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 

 nucleins, it is clear that we camiojb 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, 

 ornithuric acid, which is a condensation product of benzoic acid with 

 diainidovalerianic 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 9 H 5 KCOOH). 



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, Wien. med. 

 Wclinschr., 1893, Nos. 47 and 48. 



