638 THE CHEMISTR Y OF THE URINE. 
waste products which leave the tissues are the same in birds and 
mammals. In the liver of the former these products are prepared for 
excretion by a change into the form of uric acid, while in the latter 
the hepatic influence produces urea. There is a great preponderance of 
experimental evidence to show that when uric acid is administered to 
mammals it is converted into urea before excretion, and that when urea 
is given to birds the converse change occurs. The contention of Haig, 
that when uric acid is taken by the mouth (in man) it is excreted 
unchanged, is not supported by other observers. 
As to the small quantity of uric acid found, nevertheless, in the 
urine of mammals, if we accept the theory of its exclusive origin from 
nueleins, it is clear that we cannot look upon it as in any sense 
physiologically akin to the main part of the normal excretion of birds, 
for this must represent the waste nitrogen of the tissues as a whole. 
But this theory apart, the view is plausible, and indeed it cannot be said 
to be yet disproved, that we have in the mammalian uric acid a vestigial 
relic of the earlier type of excretion — •" something analogous with the 
vermiform appendix, the ductus arteriosus, or the ear-point." The 
actual proportion present in the urine of different mammals is very 
variable. In most animals the relative amount is less than in man, but, 
except occasionally in the cases of the cat and dog, it has never been 
found to be absent. The presence of the small amount of uric acid in 
the urine of mammals is paralleled by the existence of minute quantities 
of urea in that of birds and reptiles. 
Creatinin has been found wherever looked for in the urine of various 
species of mammals, but is said to be absent from the excretion of birds. 
Hippuric acid is represented in birds by the analogous compound, 
ornithinic acid, which is a condensation product of benzoic acid with 
diamidovalerianic acid. An aromatic acid, apparently peculiar to the 
urine of dogs, is known as kynurenic acid, and has the composition of 
an oxychinolin-carboxylic acid (OH.C,,H 5 N.COOH). 
The large proportion of hippuric acid in, and the absence of 
ammonium salts from, herbivorous urine, have been shown in previous 
sections to be, in common with the alkaline reaction of the fluid and 
its richness in salts, a direct effect of diet. 
Of the urinary pigments in the lower animals we have no accurate 
knowledge. 
It is impossible at present, owing to the wide gaps in our knowledge, 
to take any broad view of the comparative chemistry of the urine. A 
series of analyses are much needed, from the results of which we could 
form some judgment as to the line of evolution which has led from 
the simple renal excretions of the invertebrates to that most complex 
of physiological fluids — mammalian urine. 1 
1 See on the subject of the comparative chemistry of the urine, Rywosch, JFicn. mcd. 
Wchnschr., 1893, Nos. 47 and 48. 
