302 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the other tissues. We must suppose that in a starving animal the fat and the 

 proteid material, particularly that of the voluntary muscles, pass into solution 

 in the blood, and are then used to nourish the tissues generally and to supply 

 the heat necessary to maintain the body-temperature. Examination of the 

 excreta in starving animals has shown that a greater quantity of proteid is 

 destroyed during the first day or two than in the subsequent days. This fact 

 is explained on the supposition that the body is at first richly supplied with 

 " circulating proteid " derived from its previous food, and that after this is 

 metabolized the animal lives entirely, so*far as proteid-consumption is concerned, 

 upon its " tissue proteid. " The general fact that the loss of proteid is great- 

 est during the first one or two days of starvation has been confirmed recently 

 upon men, in a number of interesting experiments made upon professional 

 fasters. For the numerous details as to loss of weight, variations of tempera- 

 ture, etc., carefully recorded in these latter experiments, reference must be made 

 to original sources. 1 It may be added, in conclusion, that the fatter the body is 

 to begin with, the longer will starvation be endured, and that if water is con- 

 sumed freely the evil effects of starvation, as well as the disagreeable sensations 

 of hunger, are very much reduced. 



Potential Energy of Pood. The chemical changes occurring in the body 

 are accompanied by a liberation of energy in different forms for example, as 

 heat, electricity, and mechanical work. By far the most of this energy takes 

 the form, directly or indirectly, of heat. Even when the muscles are apparently 

 at rest we know that they are undergoing chemical changes which give rise to 

 heat. When a muscle contracts, the greater part (four-fifths) of the energy 

 liberated by the chemical change takes the form of heat; a much smaller 

 part (about one-fifth as a maximum) may perform mechanical work, which 

 in turn, as in the case of the respiratory muscles and the heart, may be con- 

 verted to heat within the body. Roughly speaking, an adult man gives off 

 from his body in the course of twenty-four hours about 2,400,000 calories of 

 heat (1 calorie = the heat necessary to raise 1 cubic centimeter of water 1 C.). 

 This supply of heat is derived from the metabolism or physiological oxidation 

 of the proteids, the fats, and the carbohydrates which we take into the body in 

 our food. By means of the oxygen absorbed through the lungs these substances 

 are burnt, with the formation of CO 2 , H 2 O, and urea or some similar nitrog- 

 enous waste product. In the long run, then, the source of body-energy is found 

 in the potential energy contained in our food. Our energy-yielding foods 

 proteids, fats, and carbohydrates are more or less complex bodies which are 

 built up originally by plant organisms with the consumption of solar energy ; 

 when they are burnt or otherwise destroyed, with the formation of simpler 

 bodies (such as CO 2 or H 2 O), the contained potential energy is liberated in the 

 form of heat, and this is what occurs in the body. From the standpoint of the 

 law of conservation of energy it is easy to understand that the amount of 

 available energy in any food-stuff may be determined by burning it outside the 

 body and measuring the quantity of heat liberated. If a gram of sugar is 

 1 Virchow's Archiv, vol. 131, supplement, 1893, and Luciani, Das Hunyern, 1890. 



