KIDNEY AND SKIN AS EXCRETORY ORGANS. 755 



animals in these investigations survived this operation for some time, 

 but they died finally, showing a series of symptoms which indicated 

 a deep disturbance of the nervous system. It was found that the 

 symptoms of poisoning in these animals could be brought on before 

 they developed spontaneously by feeding the dogs upon a rich meat 

 diet, or with salts of ammonia or carbamic acid. Later investiga- 

 tions* showed that in normal animals the ammonia contents of the 

 blood in the portal vein are from three to four times what is found 

 in the arterial blood, but that after the operation described the 

 ammonia in the arterial blood increases and at the time of the de- 

 velopment of the fatal symptoms reaches about the percentage 

 which is normal to the blood of the portal vein. It would seem 

 from these investigations that the liver stands between the portal 

 circulation and the general systemic circulation and protects the 

 latter from the comparatively large amount of ammonia compounds 

 contained in the portal blood by converting these compounds to 

 urea. If the liver is thrown out of function, ammonia compounds 

 accumulate in the blood and cause death. Similar ammonia salts 

 are probably formed in other active proteid tissues, since the per- 

 centage of ammonia in the tissues is considerably greater than in 

 the blood, and these compounds also are doubtless converted to 

 urea in the liver, in part at least. As to the origin of the ammonia 

 compounds, there is little direct evidence. They come, in the long 

 run, of course, from the nitrogenous foodstuffs, proteids and al- 

 buminoids. Drechsel supposes that the proteids first undergo hydro- 

 lytic cleavage, with the formation of amido-bodies, such as leucin, 

 tyrosin, aspartic acid, glycocoll, etc.; that these bodies undergo 

 oxidation in the tissues, with the formation of NH 3 , C0 2 , and H 2 0; 

 and that the NH 2 and C0 3 then unite synthetically to form am- 

 monium carbamate, which is carried to the liver and changed to 

 urea. It is a very significant fact that the relative and absolute 

 amount of urea nitrogen in the urine varies directly with the amount 

 of proteid taken as food, while other nitrogenous constituents of 

 the urine (creatinin, purin bases) are practically not affected by the 

 food, if care is taken to have the food free of these substances to 

 begin with. Folin has laid emphasis upon this fact,f and suggests, 

 therefore, that most of the urea may come directly from proteid of 

 the food which is hydrolyzed during digestion and absorption (action 

 of trypsin and erepsin) into simpler amido-acids. These amido- 

 bodies by further hydrolysis and oxidation may be converted, so 

 far as their nitrogen is concerned, into ammonia compounds and 

 eliminated at once as urea b}^ the liver without entering into tissue 



* Nencki, Pawlow, and Zaleski, ibid., 37, 26, 1895; also Nencki and 

 Pawlow, "Archives des sciences biologiques, " 5, 213. 



t Folin, "American Journal of Physiology," 13, 117, 1905. 



