1893.] 



Urea in the Blood of Birds, fyc. 



479 



doned, and the difficulty of explaining the above-mentioned changes 

 remained as before. 



Another explanation was that the occurrence of urea or uric acid 

 is dependent on the character of the food, and even at the present 

 time it is often assumed that a highly nitrogenised food, such as meat, 

 leads to the production and elimination of a large amount of uric 

 acid. The fallacy of this I showed in a paper read before this Society 

 in 1886, in which I proved conclusively that mammals living on raw 

 meat threw out infinitely less uric acid in proportion to their weight 

 than small birds fed on canary seed and water. In that paper it was 

 shown that the lion and the tiger eliminated probably less than a 

 hundred thousandth part of their weight of uric acid per diem, 

 whereas a small bird would eliminate as much as an eighty-fifth of 

 its own weight. The amount of uric acid cannot, therefore, depend 

 on the mere nature of the food. 



There are, however, physiologists who do not consider it necessary 

 to assume that urea must previously have existed as urate of am- 

 monium, and some few have hinted that there may even be a synthe- 

 sis in the production of uric acid. The whole subject is, however, in 

 a state of great uncertainty, and, as Dr. Michael Foster has stated in 

 his ' Text-book of Physiology,' m The whole story of proteid meta- 

 bolism consists at present mostly of guesses and of gaps." 



The difficulties in connexion with the formation of uric acid in the 

 animal body have been before my mind for many years, and the aim 

 of this paper is to suggest a solution which, in the first place, accords 

 with facts which have been long established, and, in the second place, 

 with others which I have recently discovered and proved experi- 

 mentally. 



It has always been difficult to me to believe that the metabolism of 

 nitrogenised tissues should differ so completely in animals which are 

 so closely allied in most other respects as the toad and the lizard, but 

 which excrete their nitrogen, the one as urea, the other in the form 

 of urate of ammonium, their diet being identical, and I think it will 

 be found that the assumption that urea must be a product of the 

 oxidation of urate of ammonium, has had much to do with the diffi- 

 culties which have arisen in connexion with this subject. 



Let us now consider certain points in the physiology of urea and 

 uric acid, beginning with urea. 



In mammalia, including man, it has been ascertained that urea is 

 always present in the blood, and, though the quantity may be small, 

 it is sufficient, nevertheless, to be measurable. I have recently ob- 

 tained it not only from the blood of man, but also from that of the 

 ox, the sheep, the goat, and the dog. And I have also confirmed a 

 statement made by M. Picard that the blood of the renal artery is 



2 l 3 



