CONCLUSIONS. 659 
2. By the velocity of How through the capillaries. 
.".. By the permeability of the capillary wall and the glomerular 
epithelium. 
This watery transudate is concentrated and altered on its way through 
the tubules, in consequence of the absorption of water, and probably of 
certain of its crystalloid constituents. This absorption must be due to 
the active intervention of the cells, since the osmotic pressure of the 
urine is considerably higher than that of the blood pressure. Diuretics 
may act in two ways. The saline diuretics increase the pressure and 
velocity of the blood in the glomerular capillaries, not only by increasing 
the volume of the circulating fluid, but also probably by a direct dilator 
action on the afferent vessels of the glomeruli. A similar local dilator 
effect is produced by drugs such as caffein or theobromin : but in these 
cases the drugs probably exert a paralysing influence on the absorbing 
mechanism of the kidney, i.e. the cells of the convoluted tubules, so that 
the glomerular transudate may undergo little change on its way to the 
ureter and bladder. 
One of the strongest arguments in favour of this modified Ludwig hypo- 
thesis is the fact that the more we augment the flow of urine, whether by 
caffein, saline diuretics, or production of hydraernic plethora, the more nearly 
does its osmotic pressure, saline constitution, and reaction approximate that of 
the blood plasma. It would seem that in the glomeruli we have an apparatus 
which, like the capillaries of the abdominal viscera but in a still higher degree, 
reacts to changes in the intracapillary pressure, and so serves to regulate 
accurately the amount of fluid circulating in the blood vessels. 
Whether we look upon the cells of the convoluted tubules as secre- 
tory or absorptive in function, we have at present no evidence that the 
cellular covering of the glomeruli acts otherwise than passively in the 
production of the glomerular part of the secretion. It must be remem- 
bered, however, that under certain circumstances, as after ingestion of large 
quantities of fluid, the osmotic pressure of the urine may fall below that 
of the blood plasma, Dreser 1 interprets this as pointing to an activity 
of the glomerular epithelium. I have shown above that it may equally 
well be explained by assuming an absorption of salts by the water-logged 
tubule cells, or an active excretion of water by these cells. I may 
mention here that I. Munk and Senator, 2 as a result of researches carried 
out for the most part on the excised kidney, have come to a conclusion 
analogous to that just stated, namely, that in the production of urine we 
have a co-operation of physical and physiological factors. According to 
these authors, water and part of the urinary salts (especially NaOl) 
are transuded through the glomeruli in direct consequence of the blood 
pressure, i.e. by a process of filtration, although the rapidity of the blood 
flow is at least of equal importance with its pressure. The specific 
urinary constituents — urea, uric acid, hippuric acid, etc., together with 
another portion of the urinary salts (XaCl, sulphates and phosphates) 
— are secreted by the active intervention of the cells of the tubules, 
especially the convoluted tubules. These substances are secreted in a 
dissolved condition, and must, therefore, take a certain. amount of water 
with them. 
The influence of the nervous system on the secretion of urine. 
— The discovery by Berkeley 3 of a distribution of nerve-endings to the 
1 Loc. tit,. 2 Virchov/s Archiv, 1888, Bd. cxiv. S. 1. 3 Loc. tit. 
