EXCRETION 



463 



;:ause and effect, and that the formation of urine was accomplished entirely 

 py purely physical forces. 



The progress of physiologic investigation, however, has thrown some 

 lloubt on the validity of this physical interpretation. 



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

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

 :ess of secretion on the part of the epithelial cells both of the glomerulus and 

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

 }lood normal diffusible constituents when in excess, and abnormal con- 

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

 it 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- 



JV M 



FIG. 208.-THE MECHANISM FOR THE SECRETION OF THE URINE. I A, interlobular artery ; 

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

 !j contorted tubule. 



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

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

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

 jsented the following experiment: The spinal cord of an anima is divided u 

 (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 1 

 blood-vessles washed out with alcohol for the purpose of preciprta ing 1 

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

 of the cortex alone (Fig. 209). Microscopic examination revea the fee 

 that the blue stain is due to the deposition of the pigmen in the ^lumen a 

 in the lumen border of the cells of the convoluted tubules (Fig. 210) and 



