CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 759 



the influence of chemical agents, that change by which it is 

 converted into leucin, the leucin, which appears in some consider- 

 able 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 

 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 con- 

 stituent 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 knowledge on this point is limited, but we have 

 ventured to refer to the point since it further illustrates the 

 complexity of proteid metabolism. 



492. In speaking of urea ( 401) 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 trans- 

 formation of many cyanogen compounds, as well as the large store 

 of potential energy existing in cyanogen itself, the heat of com- 

 bustion of which is very large, and contrasting these properties 

 with those of ammonia and the ammonia 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 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 cyanamide and sarcosin, and other facts. But 

 the matter, though it deserves to be borne in mind, is too obscure 

 to be dwelt on here. 



493. 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 



