SECRETION AND EXCRETION 



331 



outlet into the urinary tubules which have, at first, a convo- 

 luted course covered with a thick layer of cells and collect into 

 larger and larger tubules (Fig. 

 51), which ultimately discharge 

 the urine into the pelvis of the 

 kidney. 



The phenomenon of urine secre- 

 tion consists in separating water 

 and crystalloids from a solution 

 containing colloids and crystal- 

 loids (blood), though, usually, 

 in a much higher concentration 

 and with a different proportion 

 of crystalloid constituents than 

 occurs in the blood. 



Without going into the vari- 

 ous theories of urine secretion, 

 we shall here sketch those which, 

 from physiological and patho- 

 logical investigations, seem most 

 probable. According to this the 

 glomeruli are & filtration apparatus 

 in which water and crystalloids 

 are filtered off while the colloids 

 are held back. From this ultra- 



Convolutecf 

 Tubule 



Vascular 

 Plexus 

 (Glomerulus) 



Bowman's 

 Capsule 



.-Vas defferens 



"Twig of Renal Artery 

 FIG. 50. Glomerular structure. 



FIG. 51. Diagram of urinary drainage 

 from Bowman's capsule. (From G. 

 Ludwig.) 



filtrate which contains the crystalloids in no higher concentration 

 than blood, later, possibly in the first portion of the urinary tubules, 



