756 PHYSIOLOGY OF DIGESTION AND SECRETION. 



formation at all. In this way the large proportion of ammonia 

 compounds in the portal blood after digestion may be explained. 

 Granting that a portion, perhaps a large portion, of the urea arises 

 from this early hydrolysis of the proteid of the food, we must admit 

 also at present that ammonia compounds may be formed in the tis- 

 sues of the body generally, probably by a similar process of hydrol- 

 ysis followed by oxidation. 



2. It is stated (Kossel and Dakin*) that a ferment (arginase) 

 may be extracted from the liver which is capable of splitting arginin 

 into urea and ornithin (diamidovalerianic acid). Since arginin is 

 one of the diamido-bodies formed by the hydrolysis of the proteids 

 during digestion, it is possible that some of the urea has this origin. 

 The fact lends some probability to the view suggested above that 

 much of the nitrogen of proteid food may be converted to urea before 

 entering the general circulation. 



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

 the urine. This fact proves that other organs have the power of 

 forming urea, but what these other organs are and by what process 

 they make urea are points as yet undecided. It seems as though 

 the urea-forming power of the liver is shared by some of the other 

 tissues, just as its glycogenic functions are. 



Origin and Significance of the Purin Bodies (Uric Acid, 

 Xanthin, Hypoxanthin) . These bodies are related chemically, 

 and appear also to have a common physiological significance. Their 

 chemical relations have been described by Emil Fischer, to whom 

 we owe the term purin bodies. Fischer pointed out that these and 

 other substances belonging to this group have a common nucleus: 

 N C 



C C K which he named the purin nucleus. The 



I I > 

 N C N x 



hydrogen compound of this nucleus would be designated as purin, 



N = CH 



and would have the formula: HC C NH , C 5 H 4 N 4 . Addi- 



II II \ nw 

 N - C - N^ CH 



tion of an atom of oxygen gives hypoxanthin, C 5 H 4 N 4 O: 

 HN CO 



HC C NH 



j^>CH. Addition of two atoms of oxygen gives xan- 

 HN CO 



thin, C 5 H 4 N 4 O 2 : CO C NH 



H'N C NX 011 ' ^^ addition of three atoms 

 * Kossel and Dakin, " Zeitschrift f. physiol. Chemie," 42, 181, 1904. 



