650 THE SECRETION OF URINE. 
Heidenhain, however, points out that we have no right to conclude that 
the secretion of urine has ceased when the mercury column no longer 
rises. This stage in fact corresponds merely to the point at which the 
continued secretion of urine is balanced by the reabsorption of the urine 
from the tubules, in consequence of the abnormal pressure within 
them. 
It must be confessed that we have no very definite evidence that 
such a reabsorption takes place. It is true that the kidney becomes 
cedematous in consequence of the ligature, but the cedema fluid was stated 
by Ludwig to consist of lymph and not of urine ; and it has been shown 
that increased pressure in the urinary tubules causes them to press on 
the adjoining veins, so that the escape of blood from the kidney is 
hindered, and ordinary oedema results. Fresh investigations on this 
matter are much to be desired, since the only analyses we have of the 
oedema fluid and retained urine are those of Hermann, one of the earliest 
observers on the subject. The urine, which is secreted under pressure 
and which distends the pelvis and ureter, is light in colour, of low 
specific gravity, and contains very little urea. If, after some time, the 
ligature round the ureter be relaxed, the result is at once a copious 
secretion of watery urine. In man a similar fluid is well known to be 
excreted in cases where there is a chronic obstruction of the ureter. 
The concentration of the urine. — We have now to consider the 
second part of Ludwig's theory, according to which the dilute urine 
transuded through the glomeruli is concentrated on its passage down 
the tubules, by the absorption of its water. This absorption takes place 
in consequence of the fact that the lymph surrounding the tubules is 
more concentrated than the urine. A cogent objection to this hypo- 
thesis was raised in 1859 by Hoppe (Hoppe-Seyler), who showed that, 
if urine were separated by an animal membrane from blood serum of 
the same animal, there was a flow of water from serum to urine. 1 The 
tendency of this urine, therefore, in passing down the urinary tubules, 
would have been to become more dilute, in consequence of osmotic 
interchanges between it and the serum. At this time our knowledge 
of the factors and forces involved in the interchange of water and sub- 
stances in solution across animal membranes w T as meagre and inexact ; 
and it is only quite recently that we have acquired the necessary data 
for testing the truth of Ludwig's hypothesis and the fitness of Hoppe- 
Seyler's objections. 
Pf effer 2 showed that the osmotic attraction of any solution for water 
might be determined by measuring its osmotic pressure, and first pointed 
out how enormous these pressures were in the case of even relatively 
dilute salt solutions. Van t' Hoff later on pointed out that the osmotic 
pressure of a solution was proportional to the number of molecules this 
contained, and was therefore a colligative property (Ostwald), like certain 
other properties of solutions — such as the diminution of the freezing 
point and of the vapour tension and the elevation of the boiling 
point. 
Since these properties of a solution are proportional to one another, 
we need only know one to determine any of the others. This fact is of 
importance when w r e wish to determine the osmotic pressure of animal 
fluids, since we can substitute for the difficult and inexact determination 
1 Firchow's Archil', 1S59, Bd. xvi. S. 412 (quoted by Heidenhain). 
- "Osmotische Untersachungen," Leipzig, 1877. 
