EXCRETION. 487 



the capillaries surrounding the convoluted tubes, but the return of the 

 urea and similar substances is prevented by the secretory epithelium of 

 the tubules. The first theory is, however, more strongly supported by 

 direct experiment. 



By using the kidney of the newt, which has two distinct vascular 

 supplies, one from the renal artery to the glomeruli, and the other from 

 the renal-portal vein to the convoluted tubes, Nussbauin has shown that 

 certain substances, e.g., peptones and sugar, when injected into the 

 blood, are eliminated by the glomeruli, and so are not got rid of when 

 the renal arteries are tied; whereas certain other substances, e.g., urea, 

 when injected into the blood, are eliminated by the convoluted tubes, 

 even when the renal arteries have been tied. This evidence is very 

 direct that urea is excreted by the convoluted tubes, that is to say, if it 

 is certain that ligature of the renal artery assists the circulation through 

 the glomeruli, which, however, is denied by Adami. 



Heidenhain also has shown by experiment that if a substance (sodium 

 sulph-indigotate), which ordinarily produces blue urine, be injected 

 into the blood after section of the medulla which causes lowering of the 

 blood-pressure in the renal glomeruli, that when the kidney is examined, 

 the cells of the convoluted tubules (and of these alone) are stained with 

 the substance, which is also found in the lumen of the tubules. This 

 appears to show that under ordinary circumstances the pigment at any 

 rate is eliminated by the cells of the convoluted tubules, and that when 

 by diminishing the blood-pressure, the filtration of urine ceases, the 

 pigment remains in the convoluted tubes, and is not, as it is under 

 ordinary circumstances, swept away from them by the flushing of them 

 which ordinarily takes place with the watery part of urine derived from 

 the glomeruli. It therefore is probable that the cells, if they excrete 

 the pigment, excrete urea and other substances also. But urea acts 

 somewhat differently to the pigment, as when it is injected into the 

 blood of an animal in which the medulla has been divided, and the 

 secretion of urine stopped, a copious secretion of urine results, which 

 is not the case when the pigment is used instead under similar condi- 

 tions. The flow of urine, independent of the general blood-pressure, 

 might be supposed to be due to the action of the altered blood upon 

 some local vaso-motor mechanism; and, indeed, the local blood-pressure 

 is directly aifected in this way, but there is reason for believing that 

 part of the increase of the secretion is due to the direct stimulation of 

 the cells by the urea contained in the blood. 



To sum up, then, the relation of the two functions: (1.) The process 

 of filtration, by which the chief part, if not the whole, of the fluid is 

 eliminated, together with certain inorganic salts and possibly other 

 solids, is indirectly dependent upon blood-pressure, is accomplished by 

 the renal glomeruli, and is accompanied by a free discharge of solids 

 from the tubules. (2.) The process of secretion proper, by which urea 



