464 THE METABOLIC PKOCESSES OF THE BODY. 



ical agents, that change by which it is converted into leucin, the leucin 

 which appears in some considerable quantities is accompanied by tyrosin, 

 which appears in smaller quantities, as well as by other bodies. The almost 

 constant appearance of tyrosin as a result of the decomposition of proteid 

 material leads one, as we have previously said, to the conception that some 

 representative of the aromatic series enters into the constitution of proteid 

 substance ; and it is possible that the hippuric acid of flesh-eating animals 

 derives its benzoic acid constituent from this aromatic radicle of proteid 

 matter. Tyrosin itself does not appear in the body as a normal product of 

 proteid metabolism, and we are therefore led to infer that in proteid meta- 

 bolism the aromatic radicle takes on some other form. Whether, as in 

 tyrosin, the aromatic (phenyl) nucleus is associated with an ammonia repre- 

 sentative or no, we do not know. But if it is, then, since neither tyrosin nor 

 any similar body is a constituent of normal urine, the ammonia constituent 

 is somewhere dissociated from the phenyl one ; and while the former con- 

 tributes to the stock of urea, the latter is either discharged by the urine as 

 hippuric acid, having as we have seen effected in the kidney a new associa- 

 tion with the ammonia representative, glycin, or leaves the body as one or 

 other of the urinary phenyl compounds, or possibly may be oxidized some- 

 where into carbonic acid and water. Our knowledge on this point is 

 limited, but we have ventured to refer to the point since it further illus- 

 trates the complexity of proteid metabolism. 



406. In speaking of urea ( 337) we alluded to its relations to the 

 cyanogen compounds. Bearing in mind the peculiarly large amount of 

 energy set free as heat during the isomeric transformation of many cyanogen 

 compounds, as well as the large store of potential energy existing in cyanogen 

 itself, the heat of combustion of which is very large, and contrasting these 

 properties with those of ammonia and the ammonia compounds, we cannot 

 help being tempted toward the view that in the actual living structure the 

 nitrogen exists in the form of cyanogen compounds, and that in the passage 

 to dead nitrogenous waste, during which energy is set free, the cyanogen 

 compound changes to the amide or other ammonia representative. And there 

 are several facts which lend support to such a view, such as the presence of 

 sulphocyanates in saliva and urine, which we may look upon as a sort of 

 leakage of cyanogen factors, the artificial production of kreatinin out of 

 cyamide and sarcosin, and other facts. But the matter, though it deserves 

 to be borne in mind, is too obscure to be dwelt on here. 



407. We may now briefly sum up the varied discussions which have 

 occupied us in the present section. 



Urea is the main end-product of proteid metabolism. Unlike hippuric 

 acid and some other constituents of urine, urea is simply excreted by the 

 kidneys, being brought to them in the blood, they apparently, beyond the 

 simple act of excretion, doing no more than merely contributing to the 

 stock of urea in so far as they are masses of proteid material undergoing 

 proteid metabolism as part of their general life. What are the immediate 

 antecedents of urea we do not clearly know ; but it is probable that they 

 are not one but several and indeed possibly many. We have reason to 

 think that urea may be formed out of amides or amido-acids, or out of 

 ammonia itself by a synthetic process; and we have indications that this 

 synthesis is effected in the liver by the agency of the hepatic cells. But we 

 do not know whether this synthesis bears only on particular nitrogen-hold- 

 ing substances of food or of the body, or whether it comes into play in the 

 normal metabolism of proteid material. If the kreatin which is so con- 

 spicuous a constituent of muscular and nervous structures is a stage in the 

 direct line to urea, then the synthesis would affect only the sarcosin which 



