478 



Sir A. Garrod. On the presence of [May 18, 



(emotional, &c.,) from the higher parts of the central nervous 

 system, and probably also, as far as a diminution of its activity is 

 concerned, by the influence of some portion of the nervous mechan- 

 ism concerned in the execution of muscular effort. And, as far as 

 we know, it appears to be mainly upon the mediation of the vagus 

 centre that the most important changes in the cardiac rhythm, in 

 so far as determined by nervous influences, are primarily dependent. 



V. " On the presence of Urea in the Blood of Birds, and its 

 bearing upon the Formation of Uric Acid in the Animal 

 Body." By Sir Alfred Garrod, M.D., F.R.S. Received 

 May 2, 1893. 



Some experiments upon which I have recently been engaged have 

 yielded results which appear to be inconsistent with any explanation 

 yet advanced of the mode of formation of uric acid in the animal body, 

 and to necessitate a new theory of its formation. These results I 

 propose to discuss in the present paper, but before doing so it will be 

 well to refer briefly to the different views upon this subject which 

 have been from time to time advanced. 



Until the year 1847 it was commonly supposed that uric acid was 

 formed in the kidneys themselves, none having ever been detected in 

 the blood, but in that year I succeeded in demonstrating the presence 

 of uric acid in the blood of gouty subjects, which led to the conclu- 

 sion that uric acid was formed in certain other organs and tissues of 

 the body, and was merely eliminated by the kidneys. 



It is now generally supposed that in mammals urea is produced as 

 the ultimate product of the metabolism of nitrogenised tissues, the 

 formation of a soluble urate being an intermediate stage of the meta- 

 bolism ; but that in birds the nitrogen is eliminated in the form of 

 urate of ammonium without having undergone the further change 

 into urea. Many efforts have been made to explain why these 

 changes should be so different in animals of these two classes. A 

 view once very popular and supported by great authority was that 

 the difference could be accounted for by the amount of oxidation in 

 the system ; for example, it was assumed that, as respiration in the 

 cold-blooded animals is slow and imperfect, the uric acid is not broken 

 up, but is eliminated in the form of urate of ammonium ; whereas in 

 hot-blooded animals, such as the mammalia, the oxidation processes 

 are more active and urea is produced. Those who held this opinion 

 had in mind only the mammal and the reptile; altogether overlook- 

 ing the fact that the bird, which throws out uric acid in the same way 

 as the reptile, has even hotter blood and a more active respiration 

 than the mammalia themselves. Hence this view had to be aban- 



