402 EXCRETION. 



of the excretions. The composition of the urine, also, will be found to be exceedingly 

 complex, and its various ingredients bear the closest relation to the processes of nutrition 

 and disassimilation ; all of which considerations render it of the greatest importance to 

 ascertain the precise mode of its formation and to study all the conditions by which this 

 process may be modified. In the present state of our knowledge, we must certainly re- 

 gard the excrementitious constituents of the urine as formed essentially in the system at 

 large, being merely separated from the blood by the kidneys ; and a consideration of these 

 effete principles belongs to the subject of nutrition. It remains for us, then, in this con- 

 nection, 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 crystallizable, 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 defi- 

 nite idea of the mechanism of its production were made by Prevost and Dumas, in 1821. 

 At the time these experiments were made, chemists were not able to detect urea in the 

 normal blood ; but Prevost 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 disturbance supervened, when the presence of urea in the blood could be easily 

 determined. These observations were confirmed and extended by Segalas and Vauque- 

 lin, 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 kid- 

 neys, giving its proportion to the weight of blood employed. Since that time, as the 

 processes for the determination of urea in the animal fluids have been improved, this 

 substance has been detected in minute quantity in the normal blood. Picard carefully 

 estimated and compared the proportions of urea in the renal artery and the renal vein, 

 and he found that the quantity in the blood was diminished by about one-half in its passage 

 through the kidneys. Still later, urea has been found by Wurtz to exist in the lymph 

 and chyle in larger quantity, even, than in the blood. These facts, which have been 

 almost universally regarded as established, have led physiologists to adopt the view that 

 the peculiar excrementitious principles found in the urine are not produced by the kid- 

 neys, but are formed in the system by the general process of disassimilation, are taken up 

 from the tissues by the blood, either directly or through the lymph, and are merely 

 separated from the blood in the kidneys ; and it has consequently been pretty generally 

 assumed that nearly, if not all, the constituents of the urine preexist in the circulating 

 fluid. There is, indeed, no well-defined principle in the urine that has not been actually 

 demonstrated in the blood. As an additional argument in favor of this view of the 

 mechanism of urinary excretion, it has been ascertained that, when the kidneys are 

 interrupted in their function, there is a tendency to the elimination of the excrementitious 

 principles of the urine by the lungs, skin, and alimentary canal ; and that these matters 

 accumulate in the blood only after this vicarious effort has failed to effect their complete 

 discharge. These ideas have seemed to be so completely justified by facts, that they have 

 been applied to the mechanism of excretion by other organs, such as the skin and the liver ; 

 but, within a few years, the older observations with regard to nephrotomized animals 

 have been discredited. It has been asserted, as the result of experiment, that urea and 

 the urates do not accumulate in the blood after removal of the kidneys, and that this 

 only occurs when both ureters have been tied. The experiments upon which this idea 

 is based have been applied mainly to the pathology of uraemic intoxication, but it is evi- 



