KIDNEY AND SKIN AS EXCRETORY ORGANS. 843 



and block the uriniferous tubules. According to the antagonistic 

 theory of Heidenhain, blocking the veins suppresses the secretory 

 activity of the glomerular epithelium by depriving it of oxygen and 

 the chance for removal of CO 2 , that is, by producing local as- 

 phyxia. The latter explanation seems the simpler of the two, and it 

 is very strongly supported by the opposite experiment of clamping 

 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- 

 damped, 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 a contrary result. 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 volume of blood which passed by the cells in a 

 given time and thereby would vary the quantity of gxygen sup- 

 plied and of carbon dioxid removed, and also the quantity of chem- 

 ical substances in the blood which may act as chemical stimuli to 

 the cells. An important fact, which seems at first sight to show a 

 direct influence of pressure, is that when general arterial pressure 

 falls below a certain point, about 40 mm. 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 cervical or thoracic region. But here again the great vascular 

 dilation 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 



