432 THE HUMAN BODY. 



ing 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 dur- 

 ing rest which then appears in the carbon dioxide of the ex- 

 pired air. 



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 

 replace 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, may 

 non-nitrogenous foods be plastic, but it is certain that if 

 any foods are oxidized at once before being organized into 

 a tissue, proteids arc under certain circumstances; namely, 

 when they are contained in excess in a diet. If an animal 

 be starved it is found that its non-nitrogenous tissues go 

 first; an insufficiently fed animal loses its fat first, and if it 

 ultimately dies of starvation is found to have lost 97 per cent 

 of its adipose tissue and only about 30 per cent of its 

 proteid-rich muscular tissue, and almost none of its brain 

 and spinal cord; all of course reckoned by their dry weight. 

 It is thus clear that the proteids of the tissues resist oxida- 

 tion much better than fat does. But, on the other hand, if 

 a well-fed animal be given a very rich proteid diet all the 



