754 PHYSIOLOGY OF DIGESTION AND SECRETION. 



the derivation of urea from proteids and albuminoids will be known. 

 The results of this work may be stated briefly as follows: 



1 . Urea arises from proteids by a process of hydrolysis and oxida- 

 tion, with the formation eventually of ammonia compounds, which 

 are then conveyed to the liver and there changed to urea. Drechsel 

 has suggested that ammonium carbamate forms one at least of the 

 ammonia compounds that are converted to urea, and gives the fol- 

 lowing evidence for this view. In the first place, Drechsel found 

 carbamic acid in the blood of dogs, and Drechsel and Abel have 

 shown that it occurs normally in the urine of horses as calcium car- 

 bamate. Abel has shown, also, that it may be found in the urine 

 of dogs or infants after the use of lime-water. Drechsel has shown, 

 further, that ammonium carbamate may be converted into urea. 

 If one compares the formulas of ammonium carbamate and urea, 

 it is seen that the former may pass over into the latter by the loss 

 of a molecule of water, as 



Ammonium Urea, 



carbamate. 



Drechsel supposes, however, that this dehydration is effected in an 

 indirect manner; that there is first an oxidation removing two 

 atoms of hydrogen, and then a reduction removing an atom of oxy- 

 gen. He succeeded in showing that when an aqueous solution of 

 ammonium carbamate is submitted to electrolysis, and the direc- 

 tion of the current is changed repeatedly so as to get alternately 

 reduction and oxidation processes at each pole, some urea is pro- 

 duced. These facts show the existence of ammonium carbamate 

 in the body, and the possibility of its conversion to urea. It remains 

 possible, however, that other salts or compounds of ammonia may 

 likewise be converted normally to urea by the liver, since it has been 

 shown experimentally in artificial circulation through this organ 

 that salts such as ammonium carbonate, or even such complex 

 ammonia compounds as leucin and glycocoll, may give rise to urea. 

 Experiments made by Hahn, Pawlow, Massen, and Nencki* 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 con- 

 tents. 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, and the only blood that the organ received was through 

 the hepatic artery. If, now, this artery was ligated or the liver 

 was cut away, as was done in some of the experiments, then the 

 result was practically an extirpation of the entire organ. The 



* " Archiv f. experimentelle Pathologie u. Pharmakologie, " 32, 161, 1893. 



