1198 PHYSIOLOGY 



or water, the cells of the convoluted tubules may take up the substance, 

 which is in excess, from the surrounding lymph and accumulate it in vacuoles, 

 which are discharged on the inner surface of the cells into the lumen of the 

 tubules. 



(2) Besides this specific secretory activity of the cells of the convoluted 

 tubules, the tubules as a whole are certainly endowed with the power of 

 absorbing both water and dissolved substances from the fluid in their lumen. 

 Whether this absorptive power is limited to the cells of Henle's loop, as 

 was first suggested by Ludwig, or occurs also in the cells of the convoluted 

 tubules, as might be imagined from the close analogy between the structure 

 of these cells and that of the intestinal epithelium, we have not sufficient 

 evidence to decide. We do know however that the quality of the absorp- 

 tion is strictly regulated according to the needs of, the organism, so that the 

 constituents which are precious are reabsorbed for service in the body, while 

 those which are in excess or are of no value to /the organism are allowed to 

 pass out into the ureters. The process of resorption is indeed, as is shown 

 by Cushny's experiments, largely dependent on the physical qualities of 

 the substances undergoing absorption, and especially on the permeability 

 of the renal cells to these substances. The physical conditions are however 

 subordinated to the physiological, so that a salt so diffusible as potassium 

 iodide is left in the fluid, while sodium chloride may be reabsorbed in large 

 quantities. 



The necessity for the endowment of the tubular epithelium with a resorp- 

 tive function as well as any secretory function it may possess is determined 

 by the presence at the beginning of the tubule of a mechanism the glo- 

 merulus, devoid of the fine selective power or chemical sensibility which 

 characterises the cells of the convoluted tubules. The production of urine by 

 the glomerulus is regulated entirely by the pressure and velocity of the blood 

 through its capillaries and by the colloid content of the blood plasma. We 

 may assume that in Bowman's capsule there Is under normal conditions a 

 constant production of a fluid, free from protein but having the same 

 crystalloid concentration as the blood plasma. With any rise of general 

 blood pressure the amount of this transudate is increased ; with any fall it is 

 diminished. The small qualitative changes, which are constantly occurring 

 in the blood as the result of the taking of food or the activity of different 

 organs, probably produce but little effect on the amount of glomerular fluid. 

 Only indirectly, as the result of these events on the general blood pressure, 

 or possibly in consequence of the production of substances having a vaso- 

 dilator effect on the renal vessels, will the amount of the urine turned out 

 by the glomeruli be affected. These structures therefore have the twofold 

 function of regulating the total amount of circulating fluid and of providing 

 an indifferent fluid which will, so to speak, flush the kidney tubules and 

 carry down any constituents excreted in a concentrated form by the cells 

 of these tubules. The constant production of a glomerular transudate 

 might result, especially in terrestrial animals, in the loss to the organism of 

 water or, under certain nutritive conditions, of substances indispensable 



