746 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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. Exact figures, however, regard- 

 ing the capillary pressure in the kidney can not be obtained; so 

 that the experiment, on the whole, gives us no satisfactory in- 

 formation regarding the theory of secretion. Dreser has used a 

 different argument to prove that the production of the water in- 

 volves the performance of work on the part of the epithelial cells. 

 He points out that in some conditions e. g., after drinking beer 

 the urine may be very dilute, as shown by the fact that its freezing 

 point may be only 0.18 C. or 0.16 C. below that of pure water, 

 that is, A = 0.18 C. or 0.16 C. (see appendix). Since blood- 

 serum has A = 0.56 C, the difference in concentration between 

 the blood and the urine in such a case of extreme dilution shows an 

 osmotic pressure in favor of the blood equivalent to A = 0.4 C. 

 Measured in mechanical units, this would indicate an initial osmotic 

 pressure of 49.08 meters of water tending to drive the water from 

 the uriniferous tubules into the blood, whereas the filtration pres- 

 sure driving the water in the other direction could not at a max- 

 imum exceed 2.72 meters of water. If this argument is valid, the 

 elimination of the water takes place against a strong opposing os- 

 motic pressure, and the energy necessary for its secretion can be 

 referred only to the activity of the epithelial cells. 



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- 



