296 



Elementary Biology. 



FlG. 158. TUBT OF CAPILt.ARIKS 

 IN A GLOMERULUS WITH RELATED 



BLOOD-VESSELS. (Quain.) 



and on the other with a small vein. The tuft and its cap- 

 sule together form a glomerulus. The space between the 

 walls is continuous with the tube of the nephridium. The 

 tube-wall is lined by secretory epithelium, which differs in 



character in different regions. 

 It pursues a very convoluted 

 course through the substance 

 of the kidney, and is in in- 

 timate relation throughout 

 with the renal capillaries. It 

 ultimately unites with other 

 tubules, and all finally open 

 into the ureter above men- 

 tioned. In passing through 

 the capillaries of the glome- 

 rulus, part of the water in the 

 blood is squeezed out into the 

 space between the two walls 

 of the sac, whence it trickles 

 down the tube, washing out 

 in its course the soluble ni- 

 trogenous waste abstracted 

 meanwhile by the secretory 

 cells from the renal capillaries 

 surrounding them. The water 

 and the nitrogenous and other 

 salts contained in solution in 

 it go by the name of urine, 

 the most important constitu- 

 ent of which is a complex 

 nitrogenous compound known 

 as urea. 



Urine does not escape 

 directly from the ureter to the exterior. It is collected in a 

 large bilobed and very extensible sac, the urinary bladder, 

 which opens on the ventral surface of the cloaca, just in 



a, artery ; af, afferent brand i ; /, 

 capi lary tuft; ef, efferent vein; b t 

 capillaries of the efferent vein. 



