EXCRETION 



467 



apparently indicated that the former stood to the latter in the relation of 

 cause and effect, and that the formation of urine was accomplished entirely 

 by purely physical forces. 



The progress of physiologic investigation, however, has thrown some 

 doubt on the validity of this physical interpretation. 



Heidenhain brought forward a series of facts which supported the view, 

 that all the constituents of the urine are eliminated from the blood by a pro- 

 cess of secretion on the part of the epithelial cells both of the glomerulus and 

 the tubules; that these cells have the power of selection in removing from the 

 blood normal diffusible constituents when in excess, and abnormal con- 

 stituents, i.e., urea, uric acid, etc., when present in any amount, recognizing 

 at the same time that the secretor activity of these cells is modified in one 

 direction or another by the blood-pressure and the variations which it under- 



FIG. 205. THE MECHANISM FOR THE SECRETION OF THE URINE. I A, interlobular artery ; 

 av, afferent vessel; ev, efferent vessel; c, capillary vessels; v, vein; IV, interlobular vein; ct , 

 contorted tubule. 



goes from moment to moment. In other words that the secretion of urine is 

 a physiologic or vital process, rather than a physical or mechanical process. 

 As evidence that the cells of the tubules possess a selective power, he pre- 

 sented the following experiment: The spinal cord of an animal is divided in 

 the neck for the purpose of lowering the blood-pressure in the kidney below 

 the pressure at which the urine is secreted. Five to twenty c.c. of a satur- 

 ated solution of indigo-carmine are injected into the blood-vessels; after 

 intervals varying from ten minutes to one hour the animal is killed, the 

 blood-vessels washed out with alcohol for the purpose of precipitating the 

 indigo-carmine in situ. Section of the kidney shows a uniform blue stain 

 of the cortex alone (Fig. 206). Microscopic examination reveals the fact 

 that the blue stain is due to the deposition of the pigment in the lumen and 



