FOKMATION OF THE TJEENE. 163 



subject of nutrition. It remains for us, then, in this connec- 

 tion, to treat, in general terms, of the way in which these 

 substances find their way into the urine. 



The most important constituent of the urine is urea ; a 

 cry stalliz able nitrogenized substance, which is discharged by 

 the skin as well as by the kidneys. This has long been 

 recognized as an excrementitious principle ; but the first 

 observations that gave any definite idea of the mechanism 

 of its production were made by Prevost and Dumas, 1 in 

 1821. At the time these experiments were made, chemists 

 were not able to detect urea in the normal blood ; but Pre- 

 vost and Dumas extirpated the kidneys from living animals 

 (dogs and cats), and found an abundance of urea in the 

 blood, after certain symptoms of blood-poisoning had been 

 manifested. The first experiments were performed by 

 removing one kidney by an incision in the lumbar region, 

 and at the end of three or four days, after the animal had 

 recovered from the first operation, removing the other. 

 After the second operation the animals lived for from five 

 to nine days. For the first two or three days there were no 

 symptoms of blood-poisoning. Watery discharges from the 

 stomach and intestinal canal occurred after a few days, and 

 finally stupor and other marked evidences of nervous dis- 

 turbance supervened, when the presence of urea in the blood 

 could be easily determined. These observations were con- 

 firmed and extended by Segalas and Yauquelin, in 1822, who 

 presented to the French Academy of Medicine a specimen 

 of nitrate of urea extracted from the blood of a dog, taken 

 sixty hours after extirpation of the kidneys, giving its pro- 

 portion to the weight of blood employed. 3 A few years 

 later, the observations of Prevost and Dumas were con- 



1 The observations of Prevost and Dumas, Segalas, Marchand, and others, 

 have already been referred to (see p. 25). 



- SEGALAS, Sur des nouvelles experiences relatives aux proprietes medicamenteuses 

 de Puree, et sur le gendre de mort qui produit la noix vomique. Journal de physio* 

 logie, Paris, 1822, tome ii., p. 356. 



