132 PRINCIPLES OF PHYSIOLOGY 



orm a filtration apparatus by which the 

 watery constituent of the urine, holding salts 

 and other matters in solution, was filtered 

 from the blood through the thin membrane 



nto the end of the tubule It would appear, 

 however, that the process is not one of simple 

 physical filtration, but that there is a selective 

 action due to the vital activity of the epithe- 

 lium. A minute vessel passes from the 

 glomerulus or ball of capillaries, and this 

 divides again into capillaries which ramify 

 on the first portion of the uriniferous tubule. 

 The epithelium in this portion is of a peculiar 

 kind, and it has been thought that it has to 

 do with the separation from the blood of nitro- 

 genous matters. There is still considerable 

 obscurity as to the precise mechanism by 

 which urine is formed. 



69. In a healthy man about fifty ounces are 

 excreted daily. Its colour is due to a mixture 

 of pigments, chiefly urochrome and a small 

 amount of urobilin. The reaction to test-paper 

 is acid, due chiefly to the presence of the acid 

 phosphate of soda. The origin and destination 

 of the nitrogenous constituents require special 



