EXCRETION AND SEWAGE 



249 



presence is not noticed. Nearly a quart of water a day 

 thus passes off from the surface of the body in insensible 

 perspiration. In hot weather and during exercise so much 

 is produced that it accumulates in drops upon the skin. 



439. The kidneys. The main work of excretion is 

 performed by the kidneys, There are twe kidneys, one on 

 each side of the backbone, half 



covered by the two lower ribs. Each 

 kidney is bean-shaped, about four 

 inches in length, by two in breadth, 

 and one in thickness. It is com- 

 posed of millions of fine tubes made 

 up of epithelial cells; they unite, and 

 finally open into a pocket on the side 

 of the kidney. 



440. How the kidneys excrete. 

 The epithelial cells of the tubes have 

 the power to draw urea and mineral 

 substances from the blood. They 

 also extract a large amount of water 



in order to wash away the excreted matter. The excre- 

 tion runs down a tube called the ureter to the bladder. 

 About a quart and a half of a fluid called urine is thus 

 excreted daily. 



441. Kidney disease. Kidney disease usually takes the form 

 of an aggravated bilious attack. There are headaches, loss of appetite, 

 coated tongue, and great weakness. Usually the urine is diminished, 

 and contains some albumin derived from the blood. 



Urea itself is as harmless as carbonic acid gas and is as easily 

 excreted, but when oxidation is incomplete, substances are produced 

 which are as much more harmful than urea as a smoking lamp is more 

 unpleasant than one burning perfectly. When more food is eaten than 

 can be oxidized, poisons are developed from the imperfectly oxidized 

 albumin. Some are leucomaines or substances like them. The kid- 



Kidney cut across. 



