CHAPTER XII 



ANIMAL HEAT 



HEAT is produced by the metabolism of the tissues of the body. In man 

 and in such animals as are called warm-blooded, i.e., only mammals and 

 birds, there is an average body temperature which is maintained with only 

 slight variations in spite of changes in their environment. The possible 

 variations above and below this average are comparatively slight. The 

 average temperature in all mammals and birds is not the same, for, as we 

 shall see, the average temperature of man is 37 C. (98.6 F.), in some birds 

 it is as high as 44 C., while in the wolf it is said to be under 36 C. 



The average temperature of the human body in those internal parts which 

 are most easily accessible, as the mouth and rectum, is from 36.9 to 37.4 C. 

 (98.5 to 99.5 F.). In different parts of the external surface of the human 

 body the temperature varies only to the extent of one or two degrees centi- 

 grade, when all are alike protected from cooling influences; and the differ- 

 ence which under these circumstances exists depends chiefly upon the 

 different degrees of blood supply. In the axilla and in the groin, the most 

 convenient situations, under ordinary circumstances, for examination by the 

 thermometer, the average temperature is 37 C. (98.6 F.). In different 

 internal parts, the variation is one or two degrees; those parts and organs 

 being warmest which contain most blood, and in which there occurs the 

 greatest amount of chemical change, e.g., the muscles and the glands. The 

 temperature is highest when they are in a condition of activity. Those tis- 

 sues which subserve only a mechanical function and are the seat of least ac- 

 tive circulation and chemical change are the coolest. These differences of 

 temperature, however, are actually but slight, on account of the provisions 

 which exist for maintaining uniformity of temperature in different parts. 



The average temperature of a healthy body varies somewhat according 

 to age, sex, time of day, climate, etc. The mean temperature is said to be 

 slightly higher, 0.5 C., in young children and in old persons than in adults. 

 It is perhaps very slightly higher in women than in men, in warm climates 

 than in cold, in winter than in summer. It varies slightly at different times 

 in the day, especially during sleep when metabolism is at a low ebb. 



Heat-producing Organs. Heat is liberated in the body wherever 

 oxidative metabolism takes place. Of all the tissues of the body muscular 

 tissue is conspicuous for its mass and for its activity. It is evidently the great 

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