590 UREA A^TD KKEATIK [BOOK 11. 



381. In the first place we may take it for granted that 

 the urea carried to the kidney in the blood had an antecedent 

 in something which was not urea. We can hardly suppose that 

 the proteid constituent of living substance, when in the course 

 of its metabolism it ceases to be proteid, breaks up at once into 

 urea and into non-nitrogenous bodies. All we have learnt goes 

 to shew that what we call metabolism is not a single abrupt 

 change, but consists essentially in a series of changes ; and we 

 may safely conclude that proteid material in becoming urea 

 passes through phases in which the nitrogen exists in chemical 

 combinations distinct from proteid material on the one hand and 

 urea on the other. 



In the second place it is extremely probable that the series 

 of changes by which proteid material becomes urea is not the 

 same in all the tissues and on all occasions. We should natu- 

 rally expect to find the proteid material following different lines 

 of metabolism in different places or under different circum- 

 stances, the different lines all converging to the same body urea, 

 because for some reasons or other urea appears to be, in the 

 main, the most convenient form in which the nitrogen can leave 

 the blood and the body. 



We should accordingly expect to find, on the one hand, 

 various nitrogenous bodies resulting from proteid metabolism 

 in various parts of the body, and, on the other hand, arrange- 

 ments by means of which these various bodies were reduced to 

 the common form urea, preparatory to their discharge from the 

 body by the kidney. And actual observation as far as it goes 

 supports this view, though our knowledge of the whole matter is 

 very imperfect. 



382. We may turn our attention first to the metabolism of 

 the skeletal muscles, since these represent, as far as mere quan- 

 tity is concerned, by far the greater part of the proteid capital 

 of the body. We may safely infer that they furnish a large part 

 of the urea of the urine ; though undoubtedly a small mass of 

 tissue might by reason of its more rapid metabolism work over 

 a greater quantity of proteid material than a much larger mass 

 with a slower metabolism ; yet we have no reason to think that 

 the proteid metabolism of skeletal muscle, obscure though it is 

 in its nature, is so slow as to neutralize the probable effect of the 

 great bulk of muscle existing in the body. 



In dealing with the chemistry of muscle ( 59) we saw that 

 urea was conspicuous by its absence from the extract of muscle, 

 whereas a very appreciable quantity of kreatin was invariably 

 present, and indeed was the prominent nitrogenous crystalline 

 constituent of that extract. It seems difficult to resist the con- 

 clusion that kreatin is the main normal nitrogenous product of 

 the metabolism of skeletal muscles. If we accept this view, then 

 upon the fact of the presence of kreatin in, and the absence oi 



