1867.] Di*. Parkes on the Elimination of Nitrogen. 355 



The reason of these differences between Professors Fick and Wislicenus 

 and myself, is probably to be found in the short period of time during 

 which their observations were carried on, and also because the urea was 

 not determined by them in the night of the 30th to 31st of August. 



But their conclusion is certainly borne out, that on a non-nitrogenous 

 diet exercise produces no notable increase in the nitrogen of the urine, 

 although, when the whole period is considered, it does produce a slight 

 increase. 



It may now also be said that, under similar conditions, exercise pro- 

 duces no increase in the excretion of nitrogen by the bowels. 



The diminution in the amount of urea during the actual period of work, 

 as compared with the rest-period, which, if I am not mistaken, is obvious 

 in both our experiments, is a very curious circumstance. It shows, not 

 that on a non-nitrogenous diet the nerves and muscles are totally unaffected 

 by exercise, but that changes go on which either retain nitrogen in the 

 body or eliminate it by another channel. 



Is it possible that when the excess of nitrogen is restricted, the ex- 

 hausted muscle will take nitrogen from the products given off from 

 another portion of decomposing muscle, and thus the nitrogen may be 

 used over and over again ? or, after all, is nitrogen really given off in some 

 form by the skin during exercise, as formerly supposed ? 



Although it is thus certain that very severe exercise can be performed 

 on non-nitrogenous diet for a short time, it does not follow that nitrogen 

 is unnecessary. The largest experience shows not only that nitrogen 

 must be supplied if work is to be done, but that the amount must aug- 

 ment with the work. But for a short period the well-fed body possesses 

 sufficient nitrogen to permit muscular exertion to go on for some time with- 

 out fresh supply. But the destruction of nitrogenous tissues in these two 

 men is shown by the way in which, when nitrogen was again supplied, 

 a large amount was retained in the body to compensate for the previous 

 deprivation. 



I believe also that in these two men the great exhaustion of the second 

 day showed that their muscles and nerves were becoming structurally im- 

 paired, and that if the experiments had been continued there would have 

 been on the third day a large diminution in the amount of work. 



I have found that the period when a restricted supply of nitrogen 

 begins to tell on the work differs in different men ; in one experiment 

 I reduced the nitrogen in the food to one half its normal quantity in two 

 men ; in one no effect was produced on exercise in seven days, in the other 

 a lessening of active bodily work was produced in five days ; doubtless the 

 previous nutrition of the muscle would influence the time. 



Finally, it may be questioned whether the relation of elimination of 

 nitrogen to exercise can be properly determined in this manner, i. e. by 

 cutting off the supply of nitrogen. The true method would probably be 

 to supply nitrogen in certain definite amount, so that the acting muscle, 

 might appropriute at once what it required. 



