PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 59 



and-tear quota," obviously is based upon this analogy of the mechanical 

 motor and obviously implies that the catabolism of tissue protein will 

 increase with increased motor activity. The term is an unfortunate one, 

 particularly since it has attained a wide currency. 



Reviews of the literature — a critical comparison. — Since as early as 

 1855, the influence of work on protein metabolism has been a favorite 

 subject of inquiry and a considerable number of experiments have been 

 reported in the literature. However, the results secured have been 

 variable and at the present writing opinion is divided as to whether work 

 increases the catabolism of tissue proteins or is without appreciable 

 effect. From time to time these experiments have been reviewed, but it 

 appears to be difficult to reconcile them with any definite conclusion. In 

 1909 they were reviewed by Tigerstedt(^^), who was not convinced 

 that there is an increase of nitrogen metabolism resulting from muscular 

 work. On the other hand, they were reviewed by Magnus-Lev}'(^^) at 

 about the same time (1907) and the conclusion was reached that it is 

 probable that such an effect occurs. In 1917 LuskC**) concluded from a 

 study of the same literature not only that " muscular work does not 

 increase protein metabolism," but also that " the character of the protein 

 metabolism is unchanged by muscular activity." However, in 1925 Cath- 

 cartC^) again reviewed the literature, which, in conjunction with recent 

 experiments of his own, induced him to state that, in spite of this very 

 definite conclusion of Lusk, "the accumulated evidence seems to me to 

 point in no unmistakable fashion to the opposite conclusion, that muscle 

 activity does increase, if often only in small degree, the metabolism of 

 protein," although " there is no possible ground for the view that protein 

 is the source of muscular energy." But the former statement may be more 

 nearly true than Cathcart himself suspects. 



It would be far beyond the scope of this paper to inquire into the 

 many reasons why critics weigh essentially the same evidence with such 

 discordant results. It appears to be due in part to different criteria as to 

 what constitutes competent experimental evidence. Thus, Atwater and 

 Sherman ( ''2) insist that the diet during the working period should be 

 increased in its energy content to allow for the increased energy require- 

 ment; otherwise, an increase in the protein catabolism may mean simply 

 that tissue protein is being destroyed as a source of muscular energy, 

 rather than as the result of an inevitable wear on the muscular tissues. 

 Cathcart, however, is not so discriminating, and does not disregard or even 

 discount experiments in which no such assurance is given. Again, the 

 results of some experiments are considered in- toto by some, and only in 

 part by others. Shaffer's C^) well-known experiment offers a case in point. 



