44 PHYSIOLOGY CHAP. 



bound up in the molecules of living matter, is the source of the 

 disintegrative and oxidising changes that go on in all the tissues. 

 We may therefore assume that the proteins of muscle absorb and 

 combine with oxygen during rest, and pass it on during activity 

 to nitrogen-free molecules, while they once more take up fresh 

 oxygen in the resting period which follows. On this hypothesis 

 the proteins of muscle fulfil the same function as an enzyme 

 during work. But the inadequacy of this explanation is evident 

 from the fact that muscle, independently of rest or activity, is the 

 seat of an active nitrogenous metabolism, which must therefore be 

 heightened during work. Further, intense muscular work is 

 possible on an exclusively flesh diet. Voit showed that dogs can 

 be kept alive under normal conditions on an exclusive diet of 

 meat. In his latest researches (1892) Ffliiger fed a great Dane 

 of 30 kgrm. for nine months on horseflesh, which was almost free 

 of fat, and made it do hard work for weeks by dragging a heavy 

 cart for 13 km. in two to three hours. During this time the 

 animal remained exceptionally well and vigorous. Under these 

 conditions almost the whole of the energy developed in the 

 animal's muscles must be derived from disintegration of protein, 

 since the small quantity of glycogen and fat ingested is negligible. 



Nevertheless, on comparing the amount of nitrogen given off 

 by the animal in periods of work and of rest, Pfliiger could only 

 confirm the fact that it did not vary conspicuously, and that the 

 increase was never in proportion with the work performed. 



To explain this fact he assumed that the excretion of nitrogen 

 does not increase definitely after work, because though the muscles 

 consume more protein, other tissues consume less, by a sort of 

 adaptation due to the lesser amount of protein circulated. 



Verworn, however, pointed out that this hypothesis cannot 

 explain Voit's observation on the dog, that even in the fasting 

 state when the amount of circulating protein at the disposal 

 of the muscles and other tissues is minimal, nitrogen elimination 

 does not increase proportionately with hard work (making a wheel 

 revolve on its axis). 



Pfliiger suggested later that the increased disintegration of 

 protein effected by the muscle during work does not show a 

 larger excretion of nitrogen in the urine, because the nitrogenous 

 waste products are regenerated synthetically into the complex 

 molecules of protein, by combining with non-nitrogenous atoms 

 lost during the work, at the expense of nutrition, or of the reserve 

 materials. In other words, it is possible and even probable that 

 the nitrogenous products of proteolysis, which is increased in 

 muscular work, do not leave the body like the non-nitrogenous 

 products, which are excreted principally in the form of carbohydrate 

 and water, but are stored up and partially utilised again in the 

 synthetic regeneration of protein : this is to some extent analogous 



