10 



it is a well-known fact that the urine contains a much greater 

 amount of urea than does the blood and that it would be con- 

 trary to the laws of osmosis and diffusion for a fluid weak in 

 urea (the blood) to pass through a membrane a substance 

 which has accumulated in greater amount in a fluid (the 



urine) on the other side of 

 that membrane. It should also 

 be remembered that the blood 

 is constantly moving and pre- 

 sumably the fluid in the tu- 

 bules is doing the same. In 

 other words, it would seem 

 that if the conditions indicate 

 an osmometer, it varies from 

 experimental ones in that the 

 fluids on either side of the 

 membrane, are moving in a 

 definite direction. The pro- 

 teid constituents do not nor- 

 mally leave the blood on ac- 

 count of their well-known in- 

 disposition to osmosis. 

 One important fact, however, remains unaccounted for in 

 Ludwig's mechanical theory, and that is, if the uriniferous 

 tubules are stripped of their epithelial cells, as they often are 

 in disease, urea and some other nitrogenous products are no 

 longer, or only imperfectly eliminated, and become stored up 

 in the blood and produce the condition known as uremia, 

 although the conditions of an osmometer remain. It must, 

 therefore, be admitted that there is some direct or elaborating 

 action on the part of the epithelium as originally suggested 

 by Bowman, although under normal conditions, transudation, 

 diffusion and osmotic processes may occur coincidently. 



The theoretical conclusions of Bowman have been con- 

 firmed and extended by the practical researches of Heidenhain, 

 who injected indigo-carmine into the blood of animals and 

 found that it was promptly removed by the kidneys. These 

 organs were removed at suitable intervals after the injection 



Fig. 2. 

 1. Artery. 2. Afferent Vessel. 3. 

 Glomerulus. 4. Capsule of Bow- 

 man. 5. Efferent Vessel. 6. Capil- 

 lary network. 7. Uriniferous Tub- 

 ule. 8. Vein. 



