794 NITROGENOUS METABOLISM. [BOOK n. 



referred above. Urea is the chief end-product of proteid meta- 

 bolism, but that end is probably reached in several ways ; so that 

 probably a very large number of nitrogenous chemical substances 

 make a momentary appearance in the body. Some of these fail to 

 become urea, and either without or after further change make 

 their appearance in the urine. But we do not know whether their 

 appearance is accidental, the result of imperfect chemical machin- 

 ery ; or whether they, though small in quantity, serve some special 

 ends in the economy. Perhaps sometimes or with some of them 

 it is the one case, at other times or with others it is the other case. 

 It is interesting to note that nuclein readily gives rise to hypo- 

 xanthin and allied bodies ; it is not proved however that this 

 is their only or natural source in the body. 



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 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 oxidised 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 



