CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 751 



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

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

 thing 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 arid on all occasions. We should naturally expect 

 to find the proteid material following different lines of metabolism 

 in different places or under different circumstances, 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, arrangements 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. 



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

 the skeletal muscles, since these represent, as far as mere quantity 

 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 meta- 

 bolism 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 ( 62) we saw that urea, 

 save in the exceptional instances of certain cartilaginous fishes, 

 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 conclusion 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 of urea from, the muscle 

 itself, we may base the conclusion that while the muscle produces 



