1194 PHYSIOLOGY 



In a secreting gland such as the parotid, there is a marked change in the appearance 

 of the granules according as the gland is resting or actively secreting. No such changes 

 have been discovered in the granules of the renal cells, and the vacuoles that have been 

 described might be either in process of secretion or might be evidence of copious absorp- 

 tion of watery fluids from the lumen of the tubule. 



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 massive doses 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. 



Under the same circumstances masses of uric acid crystals are also 

 found in the connective tissues between the tubules. It is therefore impos- 

 sible to be certain that the granules observed within the epithelial cells 

 are in process of excretion or are being absorbed from the lumen. Modern 

 methods have failed to substantiate the older observations as to the occur- 

 rence of uric granules under normal conditions in the cells of the convoluted 

 tubules of the bird's kidney. 



Heidenhain has attempted to throw light on the excretive functions of 

 the kidney by studying the mechanism by means of which it excretes certain 

 dyestufEs, 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 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 

 dyestufi, wherever it may be turned out, into the tubules of the pyramids. 

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



