580 A MANUAL OF PHYSIOLOGY 



of all the heat produced in the body is the chemical energy of 

 the food substances. Whatever intermediate forms this energy 

 may assume whether the mechanical energy of muscular con- 

 traction ; 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. We do not know at what precise stage of 

 metabolism the chief outburst of heat takes place. But it is 

 known, as already pointed out (p. 504), that the fraction of the 

 total energy liberated in the processes 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. 398) 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 accomplished 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 pancreatic digest was found to yield, 

 when burned in the calori metric 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 the 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 combustion 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 founded on the heat of 



