ORIGIN OF UREA. 405 



for successive layers of different substances are generally found 

 in a stone that has attained any great size. The chief materials 

 found in calculi are uric acid, ammonium urate, calcium oxa- 

 late and carbonate, ammonio-magnesium phosphate, etc. 



SOURCE OF UREA, ETC. 



The question as to whether the chief materials of the urine pre- 

 exist in the blood and are therefore merely removed by the kid- 

 ney, or are manufactured by the special powers of the renal cell, 

 has been widely discussed, and though the great weight of evidence 

 is in favor of the former view, some of the experimental results on 

 the subject are rather conflicting. 



The following are the more important points in the argument : 



1. The blood does normally contain most of the important sub- 

 stances found in the urine; so they need not necessarily be made 

 in the kidney. 



2. The blood in the vessel leading to the kidney the renal 

 artery is said to contain more urea than the vessel leading from 

 it the renal vein so that the blood appears to lose urea in pass- 

 ing through the kidney. 



3. If the ureters be tied and the elimination be thus prevented, 

 urea accumulates in the blood. This can hardly be made by the 

 kidney, because 



4. If the renal arteries be tied so that no blood goes to the kid- 

 neys to effect the elaboration of urea in those organs, then the same 

 accumulation results, showing that the kidneys are certainly not 

 the only organs where urea is made. 



5. Extirpation of the kidneys also gives rise to a great increase 

 of the urea in the blood. The amount of urea in the blood 

 after nephrotomy is said to increase steadily with the time which 

 elapses after the operation, and the amount accumulated corre- 

 sponds to the amount that would normally have been excreted in 

 the same time, had the animal not been operated upon. 



6. Lastly. In some diseases which greatly interfere with, or 

 quite suppress the secretion of the kidneys, an accumulation in the 

 blood of certain poisonous or injurious materials takes place, and 



