844 PHYSIOLOGY OF DIGESTION AND SECRETION. 



of the urine in the capsule must be very low, owing to the fact that 

 the secretion drains away as rapidly as it is formed. If the ureter 

 is occluded, however, the pressure of the urine will increase, and the 

 filtration pressure P p will diminish. When this experiment is 

 performed and the pressure in the ureter is measured by a manom- 

 eter, it is found to rise to 50 or 60 mms. of mercury and then to 

 remain stationary. This fact might be explained by supposing 

 that when p = P the secretion stops on account of the failure of 

 the filtration pressure. Little weight, however, can be given to 

 this argument, since it is quite possible that under these condi- 

 tions the urine may still continue to form, but be reabsorbed under 

 the high tension reached. The experiment simply serves to show 

 the secretion pressure of the urine, and the fact that this pressure 

 rises as high as 50 to 60 mms. mercury, while the capillary pressure 

 is probably somewhat lower, would rather serve as an argument 

 against the filtration theory. Moreover, experiments show * that 

 when a certain moderate resistance is established in the ureters 

 (p = 10 cms. H 2 O) the flow of urine is actually increased, a fact 

 entirely opposed to the mechanical theory. 



Function of the Convoluted Tubule. By the term convoluted 

 tubule is meant here the entire stretch from the glomerulus to the 

 straight tubules. Its epithelium varies in character; its cells are 

 distinguished in general, as contrasted with the glomerular epithe- 

 lium, by a relatively large amount of granular protoplasm. The 

 question of interest at present in regard to this epithelium is whether 

 it is secretory or absorptive. The original view of Ludwig that 

 diffusion takes place in these tubules between the urine and the 

 blood (lymph) in accordance with simple physical laws and that 

 by this action alone the dilute urine is brought to its normal concen- 

 tration must be abandoned. The mere fact that the urine may be 

 more concentrated in certain constituents than the blood is suffi- 

 cient evidence that other factors must co-operate. Those who be- 

 lieve that the main function of the tubules is absorptive are obliged 

 to regard this process as physiological, as a selective absorption 

 depending upon the living structure and properties of the epithelial 

 cells. The kind of evidence upon which this view is based is some- 

 what indirect; a single example may suffice. Cushny statesf that 

 if certain diuretics for example, sodium chlorid and sodium sul- 

 phate are injected simultaneously into the blood and in such 

 amounts that an equal number of anions (Cl and SO 4 ) is pres- 

 ent, the quantities that are excreted in the urine during the next 

 hour or two follow different curves and vary independently of their 

 concentration in the plasma. While this independence might be 



* Brodie and Cullis, " Journal of Physiology," 1906, xxxiv., 224. 

 t "Journal of Physiology," 27, 429, 1902. 



