COM/'. I A'. I TIVE CI1EMISTR Y OF THE URINE. 637 
benzoic arid, 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 (XII,), or more typically in compounds containing 
the derived amido ( - NH 2 ) or imido ( =NH) groups. ( lompounds con- 
taining the other fundamental form of organic nitrogen, the cyanogen 
type (-C = N, or - N i 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. 
Comparative 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 Roberts 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 " (Roberts). 
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 
