844 PHYSIOLOGY OF DIGESTION AND SECRETION. 



body is not entirely known, but three sources of origin may be 

 stated with some positiveness. 



1. Urea arises from ammonia salts which, in the liver, are con- 

 verted to urea by a process equivalent to dehydration. It has long 

 been known that when ammonium carbonate is added to blood 

 perfused through a liver it is converted to urea.* The reaction 

 may be represented as follows: 



co<8ra:-2ao-co<NH< 



Ammonium carbonate. Urea. 



Moreover, the experiments made by Hahn, Pawlow, Massen, and 

 Nenckif show that in dogs removal of the liver is followed by a 

 decrease in the amount of urea in the urine and an increase in the 

 ammonia contents. In these remarkable experiments a fistula 

 (Eck fistula) was made between the portal vein and the inferior 

 vena cava, the result of which was that the whole portal circulation of 

 the liver was abolished, the organ receiving blood only by way of 

 the hepatic artery. If now the latter artery was ligated and the 

 liver was cut away as far as possible, the result was practically a 

 complete extirpation of the organ. Later investigations J showed 

 that in normal animals the ammonia contents of the blood of the 

 portal vein may be three to four times as great as in arterial blood, 

 but that after removal of the liver the ammonia in the general 

 circulation increases to a point equal to that observed for the 

 portal blood and produces symptoms of poisoning which may 

 result fatally. It would seem, therefore, that the liver protects the 

 body from the poisonous action of the ammonia compounds by 

 converting them to urea, while in the same process some of the 

 CO2 formed in the body is neutralized and prepared for excretion. 

 Now in the normal digestive hydrolysis of proteins brought 

 about by the successive action of pepsin, trypsin, and erepsin the 

 protein material is split largely or entirely into its constituent 

 elements and its nitrogen appears mainly in the form of the 

 amino-acids, but to some extent also probably as ammonia. In 

 addition, there is evidence that some ammonia is formed in the 

 large intestine, as the result of the action of the putrefactive 

 bacteria. The ammonia produced in these ways is probably 

 carried to the liver and there converted to urea. In what form the 

 ammonia exists in the blood is not positively known; it may be 

 present as a carbonate or possibly, as some observers have thought, 

 as a carbamate. Ammonium carbamate might be changed to 

 urea according to the following reaction : 



* Schroeder, "Archiv f. exp. Pathol, u. Pharmakol.," vols. xv. and xix., 

 1882, 1885. 



t See "Archiv. f. exp. Pathol. Pharmakol.," 1893, xxxii., 161. 



i See Nencki and Pawlow, "Archives des sciences biologiques, " v., 213. 



