This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
The alimentary 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 constructed to absorb dissolved nutritive substances; it soaks them up and passes them into blood or lymph vessels. Imbedded 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 alimentary mucous membrane, is spoken of as its digestion.
What is the alimentary canal? By what is it lined? 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 it 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?
Glands are hollow organs which make or secrete peculiar fluids and pour them out on some free surface of the body. They are very widely distributed; we find, for example, digestive glands (of several kinds) opening into the digestive tube, perspiratory glands opening in the skin, tear glands or lachrymal glands pouring out their secretion on the eyeball. Different glands have their cavities lined by different kinds of cells, and produce different secretions. In general arrangement all glands are built on one or other of two primary structural plans, known as the tubular and the racemose.