636 THE CHEMISTR Y OF THE URINE. 



even dominant share in the processes of constructive metabolism : that, 

 namely, of dehydrolysis the synthesis of larger molecules by a con- 

 jugation of smaller, associated with elimination of the elements of 

 water. This process is known to chemists as one of " condensation." 

 In destructive metabolism, on the other hand, the converse process 

 of hydrolysis is an important factor. 



While of most obvious importance in the physiology of plants, 

 in which constructive metabolism starts from a lower chemical level, 

 " condensation " is also prominent in all constructive processes of which 

 we have any accurate knowledge in the animal organism. Being thus 

 in general so characteristic of assimilative processes, it is remarkable 

 how frequently dehydrolytic synthesis interrupts the course of meta- 

 bolic breakdown and reappears as a final step before excretion. 



We have dealt with a typical instance of this in the formation of 

 hippuric acid from benzoic acid and glycine in the kidney, and we have 

 seen that a like process occurs in the production of ethereal sulphates, 

 and the conjugate compounds of glycuronic acid. We may note, too, 

 that many substances introduced experimentally into the body undergo 

 kindred conjugations before excretion. 



The theory of the origin of uric acid and the alloxuric bases from 

 nucleins, does not perhaps predicate any synthetic step in the produc- 

 tion of this group of excretives ; but however far-reaching may be the 

 truth of this theory, it must be admitted that there is much reason for 

 ascribing the origin of at least some fraction of these substances, as 

 found in the urine, to conjugative processes in the liver and kidney. In 

 birds there can be little doubt that uric acid arises by a synthetic 

 change. 



In the most important of the chemical changes antecedent to 

 excretion the formation of urea from ammonium carbonate in the 

 mammalian liver we have a process which I venture to think belongs 

 essentially to the same chemical picture. Though not resulting, properly 

 speaking, in a synthesis, the dehydrolysis which here occurs is a chemical 

 change against the line of least resistance, and suggests an influence of 

 the same type as that producing the synthetic results just discussed. 

 Without misuse of the vague and somewhat discredited terms " organic " 

 and " inorganic," we are entitled to look upon the dehydrolysis of am- 

 monium carbonate as a return from the latter to the former category ; 

 the excretive stands physiologically on a higher level than its precursor. 



To complete whatever of suggestion these considerations may contain, 

 we may note finally the dehydrolysis which creatin suffers before ex- 

 cretion as creatinin. Even here we meet with a change which, for the 

 conditions under which it occurs, is one from a more stable to a less 

 stable substance. 



It would seem that just before excretion there occurs an arrest of 

 the normal processes of down-grade metabolism (in which hydrolysis 

 goes hand in hand with oxidation, resulting in a series of compounds of 

 increasing stability), and a brief return to dehydrolysis and to the type 

 of constructive processes. It is at any rate interesting to observe that 

 the renal excretives are as a class more complex or less stable than 

 their immediate precursors in the body. When the urine decomposes, 

 under the destructive influence of enzymes derived from micro- 

 organisms, many of the precursors reappear ; the urea again becomes 

 ammonium carbonate ; hippuric acid and its analogues give place to 



