KIDNEY AND SKIN AS EXCRETORY ORGANS. 775 



tion of urea from proteins and albuminoids will be known. The 

 results of this work may be stated briefly as follows: 



1 . Urea arises from ammonia salts which in the liver are converted 

 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 concerted to urea.* The reaction may be 

 represented as follows: 



Ammonium carbonate. Urea. 



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

 Nencki f 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. Now in the normal digestive hydrol/sis 

 of proteins brought about by the successive action of pepsin, trypsin, 

 and erepsin the evidence at present indicates that the protein 

 material is split largely or entirely into its constituent elements 

 and its nitrogen appears mainly in three forms as ammonia, as 

 monamino-acids, and as diamino-bodies. The ammonia produced 

 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 : 



m . 



NH 



Ammonia salts may arise similarly in the other protein tissues 



* Schroeder, "Archiv f. exp. Pathol. u. Pharmakol./' vols. xv. andxix., 

 1882, 1885. 



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

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



213. 



