ENERGY CHANGES IN ORGANISMS 485 



mammals is about 39°C. (102°F.) but that of the monotremes is only 25°C. 

 (77°F.). The temperature of the monotremes is also more variable than 

 in other mammals. Birds average about 5°C. (9°F.) higher than 

 mammals. 



507. Temperature of the Human Body. — The normal body tempera- 

 ture of man is 37°C. (98.6°F.), taken in the mouth or arm pit. The 

 temperature of a healthy individual, however, varies somewhat during 

 the day, being highest at the end of a day of activity and lowest at the 

 close of a night of rest. It also varies in different parts of the body, 

 in the liver rising to nearly 42°C. (107°F.) and in muscles varying from 

 normal to 40.5°C. (105°F.), depending upon the degree of activity. 

 In the skin the temperature approximates that of the surrounding air; 

 thus at the one extreme it may approach freezing, while at the other it 

 may rise to 34°C. (93°F.), though normally not higher, even on a hot 

 day. 



Below an air temperature of 60°F. the heat of our bodies is main- 

 tained by increasing the rate of heat production and by heat-conserving 

 clothing. Between 60° and 70°F. regulation on the part of the body is 

 accomplished largely by changes in the caliber of the peripheral blood 

 vessels and control of radiation. From 70° to 98.6°F. the temperature is 

 regulated by evaporation from the surface, combined with flushing of 

 the skin. Above the normal body temperature maximum radiation and 

 abundant perspiration are the most effective means of control, supple- 

 mented by modification of the diet and reduction of muscular activity. 



From this discussion it is clear that excessively high summer tempera- 

 tures, those from 100° to 105°F. or more, place a heavy burden on the 

 heat regulatory mechanism of the body and that the strain increases as 

 the temperature rises. It is also evident why excessive temperatures are 

 more easily borne in dry climates or in dry weather, with rapid evapora- 

 tion, than in humid climates or when there is a high degree of humidity 

 in the air, tending to decrease evaporation. 



