vin THE EXCRETION OF URINE 445 



the blood pressure and rate of circulation, which produces increased 

 filtration through the walls of the uriniferous tubules. According 

 to these authors the water of the urine is eliminated not exclusively 

 from] the glomeruli, but from the tubules also (at least in some 

 part of the latter, e.g. in Henle's loops), an analogy with what 

 occurs in the sweat glands, where (as we shall see in the next 

 chapter) there is an abundant secretion of water without any 

 vascular apparatus similar to that of the renal glomeruli. 



De Bonis (1906), a pupil of Galeotti, who partially repeated the 

 experiments of his master, and made use of Bottazzi's method for 

 eliminating the activity of the tubular apparatus, arrived at the 

 following results : 



The diuresis observed in dogs immediately after intravenous 

 injection of hypertonic solutions depends on the glomeruli, since 

 it takes place in kidneys in which the tubules had been injured. 

 On the other hand, this diuresis is probably in close relation with 

 the rate of circulation and pressure of the blood within the 

 glomerulus. 



In the normal kidney the first period of diuresis, with low 

 concentration of urine, is followed by intense osmotic activity on 

 the part of the tubular epithelium, during which numerous saline 

 molecules are admitted into the glomerular filtrate. A rapid 

 increase in the molecular concentration of the urine then appears, 

 while the amount of urine simultaneously diminishes. In the 

 injured kidney, on the contrary, where the damaged epithelium is no 

 longer capable of performing so much osmotic work, the molecular 

 concentration of the urine is always low, and differs little from 

 that of the blood. 



Hence we may conclude that the glomerulus also represents 

 an organ which regulates the osmotic pressure of the blood, since 

 while it normally passes a fluid which is hypotonic in comparison 

 with the blood, a fluid almost isotonic with the latter filters 

 through whenever the blood contains an excessive quantity of 

 osmotically active substances, i.e. each time the body feels the 

 need of clearing out these substances so as to bring the osmotic 

 pressure of the blood back to its normal value. 



Accordingly we must regard the membrane which lines the 

 glomerulus not as a filtering membrane, the permeability of which 

 is invariable, but as a membrane that is variously permeable, 

 according to the needs of the body as indeed must be admitted 

 for all living membranes. 



Lastly it has been attempted to solve the fundamental problem 

 of the function of the two renal systems by artificial circulation 

 through the excised kidney, but without success, since it has so 

 far proved impossible to keep the isolated kidney sufficiently alive 

 for it to yield a secreted fluid similar to that which it excretes 

 normally. Pfaff and Vejnx Tyrode (1903) showed that defibrinated 



