THE KIDNEY. 221 



seen that the rod-epithelium alone excretes the indigo-salt, and 

 it may be presumed, therefore, that the function of the glome- 

 ruli is to act as a filter for the fluid portion of the urine. Thus 

 the salt is washed from the convoluted into the collecting 

 tubules, and thence into the pelvis of the kidney. The action 

 of absolute alcohol on a solution of sulphindigate of soda is to 

 precipitate that salt. It is this action within the kidney which 

 fixes the dye, as above set forth. 



Beautiful specimens may also be obtained by various modi- 

 fications of the above process. 



The following formulae and results are given by Heidenhain : 



1. Babbit or dog ; section of spinal cord, injection of only 5 c.c. of the in- 

 digo solution, the animal being killed after ten minutes. Result : pyramidal 

 portion and boundary layer free from indigo-blue. In the cortical substance, 

 some of the convoluted tubules are filled with the crystalline salt ; in the 

 greater number the epithelium is colored of a uniform blue, the nuclei possess- 

 ing the same tint ; the lumen is usually free. 



2. Same conditions as above, excepting that 20 or 25 c.c. of the solution 

 is injected. Medulla free from indigo blue. In the cortex a great many of the 

 tubules are filled with the pigment, while the epithelium is stained blue, the 

 nuclei of a deep blue color. 



3. Same conditions as in 2, excepting that the animal is killed one hour 

 after injection. Nuclei of the rod-epithelium stained deep blue, rods clear ; 

 convoluted and collecting tubules filled with crystals of pigment. 



Instead of using absolute alcohol for injecting the renal 

 artery, the writer's carmine-glycerine fluid may be employed. 

 After having injected the artery in this way, the kidney is 

 placed in a vessel of absolute alcohol, and divided into small 

 pieces while immersed in that fluid. The glycerine being anhy- 

 drous, prevents the diffusion of the indigo-salt within the kid- 

 ney, while the alcohol fixes the pigment. Sections should be 

 made from the cortex and medulla, and mounted permanently 

 in resinous turpentine. If the glycerine injection has been suc- 

 cessful, all the glomeruli and capillaries will be filled with a 

 transparent red mass (Fig. 101). If the indigo excretion has 

 reached the third stage, the collecting tubules in the medulla 

 will be filled with blue crystals of indigo-carmine, and the vasa 

 recta with a red mass, the two arranged in alternate rows. 

 Such specimens leave nothing to be desired in the way of 

 demonstrating the structural relations just described. 



