768 PHYSIOLOGY OF DIGESTION AND SECRETION. 



with more difficulty. On the other side, the facts that have been 

 urged in favor of the secretory hypothesis are more numerous and 

 varied, but none is entirely convincing. Some of these facts are 

 as follows: (1) It is stated that if the ureters are ligated in birds 

 the urates will be found deposited in the uriniferous tubules, but 

 never at the capsular end. (2) Heidenhain has given proof that 

 the convoluted tubules are capable of excreting indigo-carmin after 

 this substance is injected into the blood. His experiment consisted 

 essentially in injecting the material into the blood, after dividing 

 the cord so as to reduce the rapidity of secretion. After a certain 

 interval the kidney was removed and irrigated with alcohol to pre- 

 cipitate the indigo-carmin in situ in the organ. Microscopical ex- 

 amination showed that after this treatment the granules of the 

 indigo-carmin are found in the convoluted tubules, but not in the 

 capsules around the glomeruli. (3) Several observers (Van der 

 Stricht, Disse, Trambasti, Gurwitsch*) have described microscopical 

 appearances in the cells lining the tubules indicative of an active 

 secretion. They picture the formation of vesicles in the cells and 

 appearances which indicate the discharge of these vesicles into the 

 cavity of the tubules. (4) Nussbaum made use of the fact that in 

 the frog the glomeruli are supplied by branches of the renal artery, 

 while the rest of the tubes is supplied by the renal portal vein. 

 He stated that if the renal artery is ligated the glomeruli are de- 

 prived completely of blood, and that as a result the flow of urine 

 ceases. If under these conditions urea is injected into the circulation 

 it is excreted together with some water, thus proving the secretory 

 activity of the tubules with regard to urea. These results, although 

 denied at one time, have recently been confirmed and extended. f 

 (5) Dreser has shown that the acidity of the urine is due to an action 

 of the epithelium of the tubules. If an acid indicator, such as acid 

 fuchsin, is injected into the dorsal lymph sac of a frog, and an hour 

 or so later the kidneys are examined, it will be found that the con- 

 voluted tubules are colored red, while the capsular end is colorless, 

 indicating that the secretion at the latter point has an alkaline 

 reaction. The experiment shows that the acid substances in the 

 urine are produced in the convoluted tubules. The simplest 

 explanation is that they are formed by a secretory activity of the 

 epithelial cells. . (6) Study of the gaseous exchanges in the kidney 

 during diuresis, t and of the sugar output during the glycosuria 

 caused by phlorhizin tend to support the secretion hypothesis 

 to the extent that they prove an increased metabolism during 



* See Gurwitsch, 'Archiv f. die gesammte Physiologic, " 91, 71, 1902. 

 t Bainbridge and Beddard, "Journal of Physiology," 1906, xxxiv. (Proc. 

 Physiol. Soc.); also Cullis, Ibid., p. 250. 



J Barcroft and Brodie, 'Journal of Physiology," 1906, xxxiii., 52. 

 Pavy, Brodie, and Siam, ibid., 1903, xxix., 467. 



