THE METABOLISM OF PROTEINS. Ill 



body, namely, the muscles and the nuclei of the cells. This ex- 

 plains why the uric acid excretion increases after strenuous mus- 

 cular work, and why it is much above the normal when cellular 

 break-down is very excessive, as in the disease called leucocythe- 

 mia, in which there is an excess of leucocytes in the blood (see 

 p. 145). Another source of uric acid is the food when it con- 

 tains either muscle (flesh) or glands (sweetbreads), for a large 

 proportion (about half) of the ingested purins do not become 

 destroyed in their passage through the organism; but become 

 oxidized to uric acid, which is excreted in the urine. This is 

 called the exogenous in contrast to purin produced in the tissues, 

 which is called endogenous. 



There is only a trace of uric acid in the urine of mammals, but 

 in birds and reptiles most of the nitrogen is present in this form. 

 The reason is that in these animals it is important to have semi- 

 solid, instead of fluid excreta, so that the urea which results from 

 protein metabolism becomes converted into uric acid, which, 

 either free or as salts, is relatively insoluble. Uric acid is chemi- 

 cally a diureide, that is to say, it consists of two urea molecules 

 linked together by a chain of carbon atoms. The chain of carbon 

 atoms is furnished by substances not unlike lactic acid and the 

 synthesis occurs in the liver. If this organ be removed from the 

 circulation in birds, such as geese, in which the operation is 

 comparatively easy, a very large part of the uric acid in the urine 

 becomes replaced by ammonium lactate. 



The relative insolubility of uric acid and its salts, which we 

 have already referred to, makes it apt to become precipitated in 

 urine, especially on standing. It forms the orange reddish de- 

 posit, so frequently observed in summer, when on account of per- 

 spiration the urine does not contain as much water as usual. 

 Such deposits do not therefore indicate that there is an excess of 

 uric acid in the blood, but merely that enough water is not being 

 excreted to dissolve the usual amount of urates. Sometimes the 

 urate becomes deposited in the joint cartilages, particularly in 

 those of the great toe, causing local swelling and redness and 

 great pain. This is gout, and it may be most effectually treated 

 by drinking large quantities of alkaline fluids, and eliminating 



