URINARY ORGANS AND MAMMARY GLANDS. 55 



in which is a tangled network of finest arterioles. The little 

 sac narrows at the open end and from this narrowed neck con- 

 tinue the tubules. Each tubule makes certain convolutions, then 

 descends to the medullar)' portion, thence back to the cortical 

 where it makes certain other convolutions and finally terminates 

 in a larger duct through which the urine escapes to the pelvis 

 of the kidney and then into the ureter. Each tubule is thus 

 very long in proportion to its size and there are a large number 

 of them. 



Function of the kidneys: (a) Removal of useless or waste 

 and poisonous materials from the blood ; ( b ) removal of normal 

 substances from the blood when they are there in excess ; ( c ) 

 to keep the blood normally alkaline by removing from or adding 

 alkaline materials to the blood as needed. 



Hozv accomplished. — The water and salts in solution are re- 

 moved from the blood within the little sacs at the beginnings of 

 the tubules by a process of filtration under pressure, the pressure- 

 on the blood, inside the fine arterioles within the urine tubule 

 sac, being greater than the pressure on the water in the sac 

 around the arterioles. The urea, albumen, etc., are passed out 

 into the water farther along the tubule. These are taken from 

 the blood by the cells which line the tubules. 



Composition. — Urine is composed of water, salts of sodium, 

 potassium, calcium, etc., together with various organic matters 

 as urea and nric acid and various aromatic substances which 

 give the odor. Normal reaction in herbivorous animals is alka- 

 line. Specific gravity for horse is 1036, and for cow 1025. 

 Either may vary within the normal range. Horse excretes about 

 10 pints in 24 hours, and the cow about 25 pints. Within limits 

 the more nitrogen in feed of cow the greater amount of urine 

 is excreted. Urine is excreted by the kidneys continuously and 

 runs drop by drop into the bladder. 



Ureters. — These are two slender tubes which conduct urine 

 from kidneys to bladder. They arc about the size of a goose 

 quill, about one foot long and terminate at the bladder passing 

 for an inch along between the muscular and mucous coats of 

 that organ, before reaching the interior. This is to prevent back- 

 ward flow of urine into ureters. 



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