HETEBOTHERMY IN HOMEOTHEBMS 



physical dimensions in absolute terms. For example, the force re- 

 quired to move the mass of alegdoes not change with temperature, 

 but physiological processes involved in sensory detection of physical 

 forces do change with temperature. Mammals and birds ap- 

 pear to differ from the cold-blooded animal, however, in the large 

 inconstancy of temperature in superficial tissues and in the integra- 

 tion of individuality in this heterothermous condition. 



After seeing that the fingers of people adapted to cold were use- 

 fully coordinated when very cold, I have been trying to find a per- 

 tinent measure for their sensitivity. It seemed to me that terrestrial 

 animals would need constant appreciation for force and mass in 

 order to move. Stimulation by impact is a convenient test because 

 mass and the distance through which it falls can easily be varied and 

 measured. 



Cabbage seeds selected for uniform weight, about a miligram, 

 were found detectable after falling about 20 mm onto the ball of the 

 warm mid- finger. The impact of a seed of double the size was no- 

 ticeable after falling 10 mm, or the threshold for stimulation varied 

 about as the kinetic energy of the impact. Other parts of the skin 

 differed in sensitivity, and as the skin was cooled, a heavier weight 

 or longer fall was required for the impact to be detectable. 



It was easier to discharge mercury droplets weighing from 1 to 

 3 mg by Scholander's micrometer burette which, with a plunger 1.59 

 mm in diameter, measured volumetrically the drop discharged 

 through a hypodermic needle to within a few hundredths of a milli- 

 gram. The kinetic energy of the impact on the ball of my middle fin- 

 ger that I could just detect increased rather regularly about eight 

 times as my finger was cooled from 35 C to 20 C (Fig. 12). 



Keith Miller is now using small steel ballbearings for weights 

 and finding that when measured as kinetic energy of detectable im- 

 pact the threshold stimulus increases regularly in a trained subject 

 as the skin cools. Individuals differ in sensitivity and in the rate of 

 diminishing sensitivity with cold. We have not discovered whether 

 this measure of sensitivity of cold fingers will distinguish differ- 

 ences in the people accustomed to cold whom we regard as adapted. 



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