THE SECRETION OF URINE 1279 



apparently determined entirely by the circulation through the kidney 

 and having a concentration not inferior to that of the blood. The 

 passage of hypotonic urine can be ascribed to a modification of the 

 glomerular transudate as it passes through the tubules, a modifica- 

 tion due partly to the absorption of salts from the fluid, partly, perhaps 

 chiefly, to a secretion of water or extremely dilute salt solution by the 

 cells of the tubules themselves. 



Certain other observations accord with our hypothesis that in Bowman's 

 capsule a fluid is transuded having the same molecular concentration as blood- 

 plasma, and therefore considerably less concentrated than normal urine. 

 Ribbert succeeded in extirpating the whole of the medullary portion of the kidney 

 in the rabbit, leaving the cortex intact, and found in this case that during the 

 survival of the animal the urine passed was much more dilute than normal. 

 In cases where there is destruction of the tubular epithelium, while the glomeruli 

 remain intact, either in consequence of disease or, as in Galeotti's experiments, 

 as a result of poisons, we are accustomed to obtain a dilute copious urine ; and 

 the continual passage of such urine is in man regarded as a sign of one form of 

 renal disease. 



The experimental facts which we have passed in review do not 

 therefore negative the view that the glomerular epithelium plays the 

 part of a passive filter in the formation of urine, and that the energy 

 of the process by which ' urine ' is produced in Bowman's capsule is 

 entirely furnished by the heart in driving the blood at a high pressure 

 through the glomerular capillaries. 



FUNCTIONS OF THE RENAL TUBULES 



Whatever the nature of the glomerular activity, it is evident that 

 the multiform epithelium of the tubules may alter the glomerular 

 transudate, either by the absorption or by the secretion of water or 

 solid constituents. We may deal with the evidence for the occurrence 

 of these two processes separately. 



SECRETION BY THE URINARY TUBULES. Although it is 

 impossible to collect the secretion of the glomeruli apart from 

 that of the tubules, the arrangement of the blood-vessels in cer- 

 tain animals enables us to influence separately the circulation to 

 these two parts of the kidney. The amphibian kidney receives a 

 blood-supply from two sources. A number of renal arteries leaving 

 the aorta enter the kidney and supply the whole of the glomeruli, 

 the vasa efferentia from which pass, as in the mammalian kidney, into 

 the intertubular capillaries. These are also supplied with blood of 

 venous character by the renal portal vein. If all the renal arteries be 

 divided or ligatured, the glomeruli, as was shown by Nussbaum, are 

 entirely cut out of the circulation, though the tubules continue to 

 receive venous blood through the renal portal vein. Nussbaum 

 stated that ligature of all the renal arteries caused cessation of 



