EXCRETION. 429 



tubules; in other words, that the secretion of urine is physiologic 

 rather than physical. Heidenhain has brought forward a series of 

 facts which support this view. As evidence that the cells possess a 

 selective power, he presents the following experiment : The spinal cord 

 of an animal is divided in the neck for the purpose of lowering the 

 blood-pressure in the kidney below the pressure at which the urine is 

 secreted; a solution of indigo-carmine is injected into the blood- 

 vessels ; after the lapse of ten minutes the animal is killed, the blood- 

 vessels washed out with alcohol for the purpose of precipitating the 

 indigo-carmine in situ. Section of the kidney shows a uniform blue 

 stain of the cortex alone. Microscopic examination reveals the fact 

 that the blue stain is due to the deposition of the pigment in the lumen 

 and in the lumen border of the cells of the convoluted tubules and 

 the ascending limb of Henle's loop; while the epithelium of Bow- 

 man's capsule as well as the glomerular epithelium present no evi- 

 dence of pigmentation. 



Nussbaum attempted to establish the secretory power of the epi- 

 thelium in another way. In the frog the kidney receives blood from 

 two sources: the glomeruli receive their blood from the renal artery, 

 the tubules from the capillaries formed by the anastomosis of branches 

 of the efferent vessel of the glomerulus and the branches of the renal 

 portal vein. Nussbaum believed that by ligating the renal artery 

 all glomerular activity could be abolished and the part played by the 

 epithelium could be established. After so doing the flow of urine was 

 at once checked; the injection of urea at once reestablished it. This 

 fact was taken as a proof that the tubular epithelium not only ex- 

 creted urea, but water and perhaps other constituents as well. It 

 was also found that sugar, peptones, carmine, etc., which are always 

 eliminated from the blood under normal conditions, are not removed 

 after ligation of the renal artery. It was concluded from these ex- 

 periments that the secreting structures of the kidney consist of two 

 distinct systems, the glomerular and the tyimlar; the former secreting 

 water, salts, sugar, peptone, { ' etc. ; the latter^jorea^ric acid, etc. 

 These and similar facts indicate that the renal epithelium possesses 

 a secretory rather than an absorptive 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 conse- 

 quence of cell selection and cell activity; that the entire process is one of 

 secretion, though conditioned by blood-pressure, blood velocity, etc. 



Influence of Blood-pressure. Whether the elimination of the 

 urinary constituents is entirely secretory (physiologic) in character 

 or not there can be no doubt that the whole process is largely deter- 

 mined by the pressure and velocity of the blood in the glomerular 

 capillaries, or, to state it more accurately, on the difference of pres- 

 sure between the blood in the capillaries and the urine in the capsules. 



