58 PROTEIN REQUIREMENTS OF CATTLE: MITCHELL 



phases of muscular contraction (^^) is toward the view that carbohydrate 

 is the immediate source of muscular energy. The entire series of events 

 occurring during and subsequent to a muscular contraction can be 

 described, apparently, without reference to protein, amino acids, or the 

 nitrogenous extractives, with the exception of creatine, which seems not 

 to be destroyed in the process (^^). Furthermore, the so-called fatigue 

 products of muscles, in so far as available information indicates, are 

 neither nitrogenous in character nor related to protein metabolism (^*). 

 It is true that investigations are on record indicating changes in the con- 

 tent of purines (^^) and of creatine (^^) in muscle as the result of activity, 

 but the relation of these changes to the activity itself is obscure. The 

 relation is probably indirect, and the function of the non-protein nitro- 

 genous constituents of muscle relative to contraction is quite possibly 

 regulatory only. Functioning in this way it is conceivable that they may 

 not be consumable during muscular activity. 



To many investigators it has seemed almost axiomatic that muscle 

 tissue must undergo disintegration as the result of contractile activity. 

 And when experimental investigation has failed to indicate any such 

 disintegration, or at least any considerable disintegration, it has been 

 considered necessary to assume that most of the nitrogen thus degraded 

 escapes excretion in the urine by some process of reutilization in the tis- 

 sues (^''), a hypothesis apparently beyond the scope of experimental 

 enquiry. 



It seems that this view of the necessary wastage of muscle tissue during 

 activity has resulted from the analogy so often drawn between the mechan- 

 ical motor and the animal motor. In the early history of physiology this 

 analogy has served an admirable pur|)ose, and in pedagogy it is still 

 extremely useful. But, like many other analogies, it is only partially true, 

 and if pushed too far it will confuse rather than enlighten. It seems 

 apparent that one phase of motor activity to which the analogy has an 

 extremely doubtful application, if it applies at all, is the wearability of 

 the motor. The mechanical motor undoubtedly wears out at a rate that 

 bears a close relation to the amount or the intensity of the work per- 

 formed. But this wearing out is due mainly to the friction of moving 

 parts on bearings, that is, to a factor that has no l-noivn counterpart at 

 least, in the animal body. Nevertheless, the opinion prevails in many 

 quarters and is frequently expressed in print, that the animal motor also 

 must wear out, and since the substance of the animal motor is largely 

 protein, the conclusion has seemed reasonable, if not inevitable, that the 

 protein catabolism must be increased as the result of muscular work. 

 Eubner's term for the maintenance requirement of protein, the " wear- 



