CHAPTER XL 

 THE DIGESTIVE ORGANS. 



General Arrangement of the Alimentary Canal. The ali- 

 mentary canal is a tube which runs through the body from 

 the lips to the posterior end of the trunk. It is lined by a 

 soft reddish mucous membrane (easily seen inside the mouth), 

 which is but a redder and moister sort of skin. 'Outside 

 the mucous membrane are connective tissue and muscular 

 layers, which strengthen the digestive tube and push the 

 swallowed food along it. The mucous membrane is con- 

 structed to absorb dissolved nutritive substances; it soaks 

 them up and passes them into blood or lymph vessels. Im- 

 bedded in this mucous membrane, or lying outside it, are 

 hollow organs called glands; these glands make liquids 

 which alter chemically many substances which we eat, and 

 turn them from things which cannot be absorbed by the 

 mucous membrane into things which can. The whole series 

 of changes which any food material undergoes, between its 

 reception by the mouth and its absorption by the aliment- 

 ary mucous membrane, is spoken of as its digestion. 



Various foodstuffs undergo different kinds of changes 



"What is the alimentary canal? By what is it liiied? What are 

 found outside the mucous membrane of the digestive tube? What 

 are their uses? With reference to what object is the alimentary 

 mucous membrane constructed? What does iit do with the nutriment 

 it absorbs? What is the function of the glands of the alimentary 

 canal? What is meant by the digestion of a foodstuff? 



[128] 



