CH. XL.] CALORIFIC VALUE OF FOOD 599 



temperature to fall, and just at the onset of death it may be below 

 30 C. Active muscular exercise raises the temperature temporarily 

 by about 0'5 to 1 C. Diseases may cause the temperature to vary 

 considerably, especially those which we term febrile (see p. 592). 



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 bociy, 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 follow- 

 ing 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 tempera- 

 ture of one gramme of water 1 C. : 



Hydrogen .... 34662 Fat 9400 



Carbon . . . . 8100 j Cane sugar . . . 3950 



Urea 2530 Starch .... 4160 



Albumin .... 5600 



It is, however, most important to remember that the " physiologi- 

 cal heat-value " of a food may be different from the " physical heat- 

 value," i.e., the amount of heat produced by combustion in the body 

 may be different from that produced when the same amount of the 

 same food is burnt in a calorimeter. This is the case with the pro- 

 teids, because they do not undergo complete combustion in the body, 

 for each gramme of proteid yields a third of a gramme of urea, which 

 has a considerable heat-value of its own. Thus albumin, which, by 

 complete combustion, yields 5600 heat-units, has a physiological 

 heat- value = 5600 min us one-third of the heat-value of urea (2530) 

 = 5600 846 = 4754. Eubner has recently shown that this figure 

 must be reduced to nearly 4000, as some of the imperfectly burnt 

 products of decomposition of proteids escape as uric acid, creatinine, 

 etc., in the urine, and there is a small quantity of similar substances 

 in the faeces. Any difference between the physical and physiological 

 heat-values of fats and carbohydrates may be neglected, provided all 

 the fat and carbohydrate in the food is absorbed. 



Of the heat produced in the body, it is estimated by Helmholtz 

 that about 7 per cent, is represented by external mechanical work, 

 and that of the remainder about four-fifths are discharged by radia- 

 tion, conduction, and evaporation from the skin, and the remaining 

 fifth by the lungs and excreta. This is only an average estimate, 

 subject to much variation, especially in the amount of work done. 



The following table exhibits the relation between the production 

 and discharge of energy in twenty-four hours in the human organism 



