1192 PHYSIOLOGY 



can be ascribed to a modification of the glomerular transudate as it passes 

 through the tubules, a modification which may be due either to the absorption 

 of salts from the fluid, or to a secretion of water or extremely dilute salt 

 solution by the cells of the tubules themselves. Possibly both processes are 

 involved . 



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 hi extir- 

 pating the whole of the medullary portion of the kidney hi 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, while the glomeruli remain intact, 

 there is destruction of the tubular epithelium 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. 



It is important however to remember that, however passive it may be 

 in the formation of urine, the filtering membrane is composed of living cells, 

 which may alter and lose their powers of filtration or their powers of retaining 

 the colloid constituents of the blood plasma under any influences which 

 impair their vitality. Thus obstruction of the renal artery for half a minute 

 may suppress the formation of urine in the kidneys for half to several hours, 

 and the urine, when again formed, is found to contain coagulable protein 

 (' albumin ') which can be shown to have transuded through the glomerular 

 epithelium. The filtering properties of the membrane may be impaired to 

 a lesser degree by slowing the circulation of the blood through the kidneys. 

 In the venous congestion of heart disease, the presence of albumin in the 

 urine is of frequent occurrence. The same effect on the permeability of the 

 epithelium may be produced by many kinds of poisons, mineral or microbial, 

 circulating in the blood. 



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 RENAL 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 



