4 68 TEXT-BOOK OF PHYSIOLOGY 



in the lumen border of the cells of the convoluted tubules (Fig. 207) and 

 the ascending limb of Henle's loop; while the epithelium of Bowman's cap- 

 sule as well as the glomerular epithelium present no evidence of pigmentation. 

 The physiologic action of the cells of the convoluted tubules in elimination 

 of indigo-carmine, is supposed to indicate their action 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. 



Another experiment which apparently shows the action of the epithelium 

 is the following: A solution of uric acid in piperazin is injected into the 

 blood of a rabbit. At the end of from twenty to sixty minutes the animal is 

 killed. The kidney is sectioned and examined microscopically, whereupon 

 it is found that crystals of uric acid are present in the epithelial cells of the con- 

 torted tubules, especially toward their inner border, while in the medullary 

 tubules the crystals are confined to the lumen. These facts lead to the 

 inference that under these conditions at least, the uric acid passes from the 



FIG. 206. KIDNEY OF A RAB- FIG. 207. MICROSCOPIC APPEARANCE OF 



BIT. Cortex alone stained with THE LUMEN OF THE CONVOLUTED TUBULES 



the indigo-carmine at the end of CONTAINING THE INDIGO-CARMINE. (Hei- 



one hour. (Heidenhain.) denhain.} 



blood by way of the epithelium into the interior of the tubule to be finally 

 discharged into the pelvis of the kidney. 



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 

 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, 



