EXCRETION BY THE KIDNEY. 427 
capillaries around the tubules, which inosculate to some extent 
with the vessels emerging from the glomeruli. It was found 
that when certain substances were injected into the blood they 
no longer appeared in the urine after the renal artery had been 
tied, from which it was concluded that they were secreted only 
by the glomeruli, and that the blood of the renal-portal vein 
did not find access to the glomeruli. This conclusion was a 
pretty bold leap, but there was some show of reason for it. 
More recently, however, these experiments have been demon- 
strated to be, to a certain extent, unreliable, and that the pas- 
sage of blood from the venous capillaries backward can really 
take place, to some extent, after a time. 
Theories regarding the secretion of urine may be divided 
into those that are almost wholly mechanical, partly mechani- 
cal, and purely secretory: 1. To the first class belongs that of 
Ludwig, which teaches that very dilute urine is separated from 
the blood in the glomeruli, and by a process of endosmosis and 
absorption of water by the tubular capillaries is gradually 
concentrated to the normal. 2. As an example of the second 
class is that of Bowman, who maintained that the greater part 
of the water and some of the more soluble and diffusible salts are 
separated by the glomeruli but the characteristic constituents 
of the urine by the epithelium of the renal tubules. 3. As an ex- 
ample of the third is the theory of Heidenhain, who attributed 
little to blood-pressure in itself, and much, if not the whole, to 
the secreting activity of the epithelium of the tubules more par- 
ticularly. This physiologist showed that-while ligature 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 colored 
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 exam- 
ination 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 
that the cells have been plainly 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 
