URINARY ORGANS 



69 



cavity, the Malphigian body within which is a tangled network 

 of finest arterioles, the glomerulus. The little sac narrows at 

 the open end, and from this narrowed neck continue the tubule. 

 Each tubule makes certain convolutions, descends to the medul- 

 lary portion, and thence returns to the cortical, where it makes 



other convolutions ; fi- 



RA. 



I 



L. KIDNEY 



R. KIDNEY 



URETER 



URETER 



nally terminating in a 

 larger duct through 

 which the urine escapes 

 to the pelvis of the kid- 

 ney and then into the 

 ureter. Each tubule is 

 thus very long in pro- 

 portion to its size, and 

 there are a large number 

 of them. 



Function of the kid- 

 neys. — The kidneys have 

 four functions: (a) to 

 remove useless or waste 

 and poisonous materials 

 from the blood; (&) to 

 remove excess of normal 

 substances from the 

 blood; (c) to keep the 

 blood faintly alkaline 

 by removing or adding 

 alkaline materials as 

 needed; (dS) to main- 

 tain normal volume and 

 concentration of the 

 blood. 



How accomplished. — 

 The water and salts in 



solution are removed from the blood within the little sacs at the 

 beginnings of the tubules. This probably is done by a process of 

 filtration under pressure. The urea, albumin, etc., are dis- 

 charged farther on into the water of the urine tubules. These 

 are taken from the blood in a process of secretion by the epi- 

 thelial cells which line the tubules. 



Urine.-Vrine is composed of water, salts of sodium, potas- 



Fig 32. — Urinary Apparatus in Diagram. 

 (M. B. B.) 



P, A, posterior aorta; D, D, renal arteries; 

 P, peritoneal coat of bladder; Mus., muscu- 

 lar coat; Muc, mucous coat. 



