220 OEIGIN AND VARIATIONS OF UEEA. 



food than upon any other condition. It reaches its maxi- 



Variahons in -,11 , -,. -, . 



the quantity mum under an absolute animal diet, and its minimum under 

 a non-nitrogenized one. It still appears during fasting, and 

 about to the same extent as during a non-nitrogenized diet. Its sources, 

 therefore, are partly the waste of the tissues and partly the food. 



By several observers, urea has been detected in the blood under ordi- 

 nary circumstances. After extirpation of the kidneys it has been re- 

 peatedly recognized in that of the lower animals. It is removed with 

 such rapidity by the kidneys that its quantity is probably never per- 

 mitted to exceed a fiftieth of one per cent, of the circulating blood. Its 

 origin has generally been attributed to the waste of muscular tissue, 

 though it has not yet been detected in muscle juice ; but then rt should 

 be remembered that creatine and inosic acid may produce it during their 

 descending metamorphosis. Under this view, the seat of its production 

 would be the blood itself, a conclusion which is enforced by the circum- 

 stance that caffeine also increases its amount. 



In his inaugural dissertation, entitled, " Is muscular Motion the Cause 

 Origin of the of the Production of Urea ?" Dr. John C. Draper, by experi- 

 urea. ments on the urine of persons in different conditions of motion 



and rest, and by an examination of the diurnal and nocturnal variations 

 in the amount of urea voided, compared with an invariable standard, 

 gives reasons for concluding that the differences in the amount of urea 

 excreted are almost entirely attributable to the influence of the food, an 

 individual in such a state of comparative rest as is observed during treat- 

 ment for a fractured leg not excreting by any means so much less urea 

 as might have been anticipated when compared with another individual 

 who walked thirteen miles at the rate of four and a half miles an hour. 



But, on examining the influence of food, it appears to be well marked. 

 The greatest amount of urea is excreted within a few hours after dinner. 

 Another maximum also occurs just after breakfast ; but during the eight 

 night hours far less is excreted than during the same period in the aft- 

 ernoon. 



The ingestion of food thus exercising so rapid and marked an influ- 

 ence on the quantity of urea, he refers to it as the cause of the increased 

 excretion of that substance during the course of the day rather than to 

 the increased motion of exercise then indulged in; and in view of this 

 conclusion, it becomes probable that the nitrogen of the wasting muscu- 

 lar tissues escapes, not under the form of urea through the kidneys, but 

 through the skin, or perhaps even as free nitrogen from the lungs. 



Of the variations of the sulphates, it may be observed that the aver- 

 Variations of a ge diurnal excretion of sulphuric acid per thousand parts of 

 the sulphates. man b e i n g 0.050 of a part, an increase is observed during di- 

 gestion, a diminution occurring during the night, the minimum being 



