422 COMPARATIVE PHYSIOLOGY. 



are separated by the glomeruli but the characteristic constitu- 

 ents of the urine by the epithelium of the renal tubules. 3. As 

 an example of the third is the theory of Heidenhaia, who attrib- 

 uted little to blood-pressure in itself, and much, if not the 

 whole, to the secreting activity of the epithelium of the tubules 

 mote particularly. This physiologist showed that while liga- 



Fis. 319.— Blood-pressure curve and curve of the volume of the kidney; T, time- 

 curve, intervals indicate a quarter of a minute; A, abscissa (Stirling, ^ter Key). 



ture of a vein raised the blood-pressure within a glomerulus, it 

 was not followed by any increase in the quantity of the secretion, 

 but by its actual arrest. He also showed that injection of a col- 

 ored substance (sodium sulphindigodate) into the blood, after the 

 pressure had been greatly lowered by section of the spinal cord, 

 led to its appearance in the urine ; and microscopic examination 

 showed that it had passed through the epithelial cells of the 

 tubules, not of the glomeruli. 



It is found, however, that after the removal of a ligature 

 applied to the renal artery the urine is albuminous, showing 

 plainly that the cells have been injured by the operation ; hence 

 Heidenhain's experiment described above is not valid against 

 the blood-pressure theory. Moreover, too much must not be 

 inferred from the action of foreign substances under the ab- 

 normal conditions of such an experiment. While some physi- 

 ologists claim that the glomeruli are filtering mechanisms, they 

 explain that filtration is not to be understood in its ordinary 

 laboratory acceptation, but that the glomeruli discriminate as 

 to what they allow to pass, yet they in no way explain how 

 this is done. They make the whole process depend on blood- 

 pressure, and attribute little special action to the flat epithelium 

 of the Malpighian capsules. 



Though we can not admit the full force of Heidenhain's ex- 

 periments as he interprets them, we still believe that his views 

 are most in harmony with the general laws of biology and the 



