11 



and carefully examined under the microscope. In no instance 

 did he find any of the indigo in the capsules of Bowman, but 

 it was found abundantly in the cells lining certain portions of 

 the tubules, and in the lumen of the tubules near these cells. 

 Similar experiments with the urate of sodium showed that it 

 likewise was secreted at the same place and in the same 

 manner. 



Further investigations have tended to diminish the impor- 

 tance of the "mechanical" factor and to develop a selective 

 or secretory function for the cells, even including those of the 

 glomeruli, by virtue of which the cells will permit certain 

 substances to pass through and prevent others, among the 

 latter, albumin. The view is advanced that the passage of 

 the fluid through the glomerulus is not a mere transudation, 

 but a matter of selection also. 



The selective action of the renal cells is both qualitative 

 and quantitative. As an example of the former, it is shown 

 by experiment that if some egg albumin be injected into the 

 blood, it is promptly eliminated by the kidneys. Egg albumin 

 is not very markedly different from the serum albumin of the 

 blood ; both are indiffusible, but the renal cells recognize the 

 former as a foreign substance and immediately separate it 

 from the blood. The sugar, in normal quantity in the blood, 

 although a diffusible substance is not selected. Urea, which 

 is also diffusible, but existing in the blood in much smaller 

 amount than sugar, is selected and appears in the urine. Why 

 this should be so it is difficult to explain, although it is a well- 

 known fact that the sugar serves as a food for the tissues and 

 is needed by the system, while urea is a waste product and 

 would be detrimental to the system if not eliminated. 



That the cells exert a quantitative selection is shown by 

 the fact that when sugar is present in the blood in excessive 

 amount, 3 parts per 1000 or over, the excess is promptly 

 eliminated. 



Diet influen(3es the reaction of the urine. A vegetable 

 diet favors an alkaline reaction ; a diet of flesh favors an acid 

 reaction. The urine of the herbivora is therefore alkaline, 

 while in the carnivora and omnivora the urine is mainly acid, 

 although influenced to some extent by the kind of food eaten. 



