STRUCTURE OF THE- KIDNEY 2OI 



tivity of the kidneys by the nervous system; but that influ- 

 ence is probably exerted only through vaso-constrictor and 

 vaso-dilator fibers to the vessels. 



Assuming for the present that nearly all the constituents 

 of urine preexist in the blood and are simply taken out of 

 the circulation in the kidney, it may be stated that, for the 

 most part, the water and salts are extracted by the cells of 

 the Malpighian bodies, while the urea and related nitrogen- 

 ous solids are removed by the cells of the convoluted tubes ; 

 so that the specific gravity of the fluid is raised in passing 

 down the tubes. While the histology of the kidney, and 

 especially the arrangement of the glomeruli, is most favor- 

 able for the exercise of simple osmosis, and while this pro- 

 cess is doubtless mainly responsible for the phenomena 

 which occur, it seems highly probable that the cells them- 

 selves modify osmotic action by taking an active part in the 

 secretion of urine. They undoubtedly exercise a selective 

 affinity accounting for the different materials handled by the 

 glomeruli and the tubes. Moreover, morphological changes 

 in the tubal cells during activity have been microscopically 

 demonstrated. Vesicles are described as forming in the body 

 of the cell, approaching the lumen, bursting and discharging 

 their contents which are supposed to include the urea and 

 such other materials as may be here extracted from the 

 blood. 



As regards the elimination of water and salts by the 

 glomerular epithelium, it must also be admitted that the 

 cells take some obscure but active part. Were this only an 

 osmotic process the amount eliminated would vary exactly 

 as the pressure. While usually a rise in renal blood-pressure 

 is accompanied by an increased flow of urine and a fall by 

 a correspondingly decreased flow, the rule does not always 

 hold good. For instance, compression of the renal vein 

 raises the pressure but does not increase the amount of 

 urine. 



Another fact, which seems almost if not quite as invari- 



