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.
Some of the foodstuffs which we eat are already in solution and ready to soak at once into the lymphatics and blood-vessels of the alimentary canal; others, such as a lump of sugar, though not dissolved when put into the mouth, are readily soluble in the liquids found in the alimentary canal and need no further digestion. In the case of many most important foodstuffs, however, special chemical changes have to be brought about to make them soluble and capable of absorption. The different secretions poured into the alimentary tube act in various ways upon different foodstuffs, simply dissolving some and chemically changing others, until at last all are got into a condition in which they can be taken up into the lymph and blood-vessels for transference to distant parts of the body.