1146 PHYSIOLOGY 



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 precipitates the dyestufi), all parts of the kidney present a 

 blue colour, which is especially marked in the medulla. Under these cir- 

 cumstances the urine, which is being excreted by the glomeruli, rapidly 

 carries down the dyestufl, wherever it may be turned out, into the tubules 

 of the pyramids. In order to discover the exact locality of the cells in- 

 volved in its excretion, we must stop the glomerular transudate by some 

 means or other. This stoppage of the urinary flow can be effected in two 

 ways, viz. by section of the spinal cord in the neck, so as to reduce the 

 blood-pressure to about 40 mm. Hg., i.e. below the minimum necessary for 

 the production of urine, or by cauterising portions of the surface of the 

 kidney by means of silver nitrate. If the indigo be injected into the veins, 

 after section of the cord, and the animal be killed half an hour later, and the 

 kidneys fixed with absolute alcohol, they are found to be of a bright blue 

 colour, although no urine has been secreted. On cutting into the kidneys 

 the colour is seen to be confined to the cortex, and on making microscopic 

 sections granules of the pigment are found within the lumen and in the 

 epithelial cells of the convoluted tubules. If the kidneys have been 

 cauterised, the stain is confined to the convoluted tubules of the cortex only 

 under those areas which have been cauterised, and where the glomerular 

 functions have been abolished. It has been suggested that the appearances 

 after the injection of indigo are due, not to the secretion, but to the absorp- 

 tion of water in the convoluted tubules. A certain amount of the dyestufT 

 is thus rendered visible by becoming more concentrated, and is precipitated 

 in a granular form, as soon as the salt concentration of the fluid reaches a 

 certain height. The fact that these appearances are wanting after the 

 injection of ordinary carmine, which stains the glomeruli as well as the 

 tubules, combined with the histological facts mentioned in the last paragraph, 

 render this a somewhat forced explanation ; and we must take the results 

 of the injection of indigo-carmine as telling rather in favour of a secretory 

 than of an absorptive function on the part of the convoluted tubules. 



The question as to the secretory activity of the kidney can be attacked 

 from another side. The glomerular filtrate can contain only those crystal- 



