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 

 oedematous in consequence of the ligature, but the oedema 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 es(*ipe 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 was 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 we wish to determine the osmotic pressure of animal 

 fluids, since we can substitute for the difficult and inexact determination 



1 Virdiow's Archiv, 1859, Bd. xvi. S. 412 (quoted by Heidenhain). 



2 "Osmotische Untersuohungen," Leipzig, 1877. 



