THE SECRETION OF URINE 701 



a manometer was connected with an obstructed bile-duct. In this 

 case it is generally allowed that the stage of equilibrium marks 

 the point at which secretion and absorption exactly balance each 

 other. He suggested that the same is true for the kidney. Accord- 

 ing to Heidenhain, the manometer does not register the nitration 

 pressure of the kidney, but the pressure at which a wholly artificial 

 and abnormal reabsorption balances secretion. And considering 

 that Ludwig's own view supposes constant absorption by the 

 tubules, it would seem that Heidenhain's explanation at any rate 

 might be possible. 



It is an assumption to conclude, as Ludwig did, that, when 

 the ureter is distended with fluid under pressure, the same fluid 

 pressure must exist right up the tubule to the glomerulus. It has 

 been found impossible to inject fluid backwards from the ureter 

 up the tubules, and according to Ludwig's view the glomerular 

 nitrate has under normal circumstances two alternative routes 

 of escape. The bulk of it passes back into the blood, and only a 

 small fraction passes down the tubule into the ureter. We should 

 therefore expect that obstruction of the ureter would not dam 

 back fluid under pressure in the tubule, but would determine the 

 escape of a greater proportion into the blood, as long as that 

 route was open. Obstruction of the ureter might close this route 

 into two ways either by producing changes in the absorbing 

 epithelium, or by the accumulation of urea and other crystalloids 

 in the tubules until their osmotic pressure was equal to the absorb- 

 ing power of the epithelium. For, according to Ludwig's view, 

 urea and other specific urinary constituents are not reabsorbed 

 after being filtered off from the blood. An examination of the 

 urine which is found in the distended ureter and pelvis does not 

 throw any light on this question. It is of low specific gravity and 

 contains but little urea ; a result which may be explained equally 

 well by actual reabsorption or failure of excretion under the 

 abnormal conditions. That obstruction of the ureter may alter 

 the properties of the renal epithelium seems to be suggested by 

 the character of the urine secreted after a temporary obstruction 

 of the ureter. This urine is very copious, has a molecular con- 

 centration much below normal, and the reduction affects the 

 inorganic constituents far more than the urea. But it is impos- 

 sible to say whether this result indicates increased absorption of 

 certain constituents or their decreased excretion. 



