MAINTENANCE THE ENERGY REQUIREMENTS 285 



measuring the quantity of energy indispensable for its internal 

 work. As a matter of fact, however, absolute muscular rest 

 cannot be maintained for any considerable length of time, at 

 least during the waking hours, even by voluntary effort. The 

 horse or ox when at rest in the ordinary sense, i.e., when doing 

 no external work, is still expending a not inconsiderable amount 

 of energy in muscular activities of various sorts, some of which 

 were indicated in 3 of the same chapter (348). In particular, 

 it was stated (349) that standing as compared with lying causes 

 a very marked increase in the heat production, especially in 

 the case of cattle. When, therefore, the heat production of 

 such an animal in the fasting state is taken as a measure of the 

 energy required for its maintenance, it does not represent a 

 state of absolute rest but simply with one of relatively less 

 activity. The energy requirement for maintenance in the 

 economic sense includes not only the absolute minimum re- 

 quired for the internal work but also the amount expended in 

 various forms of incidental muscular work which are in a sense 

 unnecessary physiologically but are unavoidable practically. 

 Moreover, since the amount of this incidental work is more or 

 less variable as between different individuals and in the same 

 individual at different times, the energy requirement for main- 

 tenance is not a fixed, constant quantity whose exact value can 

 be determined, but a variable one. The purpose of investiga- 

 tion is to show the range of variation which may be expected 

 and to determine a general average value for the conditions of 

 ordinary practice. 



The maintenance requirement of swine 



377. Net energy requirement. With animals such as man, 

 carnivora or swine, having a comparatively simple digestive 

 apparatus and consuming relatively concentrated feed, the 

 fasting energy expenditure can be determined without special 

 difficulty by depriving the resting animal of feed during a rela- 

 tively short period and measuring the katabolism with the 

 aid of a respiration apparatus or calorimeter. The total 

 amount of heat produced, determined either directly or 

 by calculation, furnishes the measure of the energy expenditure 

 and therefore of the net energy requirement for maintenance. 



