MECHANISM OF THE PRODUCTION OF URINE 331 



in 1821. At the time these experiments were made, chemists were not 

 able to detect urea in the normal blood ; but Prevost and Dumas extir- 

 pated the kidneys from living animals dogs and cats and found urea 

 in the blood after certain nervous symptoms had developed. For the 

 first two or three days after the operation there were no marked symp- 

 toms ; but stupor and other evidences of nervous disturbance finally 

 supervened, when the presence of urea in the blood could easily be 

 determined. These observations were confirmed and extended by 

 Segalas and Vauquelin in 1822. Since that time, as the processes for 

 the determination of urea in the animal liquids have been improved, this 

 substance has been detected in normal blood. Picard (1856) estimated 

 and compared the proportions of urea in the renal artery and the renal 

 vein and found that the quantity in the blood was diminished by about 

 one-half in its passage through the kidneys. Still later, urea was found 

 in the lymph and chyle, in larger quantity, even, than in the blood 

 (Wurtz). It has been ascertained that urea is an active diuretic when 

 injected in small quantity into the veins of a healthy animal, and is 

 promptly eliminated. When injected into the vascular system of a 

 nephrotomized animal, however, it produces death in a very short time, 

 with characteristic symptoms. 



From a review of the important facts bearing on the question 

 under consideration, there seems to be no valid ground for a change 

 in the ideas of physiologists concerning the mode of elimination of urea 

 arid other important excrementitious constituents of the urine. There 

 is every reason to suppose that these substances are produced in 

 various tissues and organs of the body during the process of 'katabolism, 

 are taken up by the lymph and the blood and are separated from the 

 blood by the kidneys. , Urea, however, probably is formed almost ex- 

 clusively in the liver from substances of a like nature, the final process 

 of oxidation taking place in this organ. The mechanism of this will be 

 described farther on. 



Extirpation of one kidney from a living animal is not necessarily 

 fatal. When the operation is carefully performed, the wound usually 

 heals without difficulty, and in most instances the remaining kidney 

 seems sufficient for the elimination of urine for an indefinite period. In 

 a large number of experiments, the animals killed long after the wound 

 had healed presented no marked symptoms of retention of excrementi- 

 tious matters in the blood, except in one or two instances. It is a 

 noticeable fact, however, 'that in many instances they showed a change 

 in disposition, and the appetite became voracious and unnatural. These 

 animals would sometimes eat feces, the flesh of dogs etc., and, in 

 short, presented certain of the phenomena frequently observed after 



