CHAPTER LXXX 



The classification of animals into two groups warm-blooded and cold- 

 blooded according to their ability to maintain the body temperature at 

 a constant level, is more or less arbitrary. Between the two groups an- 

 other exists, represented mainly by hibernating animals, in which at 

 certain times of the year the animal is warm-blooded and at other times 

 cold-blooded. The ability of the higher mammals to maintain a constant 

 body temperature may or may not be present at the time of birth. The 

 heat-regulating mechanism of the human infant for example remains ill 

 developed for some time, so that exposure to cold is liable to lower the 

 body temperature to a dangerous degree. 



VARIATIONS IN BODY TEMPERATURE 



In animals in which the heat-regulating mechanism is fully developed, 

 there is not, even during perfect health, entire constancy in temperature 

 in the different parts of the body or in the same part at different periods 

 of the day. The average rectal temperature of man is usually stated as 

 being 37 C. (98.6 F.), but the diurnal variation may amount to 1 C., 

 being highest in the late afternoon and lowest during the night. There 

 are probably several causes for this variation, and they are in part at 

 least dependent upon the greater metabolic activities of the waking 

 hours and upon the taking of food. Apart from these influences, how- 

 ever, others which are less evident appear to operate ; for it has been 

 found that, when the daily program is reversed by night work, the usual 

 diurnal variation, although much less pronounced, still remains evident 

 even although this reversal in habit may have been kept up. for years. 

 It is of interest to note in this connection that nocturnal birds have their 

 maximum temperature at night and their minimum during the day. 



Regarding the temperature in different parts of the body, that of the 

 rectum is usually about 1 C. higher than that of the mouth, and this 

 again higher than that of the axilla. Of these three the mouth tempera- 

 ture is the most variable, for many conditions, such as mouth breathing, 

 talking, drinking cool liquids and even exposure to cold air, are sufficient 

 to lower markedly the temperature of this region. When the mouth 



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