426 



Our Surroundings 



The chief work of excretion is carried on by the kidneys, 

 which take away water containing urea and other waste matter 

 in solution. 



The kidneys are two bean-shaped organs, each about the size 

 of an ordinary potato, situated in the lower part of the abdomen. 

 They are made up of innumerable small tubes. These tubes unite 

 into one larger opening on the side of each kidney, from which the 

 water is passed into the bladder and eliminated from the body. 



The lungs throw off car- 

 bon dioxide and water vapor 

 by osmosis in the process of 

 respiration. 



The skin contains glands 

 called sweat glands that ex- 

 crete perspiration or sweat, 

 consisting of water with waste 

 matter in solution. The liver 

 assists in excretion by throw- 

 ing off poisonous wastes with 

 the bile into the intestines. 

 The bile aids in digestion by 

 breaking up fats so enzymes 



A SECTION OF HUMAN SKIN may act on them. Undigested 



material is expelled from the 

 large intestine. 



The skin is an organ both of excretion and of protection 

 and its inner layer may serve as an organ of absorption, since 

 when the epidermis, or outer covering, is removed, it permits 

 drugs or poisonous substances to pass into the blood vessels. 



The skin has two layers, the dermis, or inner, and the epi- 

 dermis, or outer covering. The dermis is a tough, elastic net- 

 work of tissue which contains blood vessels and nerves. It is 

 covered with a thin layer of cells designed to protect it. This 

 outer layer which is called the epidermis, or the cuticle, con- 

 tains neither nerves nor capillaries. The nails and the hair 

 are developed from it. Corns and callouses also are growths of 

 the epidermis. 



Epidermis. 



Duct of a sweat gland. 



Hair follicle. 



