EXCKETION 181 



blood and cause death. The lungs are essential to the re- 

 moval of this waste; the skin is not. Similarly, the perspi- 

 ration contains small amounts of urea and other wastes 

 which are removed in large quantities by the kidneys. It is 

 not necessary that the skin should remove any of these, for 

 the healthy kidney can and does, when necessary, remove 

 them. Small quantities of them appear in the perspiration 

 because they are in the blood from which the perspiration 

 is formed and because the cells of the sweat glands allow 

 them to pass through, just as the skin allows the passage 

 of carbon dioxide. . 



These considerations are of practical importance in the 

 hygiene of the skin. It is not necessary to induce perspi- 

 ration merely to remove waste products from the body. If 

 the human skin, like that of the cat and the dog, contained 

 no sweat glands, the waste products would be thoroughly 

 removed; and in cold weather, when no perspiration is 

 secreted, the excretion of waste is as complete as when in 

 warm weather perspiration is abundantly secreted. On the 

 other hand, perspiration, though not secreted to rid the body 

 of wastes, nevertheless contains wastes which accumulate 

 upon the skin. Hence the need of bathing, both as a matter 

 of health and of decency. 



The chief wastes leaving the body and their main chan- 

 nels of excretion are given in the following table, incidental 

 excretions being given in italics: 



Lungs : carbon dioxide, water. 



Kidneys : urea, uric acid, and other compounds, salts, water. 

 Intestine : bile pigments, nitrogenous compounds, etc. 

 Skin : urea, etc., salts, water. 



The structure and action of the lungs and intestine have 

 already been described, so that we have left for study the 

 kidneys and the skin. 



3. Structure of the kidneys. Each kidney is a bean-shaped 

 gland whose duct, the ureter, runs to the urinary bladder. As 



