Digestion — Respiration — Calorimetnj — Energy. 49 



The fuel value of auy food which remams after deducting these 

 three losses represents the available energy of the food, or that por- 

 tion which the animal can use for body purposes. 



70. Net energy. — The available energy of the food measures its 

 value for heat production, but, as will be shown in the next article, 

 does not represent its true value to the animal for other purposes. 

 A portion of the total available energy of any food must be ex- 

 pended in the work of masticating and digesting it and of assim- 

 ilating the digested nutrients. The energy so expended finally 

 takes the form of heat, but is not available for other purposes in 

 Ihe body, since the animal has no power to convert heat into other 

 forms of energy. That portion of the energy which remains after 

 masticating, digesting, and assimilating the food is termed the net 

 energy of the food. This net energy is used by the animal in the 

 work of the heart, lungs, and other internal organs, and in case a 

 surplus of net energy remains after satisfying the maintenance 

 requirement of the animal, such surplus may be used for produc- 

 ing fat, growth, milk, or wool, or in the performance of external 

 work, etc. 



71. Available and net energy. — There has already been given a 

 brief description of the respiration apparatus, with an example 

 study by Henneberg of the intake and outgo of an ox confined 

 therein. In recent years the respiration apparatus of the earlier 

 times has been improved by adding thereto means for accurately 

 measuring the heat given off by the animal while under study. 

 The new apparatus is styled the respiration calorimeter. The first 

 respiration calorimeter in the United States was constructed by 

 Atwater with the aid of the United States Department of Agricul- 

 ture, at IMiddletown, Connecticut. It was for human nutrition 

 studies only. The first and only respiration calorimeter for animals 

 in this country was erected some years since in a special building 

 at the Pennsylvania State College by Armsby, thru the joint efforts 

 of the United States Department of Agriculture and the Pennsyl- 

 vania Station.^ 



For many years Kellner^ of the IVIockern Station, Germany, has 

 been using the respiration chamber in animal studies. His studies 

 and those of Armsby^ with the respiration calorimeter have been 

 for the most part with the mature ox. In these investigations not 



'For a popular description of these calorimeters, see Century Magazine, July, 

 1887, and the Experiment Station Record, .July, 1904. 

 " Land. Vers. Stat., 53, 1900, pp. 440-468. 

 » U. S. Dept. Agr., Bur. Anim. Ind., Bui. 101. , 

 5 



