474 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



The mechanism concerned in the adjustment of the relations between heat- 

 production and heat-dissipation will be considered under another heading 



(p. 495). 



B. Income and Expenditure of Heat. 



Broadly speaking, the source of animal heat is in the potential energy of 

 organic food-stuffs — so little relatively being obtained from the heat of warm 

 food and drink and directly from external sources, such as the sun's rays, 

 that these sources may be disregarded. 



The researches of Rubner 1 have clearly shown that chemical changes in 

 the body constitute the source of animal heat. He made estimations of the 

 amount of heat that should be formed in the body as indicated by the ex- 

 change of ingesta and egesta, and also determined by direct calorimetry (see 

 below) the heat production in dogs under conditions of fasting and varying 

 diet. The results in the two cases are strikingly close, as will be observed 

 from the following table: 



,- re r, Quantity of heat ^ Vhv^ Per cent, dlf- 



Condition of dog. ^ calculated. Staeter. ference. 



Fasting 1193.7 calories 1180.1 calories —1.42 



Diet of Fat 1510.1 " 1495.3 " -0.97 



" Meat and Fat 3238.9 " 3223.2 " - 0.42 



« Meat 3515.3 " 3523.1 " +0.43 



These figures, which are in so close accord, are substantiated in their correct- 

 aess and import by the results obtained by Laulanie 2 in studies on guinea- 

 pigs, rabbits, ducks, and dogs. 



This potential energy of food may be converted into heat directly or indi- 

 rectly: directly, as an immediate result of chemical decomposition; and in- 

 directly, by mechanical movements, such as muscular contraction, the flow 

 of the blood, the friction of the joints, etc. About 90 percent, of the heat 

 of the organism results directly from chemical decompositions, and about 10 

 per cent, results indirectly from mechanical movements. The potential 

 energy of the food is transformed into kinetic energy (heat and work) essen- 

 tially by processes of oxidation. The energy-yielding food-stuffs enter the 

 body in the form of proteids, fats, and carbohydrates, due proteid is broken 

 up into urea, C0 2J I L<>, and various extractives; and the fats and carbo- 

 hydrates into C0 2 and IIJ). During these oxidative processes, by which the 

 potential energy of the molecules is transformed into kinetic energy, the 

 total amount of energy evolved by the complete oxidation of a given amount 

 of any substance is the same whether the processes are carried at once to the 

 final stages, that is, to the final disintegration products, or whether they pass 

 through an Indefinite number of intermediate stages, provided that the final 

 product or products are the same. In other words, the amount of heat 

 evolved by the oxidation of 1 gram of proteid into urea, C0 2 , and II.O is 

 the same when the molecule is oxidized immediately into these substances as 

 when tin' decomposition is carried through a number of intermediate stages. 

 1 Zeilschrift J. Biologic, 1893, Bd. xxx. S. 73. * Archives <!<■ Physiologie, 1898, p. 748. 



