72 



MAINTENANCE RATIONS OF FARM ANIMALS. 



ture. This being the case, it is clear that at temperatures consider- 

 ably below the critical temperature, all the metabolizable energy of the 

 feed will be of use to the body. Part of it will be available for 

 physiological uses as already explained, but the remainder, while not 

 available in this sense will nevertheless be of use as a source of heat. 



ISODYNAMIC REPLACEMENT. 



It was upon his earlier experiments (published in 1883) made un- 

 der substantially the conditions just indicated that Rubner based 

 his famous law of isodynamic replacement of nutrients which has, 

 played a large part in the discussion of nutrition problems. This 

 law may be briefly stated as follows: In amounts less than a main- 

 tenance ration the nutrients replace each other or body tissue in in- 

 verse proportion to their metabolizable energy. The quantities which 

 thus replace each other are accordingly said to be isodynamic. It 

 need scarcely be pointed out that the minimum of protein required 

 for the maintenance of the nitrogenous tissues is not included under 

 this law. Rubner was careful to limit the law to small amounts of 

 food. In his earlier publications he stated that it holds only below 

 the maintenance ration ; somewhat later he asserted 1 that it obtains 

 up to an excess of about 50 per cent over the maintenance requirement. 



These results of Rubner's have passed into the literature of physi- 

 ology and are still largely interpreted as representing the relative 

 values of nutrients, while Rubner's factors for the metabolizable 

 energy of nutrients have been extensively used in computing the 

 energy values not only of human dietaries but of stock rations as 

 well. Historically, Rubner's earlier investigations mark an epoch 

 in the science of nutrition. While similar views had previously 

 been advanced by others, Rubner appears to have been the first to 

 investigate the subject experimentally. The conception that the 

 replacement values of the nutrients could be measured by the rela- 

 tive contributions of energy which they make to the activities of 

 the body was a contribution of the first order to the study of nutri- 

 tion problems, but the exact form given it in these earlier experi- 

 ments proves to have been but a partial expression of the truth, as 

 Rubner's own later experiment, as well as those of others, have fully 

 demonstrated. (Compare pp. 26-28.) 



RELATION OF MAINTENANCE RATION TO CRITICAL TEMPERATURE. 



When its surroundings are above the critical temperature, the 

 animal is producing a surplus of heat as a consequence of its neces- 

 sary physiological activities and disposes of it by the processes of 



1 Biologisehe Gesotze, p. 20. 



