598 NITROGENOUS METABOLISM. [Book n. 



sometimes or with some of them it is the one case, at other 

 times or with others it is the other case. 



When proteid material undergoes outside the body, either 

 by the action of trypsin or as the result of decomposition or 

 under the influence of chemical agents, that change by which 

 it is converted into leucin, the leucin, which appears in some 

 considerable quantities, is accompanied by tyrosin, which ap- 

 pears in smaller quantities, as well as by other bodies. The 

 almost constant appearance of tyrosin as a result of the decom- 

 position 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 pro- 

 teid 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 metabolism the aromatic radicle 

 takes on some other form. Whether as in tyrosin the aromatic 

 (phenyl) nucleus is associated with an ammonia representative 

 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 contributes 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 association with 

 the ammonia representative glycin, or leaves the body as one 

 or other of the urinary phenyl compounds, or possibly may be 

 oxidized somewhere into carbonic acid and water. Our knowl- 

 edge on this point is limited, but we have ventured to refer to 

 the point since it further illustrates the complexity of proteid 

 metabolism. 



§ 390. In speaking of urea (§ 321) we alluded to its rela- 

 tions to the cyanogen compounds. Bearing in mind the pecu- 

 liarly large amount of energy set free as heat during the iso- 

 meric 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 con- 

 trasting these properties with those of ammonia and the am- 

 monia compounds, we cannot help being tempted towards 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 Avhich energy is set free, the 

 cyanogen compound changes to the amide or other ammonia 

 representative. And there are several facts which lend sup- 

 port 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 cyanamide and sarcosin, and other facts. But the matter, 



