DIGESTION OF FOOD IN THE MOUTH 57 



The tongue is a long, flat muscle, attached at its back 

 end only, while its front part is capable of varied and pre- 

 cise movements in every direction. 



73. Mucous membrane. The cavity of the mouth is 

 everywhere lined with a thin membrane, directly continu- 

 ous with the outside skin. It consists of a loose network 

 of cells carrying blood tubes and nerves. It is covered 

 with a layer of flat cells, called epithelium. Into the loose 

 tissue beneath the epithelium, there project pockets or 

 tubes lined with cells directly continuous with the epi- 

 thelial cells of the sur- 

 face of the mouth. In 

 health, the cells of each 

 of the tubes and of the 

 surface of the mouth pro- 

 duce just enough of a 



thin, clear liquid, called 



Mucous membrane (X 200). 



mucus, to moisten and a cells and loosely woyen fibers forming the 



lubricate the Surface Of main part of the membrane. 



the mouth. This mem- 



brane is called a mucous 



membrane. It is continued into the stomach and intestine, 



and into the windpipe and lungs. 



Mucous membrane is modified skin turned in from the surface of the 

 body to line the interior of all the cavities which communicate directly 

 with the air. Every such surface is covered by an unbroken layer of 

 epithelial cells. Wherever the epithelial cells are absent, the spot is 

 raw and sore. The epithelial cells of the surface of the mucous mem- 

 brane are designed mainly for protection ; but those which reach into 

 the tubes are set apart for the special work of producing mucus from 

 material supplied by the blood. 



74. Gland and secretion. A collection of pockets or 

 tubes lined with epithelium which forms a substance out 



