COMPARATIVE CHEMISTRY OF THE URINE. 637 



benzole acid, creatinin may again take up water, and uric acid is 

 rapidly hydrolysed. 



The urinary nitrogen, it will have been observed, always appears 

 either as ammonia (NH 3 ), or more typically in compounds containing 

 the derived amido ( - NH 2 ) or imido ( NH) groups. Compounds con- 

 taining the other fundamental form of organic nitrogen, the cyanogen 

 type ( - C = N, or - N = C), are represented only by the minute 

 quantity of potassium sulphocyanide, which is in all probability directly 

 derived from the saliva. Although a small proportion of the nitrogen 

 is excreted in aromatic compounds, it is never, in human urine, present 

 in the benzene nucleus of these, but always in side chains or accessory 

 atomic groups within the molecule. 



The carbon ring of the benzene nucleus is especially resistant to 

 oxidation in the body, the open chain of carbon atoms, proper to 

 substances of the fatty series, being much less so ; and for the most 

 part we find that the normal renal excretives do not reach such 

 molecular size as to contain as many as six carbon atoms, unless they 

 contain the aromatic nucleus. As illustrating the degree of molecular 

 complexity found in the organic urinary compounds, we may remember 

 that the molecular weight of urea is 60, that of creatinin is 113, 

 of uric acid 168, and of hippuric acid 181. The intact renal 

 epithelium, it is true, passes substances, such as the pigments, the 

 molecular weight of which is much greater than the above, but only 

 in small quantities. 



COMPAEATIVE CHEMISTRY OF THE URINE. 



In mammals, amphibia, fishes, and in certain molluscs, urea is the 

 chief end-product of nitrogenous metabolism. In birds, reptiles, and 

 arthropods, on the other hand, the nitrogen is excreted mainly in the 

 form of uric acid. In spiders and in some few other groups of inverte- 

 brates, the chief excretive has been shown to be guanin. 



A study of the renal function, from a comparative point of view, 

 offers one aspect of great interest and some difficulty, to which Sir 

 William Eoberts has called attention. It is remarkable that the wide 

 differences in the nature of the renal excretion in mammals and the 

 Sauropsida respectively, should yet be associated with almost complete 

 identity in the anatomical structure of the kidney. The kidney of 

 the bird has a glomerular mechanism identical with that of the 

 mammalian organ, and the same tubular arrangement of a secretory 

 epithelium ; and yet practically the sole function of the organ of the 

 bird, in contrast to the remarkably complex duties performed by that of 

 the mammal, is to secrete quadriurates. " The chlorides, phosphates, and 

 sulphates, the lime and magnesia salts, the pigments and the large 

 volume of water all of which figure as prominent and even essential 

 components of mammalian urine are either wholly absent from the 

 urine of birds and serpents, or are only present in such minute traces 

 as might be derived from the lubricating mucus and epithelial debris 

 with which the secretion is incidentally admixed " (Eoberts). 



The physiological differentiation, whereby soluble urea takes the 

 place of insoluble uric acid, in accordance with the needs of an 

 organism excreting a liquid urine, is now known to occur quite at the 

 final stages of metabolism. It is almost certain that, in the main, the 



