596 ANIMAL HEAT. [CH. XL. 



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 tempera- 

 ture, about 42 C. The temperature 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. Inani- 

 tion causes the temperature to fall, and just at the onset of death 

 it may be below 30 C. Active muscular exercise raises the tem- 

 perature temporarily by about 0*5 to i C. Diseases may cause 

 the temperature to vary considerably, especially those which we 

 term febrile (see p. 591). 



Although certain mechanical actions, such as friction, due to 

 movements of various kinds, may contribute a minute share in 

 the production of heat in the body, yet we have no knowledge 

 as to the actual amount thus generated. The great source of 

 heat is, as already stated, chemical action, especially oxidation. 

 Any given oxidation will always produce the same amount of 

 heat. Thus, if we oxidise a gramme of carbon, a known amount 

 of heat is produced, whether the element be free or in a chemical 

 compound. The following figures show the approximate number 

 of heat-units produced by the combustion of one gramme of the 

 following substances. A heat-unit, or calorie, is the amount 

 of heat necessary to raise the temperature of one gramme of 

 water i C. : 



