CONCLUSIONS. 659 



2. By the velocity of flow through the capillaries. 



3. 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 hydraemic 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 l 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 NaCl) 

 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 (NaCl, 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. cit. 2 Virchow's Archiv, 1888, Bd. cxiv. S. 1. 3 Loc. tit. 



