CHAP. IX.] THE CONTKACTILE TISSUES. 399 



on Thursday the nitrogenous ingesta were reduced from an average of 

 about 15-5 grams per diem to 4*228 grams; on Friday to 1'365 grams; 

 and on Saturday to '399 grams. On Saturday, Sept. 8, Mr North 

 underwent severe exertion on the tread- wheel, the previous days having been 

 days of comparative rest. On Sunday nitrogenous food was again taken 

 ad libitum. Mr North first makes the assumption that the nitrogen excreted 

 as urea is influenced by the nitrogen ingested on the previous day, rather 

 than by that ingested on the same day ; and then finds that the total nitrogen 

 excreted as urea from Tuesday to Sunday is indeed greater than the total 

 nitrogen ingested from Monday to Saturday (the work-day), but only by 1/17 6 

 grams, which is wholly insufficient to account for the loss of weight (3J 

 Ibs.) sustained during the experiment, or for the work done. 



Later experiments have led to a like result, in a majority of cases. 

 But whilst as a rule the nitrogen appears to be practically unaffected by 

 exercise, occasionally the excretion of nitrogen is decidedly increased. This 

 phenomenon, in Mr North's as in other cases, may, we think, depend upon a 

 temporary condition resembling fever engendered by the exercise, when it is 

 not due to causes already referred to. 



General Ef- ^ ma y> therefore, be regarded as established that 



feet of Mus- muscular exercise somewhat enlarges the total excretion 



cuiar Con- of nitrogen. There is no reason to doubt that this 



traction upon enlarged excretion is due, in the last instance, to the 



^tif 11 ^? 611 degradation of the nitrogenous tissues of muscle ; but 

 of the Urine. ,, , , .. ,, . , , 



the degradation is far too small to account, by me- 

 chanical equivalence, for the work done in contraction. Moreover it 

 appears that the actual elimination of waste nitrogenous matters 

 does not coincide with, or very closely follow, the period of muscular 

 contraction. Sometimes, perhaps most frequently, the immediate 

 effect of exercise is rather to dimmish the elimination of nitrogen, 

 and to postpone the enlargement of excretion for some hours, or even 

 days. This fact is well illustrated in the experiments of Parkes ; by 

 whom it was thought to be of such essential importance as to warrant 

 the hypothesis that muscle, in activity, gains rather than loses nitro- 

 gen. "When a voluntary muscle is brought into action by the 



influence of the will, it appropriates nitrogen and grows A 



state of rest ensues, during which time the effete products are 

 removed, the muscle loses nitrogen, and can again be called into 

 action by its stimulus 1 ." And this also, according to Parkes, is the 

 explanation why the elimination of urea is greater during absolute 

 rest than during light and regular labour. 



Such a hypothesis is not, however, necessary. The formation of 

 effete nitrogenous matters in muscle, and their elimination at the 

 kidneys, are separate operations conducted by different protoplasmic 

 structures. The conditions favourable to one are not necessarily 

 favourable to the other ; blood, for example, is received into muscles 

 in large quantity during contraction, and at the same moment is 



1 Parkes, "On the Elimination of Nitrogen." Proc. Roy. Soc. Lond., Vol. xvi., 

 1867-68, p. 58. 



