350 THE HUMAN BODY AND ITS FOOD 



season food. Iron compounds are needed to form a part of the blood; 

 we get them chiefly from vegetable food. Limestone and calcium 

 phosphate are needed by the bones (cf. 352). We get them in our 

 food and in hard water (cf. 132). 



Water is needed in large amounts by the body. We realize this 

 when we remember that about 70% of the body is water. We get a 

 great deal of water even in our ''solid" food (cf. 79). 



Oxygen is not usually thought of as a food, because it does not 

 enter the body by the digestive organs; but the body must have an 

 abundant supply of it all the time to remain alive. 



358. Organs of Digestion; Glands. The tube through 

 which food is passed for digestion is called the alimentary 

 canal, or digestive tract. It consists of the mouth, 

 pharynx (throat), esophagus (gullet), stomach, and the 

 small and large intestines. Altogether it is about 30 feet 

 long. 



In order to digest food, liquids are poured upon it in 

 the mouth, the stomach, and the intestines. The liquids 

 are taken out of the blood for this purpose by organs 

 called glands. The origin of glands is as follows : 



The alimentary canal, as well as the windpipe and 

 lungs, is lined with mucous membrane. This begins at 

 the lips, and is a continuation of the skin which covers the 

 outside of the body. It is pink from the network of blood 

 vessels beneath it, and produces a thin, clear liquid called 

 mucus. From this liquid it gets its name. The surface 

 of the mucous membrane is made up of a layer of flat cells, 

 called epithelial cells. Now, glands are made by the 

 folding in of epithelial cells in tubes or pockets (Fig. 278). 

 Blood vessels and vessels containing lymph, a substance 

 much like blood (cf. 379), are all about the glands; hence 

 it is only necessary for the epithelial cells to take what 



