208 THE BIOLOGY OF THE FROG chap. 



that water and other substances diffuse from the blood 

 through the walls of the capillaries of the glomeruli into 

 the renal tubules. It has been held, especially by Ludwig 

 and his followers, that practically all of the substances excreted 

 by the kidney pass through the glomeruli, and that the func- 

 tion of the tubules is to absorb the excess of water and 

 certain other materials which pass down the lumen. By 

 other physiologists it has been maintained that both the 

 glomeruli and the renal tubules are secretory, but that they 

 eliminate different products. Nussbaum's ingenious experi- 

 ments on the frog seemed to offer a solution of this problem. 

 As the glomeruli are supplied by branches of the renal 

 arteries, Nussbaum concluded that the blood supply of these 

 organs would be cut off if the renal arteries were tied. The 

 opportunity was thus presented of comparing the excretion 

 of the kidney in which the glomeruli are rendered function- 

 less with that of the normal organ. It was found that in 

 frogs with the renal arteries tied the secretion of urine was 

 much diminished in amount. Solutions of sugar, peptones, 

 and egg albumen, which when injected into the blood of 

 normal frogs soon make their appearance in the urine, could 

 not be detected, after injection into the blood, in the urine 

 of frogs whose renal arteries were ligatured, even after the 

 flow of urine was increased by the simultaneous injection of 

 urea. Nussbaum came to the conclusion that albumen, 

 sugar, and most salts are excreted by the glomeruli, while 

 urea is eliminated by the cells of the uriniferous tubules. 

 There is a source of error in such experiments, since ligat- 

 ing the renal arteries alone does not entirely cut off the 

 blood supply of the glomeruli ; there are anastomoses with 

 the genital arteries by means of which these organs may 

 receive blood in a somewhat roundabout way. Adami 

 found that some of the glomeruli became filled by injecting 



