306 PHYSIOLOGICAI. EEGULATIONS 



than the first, both during periods of rapid change, and especially 

 where one volume of tissue (superficial) is being cooled while 

 another volume (deep) is not being cooled. 



Acclimatizations have been recognized, whereby exposure to 

 given conditions yields smaller loads after repetitions than before. 

 Changes of body heat content are thereby resisted. A man im- 

 mersed in water at 5° C. for a few seconds subsequently diminished 

 the rectal temperature by 1.0° C. After 3 months of training, twice 

 as long an exposure secured less decrease of rectal temperature 

 (0.1 to 0.7° C). Less heat was also lost in the later exposures 

 (Lefevre, 1894). In periods of physical exercise in hot atmos- 

 pheres, progressively faster losses of latent heat occurred, with 

 smaller accumulations of heat in the body, during the first ten days 

 of life in the desert (Adolph, '38). Even basal heat production 

 appears to become slightly slower every summer (Gessler, '25), 

 when the organism partially transforms itself from a frequent 

 regulator for deficits to a more frequent regulator for excesses. 

 All these are acclimatizations. 



It might be possible to derive an '' absolute" value for the heat 

 content of man; this would ideally be the total heat lost in cooling 

 to absolute zero, a determination not likely to be made. Such a 

 value would be useful in comparing heat increments with incre- 

 ments of other components, for all changes of heat content might 

 then be fractions of the content that prevails during heat balance. 

 Some value for this content (in Cal./kg.) could be assumed, such 

 as 309° X 0.83 X Bo, the coefficient 0.83 being the mean specific heat 

 capacity of human tissues, and 309° K. the usual average tempera- 

 ture of the body. Assumptions concerning heat capacities may if 

 desired be avoided in studies solely of net exchanges, since the rela- 

 tions here considered hold equally if AT (increment of mean tem- 

 perature) is substituted for AH (increment of heat content per unit 

 of body weight). Rates of change of either local or integrated 

 tissue temperature (BT/At) then replace the rates of heat exchange 

 (SH/At). 



In brief, heat loads may be estimated with almost the accuracy 

 of water loads. Special circumstances of heat production and heat 

 distribution are recognized as opposing the experimental establish- 

 ment of either deficit or excess. The oppositions and the sharp 

 discomforts incident to heat loads are evidences of the narrow 

 range within which heat content is usually preserved. 



