KIDNHY AND SKIN AS EXCRETORY ORGANS. 765 



do not suffice for a satisfactory explanation of the facts.* As in 

 other similar cases, our knowledge of the physical structure and 

 chemical properties of the walls of the living cells is still very de- 

 ficient, and it seems necessary to designate these activities by the 

 indefinite term secretion. 



Function of the Glomerulus. As stated above, the structure 

 of the glomerulus is peculiar and suggestive of a special adaptation. 

 Ludwig's theory looks upon it as a filter, the pressure of the blood 

 in the glomerular capillaries driving the water and salts through 

 the endothelium of the capillaries and the glomerular epithe- 

 lium into the cavity of the urinary tubule. If we consider 

 only the water and assume that the membranes traversed are freely 

 permeable to its molecules, then it is evident that, upon this theory, 

 the quantity of urine formed will depend upon the filtration pres- 

 sure, and that this filtration pressure can be expressed by the formula 

 F=P p, in which P represents the blood-pressure in the glom- 

 erular capillaries and p the pressure of the urine in the capsular 

 end of the uriniferous tubules. Some of the interesting facts de- 

 veloped by experiment may be presented in connection with this 

 formula. According to the mechanical theory, the amount of urine 

 formed should vary directly with P and inversely with p. The 

 factor P may be increased in two general ways: First, by those 

 changes which raise general arterial pressure and therefore the 

 pressure in the renal arteries, such changes, for instance, as are 

 brought about by an increased force of heart beat or a large vaso- 

 constriction. Second, by obstructing or occluding the renal veins. 

 Experiments have been made along these lines. With regard to 

 the first possibility it has been found in general, although not invar- 

 iably, that raising arterial pressure increases the quantity of urine 

 if the means used are such as may be assumed to raise the pressure 

 in the glomerular capillaries. 



The reverse experiment, however, of raising P by blocking the 

 venous outflow fails entirely to support the theory. When the renal 

 veins are compressed the capillary pressure in the glomemli must 

 be increased, and, if the veins are blocked entirely, we may suppose 

 that the capillary pressure is raised to the level of that of the renal 

 arteries. In such experiments, however, the flow of urine is di- 

 minished instead of being increased, and indeed may be stopped 

 altogether when the veins are completely blocked. The adherents of 

 the Ludwig theory have attempted to explain this unfavorable 

 result by assuming that the swollen interlobular veins press upon 

 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 



* For discussion and literature see Magnus, "Miinchener med. Wochen- 

 schrift," 1906, Nos. 28 and 29. 



