112 ANN HAVEN MORGAN 



equivalence between chemical and tactile stimuli or degrees of 

 heat and cold. Granting this necessary inaccuracy, 45°C. heat 

 and 2°C. cold were selected as sufficient extremes to be set against 

 each other. 



There is a definite cold sense present in the frog's skin. When 

 the foot was immersed in water of decreasing temperatures, the 

 first responses occurred at 10°C. Contrasted with that of heat, 

 the interval between stimulation and response was an inconsider- 

 able period and could not be accurately taken with a stop-watch. 

 The responses to cold were of two types, a sudden rigidity of the 

 muscles of the leg, with a spreading of the toes and web, or an 

 upward jerk instantly following the contact of the toes with the 

 water. The latter action was less frequent and usually occurred 

 after stimulation by severe cold or in unusually sensitive frogs. 

 Such responses differed from heat responses only in the length 

 of the reaction time. 



The sense of cold may be wholly eliminated by cutting the 

 nerve, removing the skin, or by cocaine treatment. It can be 

 shown to be independent of heat, and the tactile and chemical 

 senses by the same treatment. In such comparisons sensation 

 to cold disappears, acid remains; cold disappears and heat and 

 pain remain, but cold remains while touch is eliminated. 



The frog's skin is indifferent to temperature of 10°C. or 15° C. 

 to 35°C., whether the stunulation be made by gradual increases 

 or whether it be given suddenly at one selected degree. 



SUMMARY 



The skin of the frog contains well-defined receptors for heat 

 and for cold. The heat receptors have a comparatively long reac- 

 tion time. The heat receptors are stimulated by 39°C. to 43°C.; 

 the cold receptors at 10°C. This response is immediate and 

 becomes more vigorous as the cold is increased. The typical 

 response to heat is an upward jerk of the leg. The typical 

 response for cold is a rigidity and tenseness of the muscles, but 

 there may be an upward jerk similar to that of the heat response. 

 Responses to heat and cold may be separated from each other 

 and from the tactile and chemical senses. 



