6 Physiology of the Kidney 



which the glomerular capillaries were presumed to be imper- 

 meable. The actuality of this process of glomerular filtration 

 is now established beyond the slightest doubt. The painstak- 

 ing investigations of Professor Richards and his co-workers 

 on the composition of the glomerular fluid in the frog and 

 other cold-blooded animals, investigations begun in 1.924 and 

 only recently published in full,"^ have afforded incontrovert- 

 ible proof of this thesis in these animals. Minute amounts of 

 fluid have been collected from the glomerular capsule of Am- 

 phibia and reptiles by means of a Chamber's micro-dissection 

 pipette, and by micro-analytical methods of high accuracy 

 these minute quantities of fluid have been analyzed for various 

 constituents. Comparison of the capsular fluid and of the 

 plasma simultaneously^ circulating in the animal has demon- 

 strated that fats and plasma proteins, or substances chemi- 

 cally combined with these large molecular aggregates, are the 

 only important constitutents of the plasma that do not pass 

 through this filtering bed. Since certain substances, such as 

 glucose and chloride, are invariably present in the capsular 

 fluid although they may be nearly or wholly absent from the 

 urine, these investigations also afford a direct demonstration 

 of the actuality of tubular reabsorption — which process is, 

 of course, a corollary of glomerular filtration. By a still more 

 admirable technique the Philadelphia investigators have been 

 able to follow the processes of tubular reabsorption, and to 

 demonstrate the specific role of the proximal and distal tu- 

 bules in the reabsorption of various substances. 



Because of the similar anatomy of the glomerulus in frog 

 and man, the above evidence for glomerular filtration could 

 by inference be transferred directly to the human kidney. 

 Actually, inferential transfer is no longer necessary, for data 

 are now available on the excretion of a variety of substances 



