INCOME AND EXPENDITURE OF ENERGY 659 



Sources of the Heat of the Body Heat Production. Some heat 

 enters the body as such from without in the food, and by radiation 

 from the sun and from fires,, The ultimate source of all the heat 

 produced in the bdy is the chemical energy of the food substances. 

 For this reason, the distinction between much of the subject matter 

 of this chapter and that of Chapter X. is scarcely even a formal 

 one. Whatever intermediate forms this energy may assume 

 whether the mechanical energy of muscular contraction ; the energy 

 of electrical separation by which the currents of the tissues are 

 produced; the energy of the nerve impulse; or the energy, be it 

 what it may, which enables the living cells to perform their chemical 

 labours it all ultimately, except so far as external mechanical 

 work may be done, appears in the form of heat. As already pointed 

 out (p. 536), the fraction of the total energy liberated in the pro- 

 cesses of hydrolytic cleavage is comparatively small. Most of the 

 heat is set free in the oxidative processes which accompany or follow 

 the hydrolytic changes. 



Thus the energy-value of a gramme-molecule (p. 420) of maltose, 

 cane-sugar, or lactose is a little more than 1,350 calories; that of the 

 two gramme-molecules of dextrose formed by hydrolysis of the maltose 

 is 1347-4 calories; that of the gramme-molecule each of dextrose and 

 levulose formed from the cane-sugar, 1349*6; and that of the gramme- 

 molecule each of dextrose and galactose formed from the lactose, 

 I343'6 calories. That is to say, the hydrolysis of these disaccharides 

 to monosacchariides, which is the first step in their metabolism, is accom- 

 plished with the liberation of very little heat. The same is true of the 

 splitting of the fats and proteins. The dried residue of a filtered pan- 

 creatic digest was found to yield, when burned in the calorimetric 

 bomb, only 10 per cent, less heat than the same weight of dry meat. 

 Much the greater part of this deficiency was accounted for by tke leucin 

 and tyrosin which had crystallized out, and the derivatives of higher 

 fatty acids in the meat, as these would be removed from the digest 

 by filtration. 



It has been shown that the law of the conservation of energy holds 

 for the animal body; in other words, there is a practically exact 

 agreement between the potential energy of the food and the kinetic 

 energy into which it is transformed in the body both during rest 

 and during work. This kinetic energy is represented by the heat 

 given off plus the heat-equivalent of any mechanical work done 

 (Atwater). In other words, the food, whether it is burned in a 

 calorimeter to simple end-products like carbon dioxide and water, 

 or more slowly oxidized in the body, yields the same amount of 

 heat, provided always that in both cases it is entirely consumed, and 

 that no work is transferred to the outside. In the body the com- 

 bustion of carbo-hydrates and fats is complete; but the nitrogenous 

 residues of the proteins urea, uric acid, etc. can be further 

 oxidized, and the remnant of energy which they yield must be 

 taken into account in any calculation of the total heat-production 



