jong, but decreased as the heat increased. 

 The range of the first type of response was only slightly modified 

 when the stimulus was very gradually applied. 



Is there a temperature sense in the skin? With regular re- 

 sponses thus secured in the foot, the next problem was to show 

 that these responses were not due to stimulus of the muscles, 

 and to prove the exclusive presence of the temperature sense 

 in the skin. 



The skin was desensitized by three different methods. A series 

 of tests paralleling the preceding was made upon frogs with 

 one foot normal and the other treated with a solution of 1 per 

 cent cocaine, upon frogs with one foot normal and the other with 

 the sciatic nerve cut, and upon frogs with one leg normal and 

 the skin removed to the ankle from the other. 



The cocaine method of desensitizing the skin has been used 

 by L. W. Cole ('10), Crozier ('16), and others. By varying the 

 length of treatment with the 1 per cent solution of cocaine, dif- 

 ferent senses in the skin can be affected or even eUminated. In 

 experiments which involved only the heat sense the foot was 

 cocained for thirty minutes immediately preceding the experi- 

 ment. It was also immersed in the cocaine solution during the 



