EXCRETION 465 



peptone, etc., the latter urea, uric acid, etc. These and similar facts in- 

 dicate that the renal epithelium possesses a secretor function. Heidenhain 

 and those who agree with him assert that even the water and inorganic salts 

 which pass through the glomerular epithelium do so in consequence of cell 

 selection and cell activity; that the entire process is one of secretion, though 

 conditioned by blood-pressure, blood velocity, etc. 



The Present View as to the Mode of Formation of the Urine. 

 The still further progress of physiologic investigation has apparently re- 

 established the view, subject to some modifications, that the gtomerulus is 

 after all but a passive apparatus permitting the filtration of water and diffus- 

 ible salts, as Bowman and Ludwig suggested, and has confirmed the view of 

 Heidenhain that the epithelium is a secretory apparatus, and in addition 

 has established the further fact that the epithelium is an absorptive apparatus 

 as well. Without presenting the details of a long series of experiments it 

 suffices to say that the results of these investigations make it reasonably 

 certain that the glomerulus permits of the passage of a filtrate, not of the 

 character and composition asserted by Ludwig, but having the character- 

 istics and composition of the blood-plasma, i.e., having the chemic composi- 

 tion and the same degree of concentration as this fluid, less its protein con- 

 tent; that as this fluid passes through the tubules it receives the organic con- 

 stituents which are secreted by the epithelium and in addition a certain 

 volume of water as well; also that it loses water and inorganic salts which 

 are returned to the blood not by diffusion as Ludwig believed but by an 

 act of absorption on the part of the epithelial cells, when these substances 

 are needed for nutritive purposes. 



The probability that the renal epithelium possesses absorptive functions 

 is indicated by a greater or less constriction of the tubules in those animals 

 in which the retention of water at least, is desirable. Thus in fish where 

 the supply of water is abundant and no need for its retention in the body 

 exists, the tubules are short, wide and free from any narrowing which would 

 retard the downward flow of the fluid; in frogs which live partly on land and 

 I which lose water readily by evaporation from the skin, the retention of water 

 is necessary, hence in these animals the tubules are long and much con- 

 stricted; in tortoises where there is no evaporation from the skin, the tubes 

 are short, wide and devoid of constrictions. In mammals the constricted 

 portion, the loop of Henle, is long and narrow, and very probably serves t 

 retard the flow of the liquid thus permitting of the absorption of water am 

 other constituents. ,. . 



Summarizing the foregoing it may be stated that the formation i of urine 

 is accomplished by the cooperation of the glomerulus and the epithelium o 



^^IgUmeruius permits the passage of a nitrate similar if not identical 

 with the blood plasma minus the protein. 



The epithelium, especially that lining the contorted portion of the tubule, 

 (i) secretes water, various salts previously in combination with protein, eg 

 phosphates, urea, uric acid, various other nitrogen-holding compounds 

 organic matters; (2) absorbs such constituents of. the urinary fluid as water 

 and inorganic salts which may be necessary to the nutrition of the body^ 

 final result of the entire process is the maintenance of the normal composition 

 of the blood and body fluids. 



