NUTRITION. 459 



living muscle material; in excessive work the same thing 

 partially occurs, decomposition occurring faster than recom- 

 position; clotted myosin is then broken up into simpler 

 bodies as kreatin, and these are somewhere turned into urea 

 and excreted. In rigor mortis all the myosinogen passes into 

 clotted myosin and causes the rigidity. A working muscle 

 takes up more oxygen from the blood than a resting one, as 

 is shown by a comparison of the gases of the venous blood 

 of the two; this oxygen assumption is not necessarily pro- 

 portionate to the carbon-dioxide elimination at the same 

 time.; for the latter depends on the breaking down of a 

 material already accumulated in the muscle during rest, and 

 this breaking down may occur faster than the reconstruction. 

 We are thus enabled, also, to understand how, during exercise, 

 the carbon dioxide evolved from the lungs may contain more 

 oxygen than that taken up at the same time; for it is largely 

 oxygen previously stored during rest which then appears in 

 the carbon dioxide of the expired air. The kreatin which 

 can always be found even in muscles suddenly killed after 

 long rest, represents the breaking down of proteid in the 

 chemical processes of the living fibres, in their vital meta- 

 bolisms, which are not necessarily similar to the special 

 chemical changes associated with a contraction. 



Are any Foods Respiratory in Liebig's Sense of the 

 Term ? We find, then, that Liebig's classification of foods 

 cannot be accepted in an absolute sense. There is no doubt 

 that the substance broken down in muscular contraction is 

 proper living muscular tissue; and if this (its proteid con- 

 stituent being retained) be reconstructed from foods con- 

 taining no nitrogen (whether carbohydrates or fats) then 

 the term plastic or tissue-forming cannot be restricted to the 

 proteids of the diet. We must rather conclude that any 

 alimentary principle containing carbon may be used to re- 

 place the oxidized carbon, and any containing hydrogen to 

 replace the oxidized hydrogen, of a tissue; and so even non- 

 proteid foods may be plastic. A certain proportion of the 

 foods digested may perhaps be oxidized to yield energy, 

 before they ever form part of a tissue; and so correspond 

 pretty much to Liebig's respiratory foods; but no hard and 

 fast line can be drawn, making all proteid foods plastic and 

 all oxidizable non-proteid foods respiratory. 



Luxus Consumption. Not only, as above pointed out, 



