THE SECRETION OF URINE 1281 



The view that a portion of the tubules is secretory in function 

 is further supported by histological examination of these structures 

 under various conditions of activity. In the cells of the convoluted 

 tubules various kinds of granules and of vacuoles may be distinguished. 

 Gurwitsch divides these vacuoles into three classes : 



(1) Large granules staining densely with osmic acid, and probably 

 rich in lecithin. 



(2) Smaller very numerous granules consisting of some form of 

 protein material. 



(3) Large vacuoles lying close to the free margin of the cells, whose 

 contents do not undergo coagulation with the ordinary fixing reagents, 

 and therefore are free from protein, fat, or mucin. These vacuoles are 

 especially marked in kidneys which are secreting at a great rate, in 

 consequence of the injection of saline diuretics or of large quantities 

 of normal salt solution. They may be regarded as excretory vacuoles 

 and consist of water or saline fluids which have been collected by the 

 cells and are being passed on by them to the lumen of the tubules. 



As a rule it is impossible to trace any definite constituent of the 

 urine on its way through the cells of the tubules. But if a solution of 

 uric acid in piperazin be injected intravenously into a rabbit, the 

 kidneys, taken twenty to sixty minutes after the injection, present 

 tubules full of uric acid concretions. In the medullary portion of the 

 kidney this uric acid precipitate is confined to the lumen of the tubules, 

 but in the convoluted tubules granules of uric acid are to be found in 

 the epithelial cells, especially towards their inner border. Since these 

 cells are able to excrete uric acid when present in abnormal quantities in 

 the blood, it is a reasonable assumption that they also undertake the 

 secretion of this substance under normal conditions. Certain observers 

 have in fact described the presence of urate granules in the cells of the 

 convoluted tubules of the bird's kidney. 



Although the larger number of the urinary constituents must 

 escape detection on their way through the cells, we can throw some 

 light on the excretory functions of the kidney by studying the 

 mechanism by means of which it excretes certain dyestuffs, such as 

 sulphindigotate of soda (' indigo carmine '). If the indigo be injected into 

 the veins, it is excreted in a concentrated form, both by the liver and 

 by the kidney, so that the urine assumes a dark blue colour. If the 

 animal be killed when the excretion of the pigment is at its height, 

 and the kidneys be rapidly fixed with absolute alcohol (which precipi- 

 tates the dyestuff), all parts of the kidney present a blue colour, which 

 is especially marked in the medulla. Under these circumstances the 

 urine, which is being excreted by the glomeruli, rapidly carries down 

 the dyestuff, wherever it may be turned out, into the tubules of the 

 pyramids. In order to discover the exact locality of the cells involved 



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