256 NUTRITION OF FARM ANIMALS 



be synthesized by the organism. It might very well be, there- 

 fore, that the minimum unavoidable protein katabolism in the 

 absence of nitrogenous feed is due to such a demand for certain 

 amino acids or other groupings, and only in part or not at all 

 to a necessary breaking down of cell proteins as a condition of 

 protoplasmic activity. Moreover, it is quite conceivable that 

 both of these views may be true ; that a part of the minimum 

 protein katabolism represents a necessary destruction of cell 

 protoplasm in the performance of its functions, while the other 

 part represents protein broken down for the sake of securing 

 certain constituents for specific purposes. 



There will be occasion to consider these possibilities further 

 in discussing the protein requirement for maintenance (398). 



2. THE ENERGY KATABOLISM IN FASTING 



341. Internal work. The body of an animal receiving no 

 feed and doing no external work is still carrying on a great 

 variety of internal activities, both mechanical and chemical. 

 Of the former, the most prominent is the muscular work of 

 circulation and respiration, together with the maintenance 

 of muscular tonus (632), while the secretory and excretory 

 activities of the various glands are typical of the latter. These 

 various bodily activities, whose due performance is essential 

 to the continued existence of the animal, may be conveniently 

 summarized in the term internal work. 



342. Measure of energy expended in internal work. In 

 the fasting animal, all the various forms of internal work in- 

 dicated in the previous paragraph are performed by means of 

 energy derived from the katabolism of the fats, carbohydrates 

 and proteins contained in the tissues. The chemical energy 

 thus utilized may undergo numerous transformations, but 

 ultimately, since it does no work upon the surroundings of the 

 animal, it assumes the form of heat. A determination of the 

 heat produced by a fasting animal in a state of rest, therefore, 

 furnishes a measure of the energy expended in internal work, 

 or of what is often called the basal metabolism. 



343. Relative constancy of energy katabolism. The results 

 recorded in i regarding the nature of the material katabolized 

 in fasting, and the way in which fat, carbohydrates and protein 



