MAINTENANCE THE ENERGY REQUIREMENTS 311 



while the additional 4.69 Therms of heat resulting from the "work of 

 digestion" will be of use in keeping the animal warm and will obviate 

 the necessity of its katabolizing body substance for that purpose. 

 All the metabolizable energy of the ration, therefore, contributes to 

 the maintenance of the animal under these conditions, part directly 

 and part indirectly, and the availability is apparently 100 per cent, 

 while the real availability for the physiological processes in the body 

 is only 56 per cent. If the experiment were made at an intermediate 

 temperature at which the fasting metabolism would be 8.0 Therms, 

 then 2.0 Therms of the heat due to the "work of digestion" would be 

 of use in maintaining the body temperature and the apparent avail- 

 ability would be 75 per cent, i.e., the result would be a mixed one. 

 Evidently, the actual expenditure of energy in the " work of digestion," 

 and its complement, the net energy, can be determined only by ex- 

 periments made above the critical temperature. 



397. Bearing on maintenance ration. The foregoing facts 

 render it apparent that in the case of an animal on a main- 

 tenance ration the external temperature may fall considerably 

 below the critical temperature for the same animal when fast- 

 ing before there is any increase in the amount of feed actually 

 required for maintenance. Only when the temperature falls so 

 low that all the metabolizable energy of the ration is being 

 utilized, directly or indirectly, to maintain the body heat will a 

 further drop in the temperature call for greater feed consumption, 

 i.e., for an increase in the maintenance ration. These considera- 

 tions may affect the computation of actual maintenance rations. 

 An example of this is afforded by Zuntz and Hagemann's 

 results upon the maintenance requirements of the horse (387). 

 According to these investigators, a horse weighing 1000 pounds 

 requires only 4.1 Therms of net energy per day for maintenance, 

 but the body also needs to be supplied with an additional 7.8 

 Therms of heat, making a total of 11.9 Therms daily, to bal- 

 ance the loss of heat from the body. If, therefore, a mainte- 

 nance ration be computed supplying the necessary 4. i Therms of 

 available energy, it still remains to be considered whether the 

 heat arising from the so-called " work of digestion " will supply 

 the remaining 7.8 Therms of heat required. If it does not, the 

 difference, according to Zuntz and Hagemann, will be made up 

 by the katabolism of body tissue, as is illustrated in several of 

 their experiments, and the ration will not maintain the animal 

 although it contains net energy equal to the fasting katabolism. 



