THE URINARY SYSTEM. 395 



injection of sodium sulphate no longer increases the amount of oxygen 

 used by the kidney, and the urine may be isotonic with the blood 

 plasma, This experiment is not quite conclusive in mammals, since the 

 glomeruli, as well as the tubules, are exposed to the poisonous action 

 of corrosive sublimate ; and it might be legitimately argued that the 

 glomeruli possess a secretory function, which is abolished in the poisoned 

 kidney. But it is possible in frogs to poison the tubules and to leave 

 the glomeruli intact ; and since, when this is done, the urine is isotonic 

 with the fluid passing through the glomeruli, the function of the latter 

 is not secretory, but is that of a filtering membrane. 



Further evidence for the secretory function of the tubules is pro- 

 vided by experiments in which indigo-carmine (sulphindigotate of soda), 

 which is a blue pigment, is injected into the blood stream. The spinal 

 cord of the animal is previously divided in the neck, so as to abolish 

 the formation of urine by the glomeruli, and to prevent the dye from 

 being carried away in the urine. The animal is killed ten minutes 

 after the indigo-carmine has been injected, and the kidneys are fixed 

 in absolute alcohol. Sections of the kidneys show the presence of 

 blue granules of the pigment in the lumen of the tubules, and in the 

 cells of the convoluted tubules, but not in Bowman's capsule or the 

 cells lining it, indicating that the pigment had been secreted by the 

 tubules but not by the glomeruli. 



Reaction of Urine. The fluid filtered off from the glomeruli has 

 the same reaction as the blood plasma, and the acid reaction of normal 

 urine is due to the activity of the cells of the tubules. This can be 

 shown by repeatedly injecting into the dorsal lymph sac of a frog acid 

 f uchsin, which is almost colourless in neutral or alkaline solutions and 

 red in acid solutions. When the kidneys are subsequently examined 

 microscopically, the glomeruli are seen to be colourless, whereas the 

 cells of the convoluted tubules are red. 



Further, the more rapidly urine is formed in the mammal, that is, 

 the greater the amount filtered through the glomeruli, the more nearly 

 does its reaction approximate to that of the blood. 



It has been thought that the acid reaction of urine is due to the 

 fact that as the glomerular filtrate passes down the tubule an absorp- 

 tion of bases, especially sodium, takes place, so that the urine becomes 

 acid. Since the urine formed by frogs after ligature of the renal 

 arteries is acid in reaction, it is probable that the acid reaction of 

 urine is due to the secretion of acid radicles by the tubules, rather 

 than to the absorption of bases. 



In the process of secretion the renal tubules do work which can 

 be approximately measured if the osmotic pressure of the blood plasma 



