278 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the adenoid tissue of the spleen, there is contained a substance which may be 

 regarded as the mother-substance of uric acid. He ventures the hypothesis 

 that uric acid represents an end-product in the metabolism of leucocytes, but 

 the view at present can be regarded only as an interesting suggestion. Among 

 birds and reptiles it has been shown that the liver is the chief producer of uric 

 acid, just as it is of urea in mammals. Extirpation of the kidneys in birds 

 leads to an accumulation of uric acid in the blood and tissues, showing that the 

 kidneys do not produce the uric acid. Extirpation of the liver, on the contrary, 

 leads to a marked diminution in the uric acid of the urine, and it is noteworthy 

 that its place is taken by ammonium salts, probably ammonium lactate. This 

 would indicate that in. these animals proteid metabolism leads in some way to 

 the formation of ammonium lactate, which is then carried to the liver and com- 

 bined synthetically to make uric acid before being excreted by the kidneys. It 

 is stated that in man also, in certain pathological conditions of the liver for 

 example, acute yellow atrophy and phosphorus-poisoning lactates are found in 

 the urine. Reasoning from analogy, we should suppose that in mammalia too 

 uric acid is formed in the liver, but there is at present no positive evidence in 

 favor of this view. Although the quantity of uric acid produced, or at least 

 eliminated, during a day is so small in the mammalia, it is needless to say that 

 the history of its formation, when completely known, will be of great import- 

 ance, not only in that it will throw additional light upon the metabolism of 

 the proteids in the body, and indeed upon the structure of the proteid molecule, 

 but also because of its bearing upon the nature of certain pathological con- 

 ditions; for it has been found that in fever, in leucaemia, and possibly in other 

 diseases, there is an increased production of uric acid. Several other nitrogenous 

 substances xanthin, hypoxanthin, guanin, and adenin forming members of 

 what is known as the xanthin group, are closely related in composition to uric 

 acid. Some or all of these substances may occur in the urine, especially xanthin 

 and hypoxauthin, whose formulas are respectively C 5 H 4 N 4 O 2 and C 5 H 4 N 4 O. 

 They are found only in minute quantities. To the extent that they occur they 

 represent so much proteid broken down in the body ; but what peculiarity in 

 metabolism leads to their formation rather than to that of uric acid, or indeed 

 urea, has not been discovered. They are found in greatest quantity in muscle, 

 and are present, therefore, in meat extracts. It is interesting in this connection 

 to call attention to the fact that theobromin (dimethyl-xanthin) and caffein 

 (trimethyl-xanthin) are closely related to the xauthin bodies. 



Creatinin. Creatinin (C 4 H 7 N 3 O) is a crystalline nitrogenous substance 

 constantly found in urine. It is closely related to creatin (C 4 H 9 N 3 O 2 ), the two 

 substances differing by a molecule of water ; the creatin changes to creatiuin 

 upon heating with mineral acids. Creatinin occurs in urine to the extent 

 of about 1.12 grams per day in man. In dogs it has been found that 

 the amount may vary between 0.5 and 4.9 grams per day according to 

 the diet, an increase in the amount of meat in the diet causing an increase 

 in the creatinin. This is readily explained by the fact that creatin is a 

 constant constituent of muscle, and when taken into the stomach it is 



