METABOLISM OF PROTEINS 573 



-easy. {The amino-acids, whatever intermediate stages they may 

 pass through,pha%ever cleavages, oxidations, or reductionsthey may 

 undergo/ yield eventually carbon dioxide, water, and comparatively 

 simple nitrogen-containing substances, which after further changes 

 appear in the urine principally as urea) and in birds and reptiles as 

 uric acid. When amino-acids are fea to mammals or introduced 

 parenterally, a very large proportion of the nitrogen appears in the 

 urine as urea. The same is true when, instead of simple amino- 

 acids, polypeptides, like glycyl-glycin, alanyl-alanin, or leucyl-leucin, 

 are given. When amino-acids are administered to birds, the great 

 bulk of the nitrogen is excreted in the form of uric acid. Whether 

 in mammals, and if so to what extent, uric acid is also one of the 

 nitrogenous end-products of the decomposition of ordinary proteins 

 or of the amino-acids which they yield, are moot questions. In 

 any case, the most important and characteristic source of the uric 

 acid in mammals and the other groups of animals whose chief 

 nitrogenous end-product is urea, is not the ordinary proteins, but 

 the nucleins which form constituents of the nucleo-proteins. 



We have no definite information as to the production of water from 

 the hydrogen of the tissues, except what can be theoretically deduced 

 from the statistics of nutrition (p. 608). A few words will be said 

 a little farther on about the production of carbon dioxide from 

 proteins ; we have now to consider the seat and manner of formation 

 of the nitrogenous metabolites. And since in man and the other 

 mammals urea contains, under ordinary conditions, by far the 

 greater part of the excreted nitrogen, it will be well to take it first. 

 f Formation of Urea.^The starting-point of all inquiries as to the 

 place of formation of urea is the fact that it occurs in the blood in 

 small amount (4 to 6 parts per 10,000 in man) 3 to 15 parts per 

 10,000 in the dog),fthe largest quantity being found when the food 

 contains most protein and at the height of digestion, the smallest 

 quantity in hunger HSchondorff). f Evidently, then, some, at least, 

 of the urea excreten in the urine may be simply separated by the 

 kidney from the blood; and analysis shows that this is actually 

 the case, for the blood of the renal vein is poorer in urea than that 

 of the renal artery, containing only one-third to one-half as much^ 

 If we knew the exact quantity of blood passing through the kidneys 

 of an animal in twenty-four hours, and the average difference in 

 the percentage of urea in the blood coming to and leaving them, 

 we should at once be able to decide whether the whole of the urea in 

 the urine reaches the kidneys ready made, or whether a portion of 

 it is formed by the renal tissue. Although data of this kind are as 

 yet inexact and incomplete, it is not difficult to see that all, or most 

 of, the urea may be simply separated by the kidney. 



If we take the weight of the kidneys of a dog of 35 kilos at 160 grammes 

 of the body- weight is the mean result of a great number of 



