1144 PHYSIOLOGY 



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

 hypotonic urine can be ascribed to a modification of the glomerular transu- 

 date as it passes through the tubules, a modification due partly to the ab- 

 sorption 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 there- 

 fore considerably less concentrated than normal urine. Ribbert succeeded in extir- 

 pating 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 certain 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 the urinary secretion, which could be 

 reinduced by injection of urea. He concluded that urea with water was 

 secreted by the tubules, whereas peptone, sugar, and haemoglobin were 

 turned out by the glomeruli. Beddard showed that these results of Nuss- 

 baum must have been due to the fact that he had not obstructed the whole 

 of the renal arteries. One or two of these small vessels will suffice to supply 

 blood to a considerable number of the glomeruli. After complete obstruc- 



