Tevtperature: Metabolic Aspects and Perception 



365 



summation occurs, in that the threshold for stimulation of large areas is less 

 than that for stimulation of single sensory endings, an area of cold receptors 

 being stimulated by a fall of 0.004°/sec., and one of warmth receptors by a rise 

 of 0.001 °/sec. for 3 seconds.'- Non-penetrating infrared is more effective as a 

 thermal stimulus than are penetrating rays; therefore the thermal sense appears 

 to depend more on the thermal gradient in the skin than on absolute tempera- 

 ture or even rate of change of temperature in the skin.'-'-* It is also possible to 

 obtain pain with thermal stimulation, but the skin receptors are not the same, 



llo 



loo - 



AIR. TEMPER.ATUR.E — DEGREES CENTIGRADE 

 lo 15 20 25 30 35 40 



40 



5o 60 70 60 90 loo 



AIR TEMPERATURE — DEGREES FAHRENHEIT 



Fig. 95. Body temperature of wrens at different air temperatures at different ages 

 after hatching. From Kendeigh."* 



as shown by the following: the pain threshold is raised and the heat threshold 

 lowered by acetylsalicylic acid; after occluding tbe blood flow to an arm for 

 some minutes only the pain sense remains, and pain shows little summation as 

 compared with heat or cold.^^ The- threshold for pain by a 3-minute thermal 

 stimulation is 0.218 cal./cm.-/sec., compared with the warmth threshold of 

 0.00015 cal./cm.-/sec.i-''^- 



The nervous pathways by which reflex responses to thermostimulation are 

 elicited are not well known. There may be local vascular reactions, largely 

 by axon reflexes, also some direct responses of blood vessels to temperature 

 change, but the principal responses of vasodilation, vasoconstriction, and hair 



