460 THE HUMAN BODY. 



may non-nitrogenous foods be plastic but, on the other hand, 

 it is certain that if any foods are oxidized at once before 

 being organized into a tissue, proteids are 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 oxidation much better than fat does. But, on the 

 other hand, if a well-fed animal be given a very rich proteid 

 diet all the nitrogen of its food reappears in its urine, and 

 that when it is laying up fat; so that then we get a state of 

 things in which proteids are broken up more easily than 

 fats. This indicates that proteid in the Body may exist 

 under two conditions ; one, when it forms part of a living 

 tissue and is protected to a great extent from oxidation, 

 and another, in which it is oxidized with readiness and is 

 presumably in a different condition from the first, being not 

 yet built up into part of a living cell. The use of proteids 

 for direct oxidation is known as luxus consumption ; how 

 far it occurs under ordinary circumstances will be considered 

 presently. The main point now to be borne in mind is that 

 while all organic non-nitrogenous foods cannot be called 

 respiratory, neither can proteids under all circumstances be 

 called plastic, in Liebig's sense. 



The Antecedents of Urea. In the long-run the pro- 

 genitors of the urea excreted from the Body are the proteids 

 taken in the food; but it remains still to be considered what 

 intermediate steps these take before the excretion of their 

 nitrogen in the urine. 



In seeking antecedents of urea one naturally turns first 

 to the muscles, which form by far the largest mass of pro- 

 teid tissues in the Body. Analysis shows that they always 

 yield kreatin, the quantity of this in muscles being practically 

 unaffected by work, and from 0.2 to 0.3 per cent of the dry 

 weight of the muscle. Since it is readily soluble and dialyz- 

 able, and therefore fitted to pass rapidly out of the muscles 

 into the blood stream, it is a fair conclusion that a good deal 

 of it is formed in the muscles daily and carried off from them. 



