CHEMISTRY OF DIGESTION AND NUTRITION. 277 



formation of NH 3 , CO 2 , and H 2 O ; and that the NH 3 and CO 2 then unite syn- 

 thetically to form ammonium carbamate, which is carried to the liver and 

 changed to urea. There is reason to believe that this formation of ammonium 

 carbamate may take place in the tissues generally. The carbamate theory is 

 at least in accord with the facts so far as known, and it is more complete and 

 satisfactory than others which have been offered. 



2. Even after the removal of the liver some urea is still found in the urine. 

 This fact proves that other organs are capable of producing urea, but what the 

 other organs are and by what process they make urea are points yet undeter- 

 mined. It seems probable that some of the ammonia compounds which are 

 now known to be formed in the tissues generally and to be given off to the 

 blood may be converted into urea elsewhere than in the liver. Just as the 

 glycogenic function of the liver-cells is shared to a less extent by other tis- 

 sues e. g. the muscle-fibres it is possible that their power of converting 

 ammonia salts to urea may be possessed to a lesser degree by other cells, and 

 for this reason removal of the liver is not followed at once by a fatal result. 

 Concerning this point, however, we must wait for further investigation. 

 Drechsel has recently called attention to a method of obtaining urea directly 

 from proteid outside of the body. His method is interesting not only 

 because it is the first laboratory method discovered of producing urea from 

 proteid, but also because it is possible that substantially the same process may 

 occur inside the body. The method consists, in brief, in first boiling the pro- 

 teid with an acid ; HC1 was used, together with some metallic zinc, so as to 

 keep up a constant evolution of hydrogen and to exclude atmospheric oxygen. 

 Among the products of decomposition of the proteid thus produced was a 

 substance termed lysatinin (C 6 H 11 N 3 O), and when this body was isolated and 

 treated with boiling baryta-water (Ba(OH) 2 ) some urea was obtained. It is to 

 be noted that in this case the urea was obtained not by the oxidation of the 

 proteid, but by a series of decompositions or cleavages of the proteid molecule. 

 Now, lysatinin occurs also in the body as one of the products of the con- 

 tinued action of trypsin on proteids (see p. 241). It is possible, therefore, that 

 by further hydrolysis this substance, when it occurs, is converted to urea, and 

 that normally a part of the urea arises from proteids by this process. 



Uric Acid and Xanthin Bodies. Uric acid, which has the formula 

 C 5 H 4 1S" 4 O 3 , is found constantly, but in relatively small quantities, in human 

 urine and in the urine of mammals generally. The total quantity in the urine of 

 man under normal conditions varies from 0.2 to 1 gram every twenty-four hours. 

 In the urine of birds and reptiles it forms the chief nitrogenous constituent. In 

 these animals it takes the place physiologically of urea in mammalia in that it- 

 represents the main end-product of the metabolism of the proteids in the body. 

 It is evident that at some point in the process the katabolism of the proteids in 

 mammalia differs from that in birds and reptiles, since in the one urea, and in 

 the other uric acid, is the outcome. Uric acid occurs in such small quantities 

 in mammals that its place of origin has not been investigated successfully. It has 

 been shown by Horbacewsky that in the lymphoid tissue generally, including 



