RESPIRATION AND EXCRETION 



79 



puscles and colloidal particles do not pass through the walls of the capil- 

 laries, but the filtrate is little different from the plasma of the blood. 

 This fluid, in the course of its long tortuous passage through the tubule, 

 returns the greater portion of its water and nearly all its nonwaste mate- 

 rials to the blood stream by way of the walls of the tubule and their 

 surrounding network of capillaries. The reduced amount of fluid that 



uriniferous tubule 



arteriole 



glomerulus— - — ^^£J&#$-j, 



Bowman's 

 capsule 



arteriole -~ycy)-J 



]narieS ^jf^ff^t 



cortex 



an 

 s*3»— undissected 

 Q^t» duct 



medulla 

 [pyramids) 



collecting 

 tubule 



renal pelvis 



renal artery 

 nd vein 



ureter 



Fig. 5.3. Longitudinal section through a kidney, with enlarged detail of malpighian corpus- 

 cle and uriniferous tubule, and (in silhouette) the relation of the corpuscle and uriniferous 

 tubule to a collecting tubule. 



reaches the lower portion of the tubule contains the dissolved nitrogenous 

 (and other) wastes and is known as urine. 



The minute quantity of urine formed in each tubule passes into a 

 larger collecting duct (each duct receives the discharge of many tubules) ; 

 and the thousands of ducts empty into the pelvis of the kidney and thence 

 into the ureter, through which the urine is propelled by muscular con- 

 tractions into the bladder. The latter is a hollow muscular organ located 

 in the lower, ventral part of the abdominal cavity. A tube, the urethra, 

 leads from the lower, funnel-like end of the bladder to an opening on the 

 outside of the body. The juncture of the bladder and urethra is closed 

 by a muscular valve, so that the urine that continuously flows into the 



