CHAPTEE XL 



ANIMAL HEAT 



AMONG the most important results of the chemical processes we sum 

 up under the term metabolism, is the production of heat. Heat, like 

 mechanical motion, is the result of the katabolic side of metabolic 

 processes ; the result, or accompaniment, that is to say, of the forma- 

 tion of carbonic acid, water, urea, and other excreted products. 



As regards temperature, animals may be divided into two great 

 classes : 



(1) Warm-blooded or homoiothermal animals, or those which have 

 an almost constant temperature. This class includes mammals and 

 birds. 



(2) Cold-blooded or poikilothermal animals, or those whose 

 temperature varies with that of the surrounding medium, being 

 always, however, a degree, or a fraction of a degree, above that of the 

 medium. This class includes reptiles, amphibians, fish, embryonic 

 birds and mammals, and probably most invertebrates. 



The temperature of a man in health varies but slightly, being 

 between 36'5 and 37'5 C. (98 to 99 F.). Most mammals have 

 approximately the same temperature : horse, donkey, ox, 37'5 to 38 ; 

 dog, cat, 38'5 to 39 ; sheep, rabbit, 38 to 39'5 ; mouse, 37'5 ; rat, 

 37'9. Birds have a higher temperature, about 42 C. The tempera- 

 ture varies a little in different parts of the body, that of the interior 

 being greater than that of the surface ; the blood coming from the 

 liver where chemical changes are very active is warmer than that of 

 the general circulation ; the blood becomes rather cooler in its passage 

 through the lungs. 



The temperature also shows slight diurnal variations, reaching a 

 maximum about 4 or 5 P.M. (37'5 C.) and a minimum about 3 A.M. 

 (36 '8 C.) ; that is, at a time when the functions of the body are least 

 active. If, however, the habits of a man are altered, and he sleeps in 

 the day, working during the night, the times of the maximum and 

 minimum temperatures are also inverted. Inanition causes the 



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