1586 PHYSIOLOGY 



The amazing adaptability of its functions to the needs of the organism 

 has been abundantly illustrated in the facts with which we have 

 dealt. Its ordinary activity is determined by the production, as a 

 result of the normal processes of metabolism, of soluble non- volatile 

 substances in every cell of the body. These substances, together 

 with the excess of water taken in with the food above that lost by 

 respiration and cutaneous transpiration, are turned out by the kidney 

 as urine. The activity of this organ must therefore be determined 

 in the first place by chemical stimuli. It must react to the slightest 

 deviation from normal of the blood composition by excreting 

 water or dissolved substances. This delicate sensibility is displayed 

 in two directions : 



(1) Under the influence of certain substances, such as urea, uric 

 acid, or water, the cells of the convoluted tubules 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 con- 

 voluted tubules, the tubules as a whole are 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 coincidently 

 with secretion 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 absorption 

 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 physio- 

 logical, 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 

 resorptive as well as a secretory function is determined by the presence 

 at the beginning of the tubule of a mechanism the glomerulus, devoid 

 of the fine selective power or chemical sensibility possessed by the cells 

 of the convoluted tubules. The production of urine by the glomerulus 

 is apparently 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 



