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TEXT-BOOK OF PHYSIOLOGY. 



in the elimination of urea and other nitrogen-holding compounds. The 

 absence of the pigment from the glomerular epithelium lends support to the 

 view, that its function is the elimination of water and highly diffusible 

 inorganic salts. 



Nussbaum attempted to establish the secretory power of the epithelium 

 in another way. In the frog the kidney receives blood from two sources: the 

 glomeruli receive their blood from the renal artery, the tubules from the 

 capillaries formed by the anastomosis of branches of the efferent vessel 

 of the glomerulus and the branches of the renal portal vein. Nussbaum 

 believed that by ligating the renal artery all glomerular activity could be 

 abolished and the part played by the epithelium could be determined. After 

 so doing the flow of urine was at once checked; the injection of urea at 

 once reestablished it. This fact was taken as a proof that the tubular 

 epithelium not only excreted urea, but water and perhaps other constituents 



FIG. 219. KIDNEY OF A RAB- 

 BIT. Cortex alone stained with 

 the indigo-carmine at the end of 

 one hour. (Heidenhain.) 



FIG. 220. MICROSCOPIC APPEARANCE OF 

 THE LUMEN OF THE CONVOLUTED TUBULES 

 CONTAINING THE INDIGO-CARMINE. (Hei- 

 denhain.') 



as well. It was also found that sugar, peptones, carmine, etc., which are 

 always eliminated from the blood under normal conditions, are not removed 

 after ligation of the renal artery. It was concluded from these experiments 

 that the secreting structures of the kidney consist of two distinct systems, 

 the glomerular and the tubular; the former secreting water, salts, sugar, 

 peptone, etc.; the latter urea, uric acid, etc. These and similar facts in- 

 dicate that the renal epithelium possesses a secretor rather than an absorptive 

 function. Heidenhain and those who agree with him assert that even the 

 water and inorganic salts which pass through the glomerular epithelium 

 do so in consequence of cell selection and cell activity; that the entire process 

 is one of secretion, though conditioned by blood-pressure, blood velocity, etc. 

 Influence of Blood-pressure. Whether the elimination of the urinary 

 constituents is entirely secretor (physiologic) in character or not there can 

 be no doubt that the whole process is largely determined by the pressure and 

 velocity of the blood in the glomerular capillaries, or, to state it more accu- 

 rately, on the difference of pressure between the blood in the capillaries and 

 the urine in the capsules. As a rule, this latter pressure is at a minimum. If 



