INCOME AND EXPENDITURE OF ENERGY 683 



Per Cent. Calorie*. 



f Evaporation of water - - 15 \ 400 



Skin \ Radiation* - - 25 80 650 



(.Conduction (and convection) 40 J 1,000 



J Evaporation of water - - 15 \ (400 



\Heating the expired air- - 2-5) I7 ' 5 \ 70 



Heating the excreta - 2-5 70 



too 2,590 



In the rabbit, according to Nebelthau, the heat lost by evaporation 

 of water is about 16 per cent, of the whole, or about half the proportion 

 in man, according to the above calculation. This is not surprising 

 when we reflect that the rabbit does not sweat, and drinks comparatively 

 little water. 



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 body 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. 544), 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. 426) 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, 

 1343-6 calories. That is to say, the hydrolysis of these disaccharides 

 to monosaccharides, 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 nitration. 



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 



* The relative amounts lost by radiation and conduction cannot bs accur- 

 ately fixed. The proportion is extremely variable. 



