246 



NUTRITION 



FIG. 132 



processes and the general results which can be described. The illustra- 

 tion shows the various parts and twists of the uriniferous tubule, but it 

 fails to describe their respective functions and what each of the several 

 varieties of epithelium making up the tubule contributes to the urine's 

 complex composition. 



Few subjects in physiology have been more actively discussed than 

 this, the matter resting on two basal presumptions, nearly opposite in 

 theory, concerning renal secretion. These presumptions are still known 

 as the theories of Bowman and of Ludwig respectively. Bowman 



supposed that the protoplasm of the cap- 

 sule named for him, largely by virtue of 

 its own secretory powers, took from the 

 capillary blood the salts and the water of 

 the urine; and that the varied epithelium 

 of the different tubular parts farther down 

 added the organic constituents, the urea, 

 hippuric and uric acids, and the rest; 

 Heidenhain has added much support to 

 this theory. Ludwig's hypothesis was 

 more mechanical so far as the working 

 of the glomeruli are concerned, for he 

 maintained that these tiny organs are 

 little more than organized filters which 

 by physical means take the urine in a 

 dilute form out of the stream of blood to 

 be condensed later by the absorption from 

 it of water in the epithelial walls of the 

 devious tubules. The research of Cushny 

 and others makes them suppose with 

 Ludwig that the water, salts, and urea 

 pass from the glomeruli into the tubules, 

 but that part of the salts and of the water 

 pass from the latter again into the circula- 

 tion. (See the figure opposite diagram- 

 ming these three theories.) A compromise 

 which recent work impels makes it likely 



that urinary excretion is accomplished by both mechanical filtration 

 and by vital secretory action in the glomerulus, but that the epi- 

 thelium of the tubules contributes perhaps numerous unknown sub- 

 stances to the urine. Here, as in the other cases, we may safely say 

 that there is mechanical filtration surely enough, but that the filter is 

 alive and selects what it shall let pass and passes nothing else so long 

 as it is normal. The trend of recent work has been to prove directly 

 that the "rodded" or striped epithelium of the tubules secretes products 

 (such for example as uric acid) into them; urea also may be seen to 

 collect in vacuoles in the epithelial cells, the former "bursting" after a 

 while. The glomerulus acts in a manner and, occasionally at least, under 



The relation of the blood-capil- 

 laries to the convoluted tubule in the 

 frog's kidney. The erythrocytes in 

 the capillary close to the outer side 

 of the tubular epithelium and the 

 nerve fibers supplying the two last 

 are obvious. (Smirnow.) 



