400 INFLUENCE OF MUSCULAR EXERCISE ON THE URINE. [BOOK I. 



diverted from the kidneys. It is therefore as probable that the 

 kidneys act ill during excessive muscular exertion, as that digestion 

 is imperfectly performed in the same circumstances. Further, 

 the formation of urea, the end*product of nitrogenous waste, takes 

 place in all probability in several stages, of which the earlier only 

 have their seat in muscle itself. This much at least is certain, that 

 muscle contains little or no urea, either at rest or after contraction ; 

 whence it must probably be concluded that if the proteids of muscle 

 contribute to the urea excreted normally, their contribution takes the 

 immediate form, not of urea,- but of some antecedent of urea. It is 

 not necessary to suppose that this antecedent form is creatine or any 

 body like creatine : indeed, as will be urged elsewhere, the tendency 

 of the experimental evidence is to render it very improbable that any 

 of the urea excreted passes through a preliminary creatine-stage ; for 

 when creatine is introduced artificially into the blood it is invariably 

 excreted not as urea, but as creatine. The form in which muscle- 

 proteid leaves the muscles, after having become effete as contractile 

 matter, may still be proteid ; in which case the whole oxidation of 

 muscle-proteid to the urea-form would occur altogether outside the 

 muscular tissues. But, if urea is not at once formed in muscle if 

 the nitrogenous waste of muscle escapes into the blood in a proteid 

 (or other) form the elaboration of the waste material into the form 

 in which it is actually excreted must go on elsewhere ; and in what- 

 ever organ this elaboration has its seat, it is very probable that the action 

 of the organ is hampered during prolonged or excessive muscular 

 exercise. 



The manufacture of urea in two stages also fully explains the 

 other fact elicited in Parkes' experiments, viz. that during absolute 

 rest the elimination of nitrogen is slightly increased, if the diet 

 remains the same. Of the nitrogen ingested as food, part is de- 

 composed more or less directly and appears in the urine at once as 

 urea, and part serves to repair the nitrogenous waste of muscles, 

 reaching the urine by a circuitous path through muscular tissue; 

 the latter portion appearing in the urine at a later date than the 

 former. If anything occurs to diminish the wasting of muscle, less 

 nitrogen is yielded up by muscle to the urine, but at the same time 

 less is called upon to repair waste, and more, therefore, passes 

 directly into the urine from the food. Thus the same quantity of 

 nitrogen should appear in the urine whether muscles be exerted or 

 not, so long as the food remains constant. But this is only true if the 

 nitrogenous waste products of muscle pass at once from muscle into the 

 urine ; which ex hypothesi is not the case. On the contrary, they are 

 intercepted by some other organ and delayed. This organ, therefore, 

 at any given moment, contains waste products derived from muscle, 

 in course of preparation for excretion as urea. If, now, muscles are 

 suddenly thrown out of employment, less nitrogen of the food is called 

 upon to repair waste of tissue and more passes directly into the urine; 

 but at the same tinie the waste nitrogenous matters which happen 



