I shall take for granted that you have a fairly plain idea of the stomach and its dependences. Physiologies can always be had, and for minute details they must be referred to. Bear in mind one or two main points: that all food passes from the mouth to the stomach, an irregularly-shaped pouch or bag with an opening into the duodenum, and from thence into the larger intestine. From the mouth to the end of this intestine, the whole may be called the alimentary canal; a tube of varying size and some thirty-six feet in length. The mouth must be considered part of it, as it is in the mouth that digestion actually begins; all starchy foods depending upon the action of the saliva for genuine digestion, saliva having some strange power by which starch is converted into sugar. Swallowed whole, or placed directly in the stomach, such food passes through the body unchanged. Each division of the alimentary canal has its own distinct digestive juice, and I give them in the order in which they occur.
First, The saliva; secreted from the glands of the mouth:--alkaline, glairy, adhesive.
Second, The gastric juice; secreted in the inner or third lining of the stomach,--an acid, and powerful enough to dissolve all the fiber and albumen of flesh food.
Third, The pancreatic juice; secreted by the pancreas, which you know in animals as sweetbreads. This juice has a peculiar influence upon fats, which remain unchanged by saliva and gastric juice; and not until dissolved by pancreatic juice, and made into what chemists call an _emulsion_, can they be absorbed into the system.
Fourth, The bile; which no physiologist as yet thoroughly understands. We know its action, but hardly _why_ it acts. It is a necessity, however; for if by disease the supply be cut off, an animal emaciates and soon dies.
Fifth, The intestinal juice; which has some properties like saliva, and is the last product of the digestive forces.
A meal, then, in its passage downward is first diluted and increased in bulk by a watery fluid which prepares all the starchy portion for absorption. Then comes a still more profuse fluid, dissolving all the meaty part. Then the fat is attended to by the stream of pancreatic juice, and at the same time the bile pours upon it, doing its own work in its own mysterious way; and last of all, lest any process should have been imperfect, the long canal sends out a juice having some of the properties of all.