KIDNEY AND SKIN AS EXCRETORY ORGANS. 745 



the renal artery. When this is done the blood-flow through the 

 kidney ceases and the secretion of urine also stops, as would be 

 expected. But when after a few minutes' closure the artery is un- 

 clamped, the secretion is not restored with the return of the cir- 

 culation. On the contrary, a long time (as much as an hour or more) 

 may elapse before the secretion begins. This fact is quite in harmony 

 with the Heidenhain theory, since complete removal of their blood 

 supply might well result in a long-continued injury to the delicate 

 epithelial cells. On the mechanical theory, however, we should 

 expect the contrary. Injury to the cells should be followed by 

 greater permeability and an increased filtration, as is found to be 

 the case with the production of lymph. These two experiments, 

 blocking the renal artery and the renal vein, seem at present to dis- 

 credit the filtration theory and to support the secretion theory. 

 If we accept this latter theory it may be asked how it agrees with 

 the experiments mentioned above upon the variations in capillary 

 pressure brought about otherwise than by obstructing the venous 

 outflow. Heidenhain has emphasized the fact that all of these ex- 

 periments involve not only a variation in capillary pressure, but also 

 in the blood-flow, and that it is open to us to suppose that the 

 effect upon the secretion of urine is dependent upon the rate of flow 

 rather than upon the capillary pressure. If we adopt this expla- 

 nation we are led again to the secretion hypothesis. Mere rate of 

 flow should not influence filtration, but might affect secretion, since 

 it would alter the composition of the blood flowing through the 

 glomeruli and also the amount of oxygen and carbon dioxid. An 

 important fact, which seems at first sight to show a direct influence 

 of pressure, is that when general arterial pressure falls below a cer- 

 tain point, about 40 mms. of mercury, the secretion of urine ceases 

 altogether. Such a condition may be brought about by surgical 

 shock, by hemorrhage, or by section of the spinal cord in the cer- 

 vical or thoracic region. But here again the great vascular dila- 

 tation causing this fall of pressure is associated with a feeble cir- 

 culation, and the effect upon the kidney secretion may well be due 

 to this latter factor. 



In addition to varying the factor P in the formula given above, 

 it is possible also to increase the factor p. Normally the pressure 

 of the urine in the capsule must be very low, owing to the fact that 

 the secretion drains away as rapidly as it is formed. If the ureter 

 is occluded, however, the pressure of the urine will increase, and the 

 filtration pressure P p will diminish. When this experiment is 

 performed and the pressure in the ureter is measured by a manom- 

 eter, it is found to rise to 50 or 60 mms. of mercury and then to 

 remain stationary. This fact might be explained by supposing 

 that when p=P the secretion stops on account of the failure of 



