When the mucous surface of the membrane is irritated by improper food, the glands become over-excited and pour out a large quantity of fluid, and diarrhoea may result. If the irritation be continued, the glands become exhausted and unable to act, and dryness, causing constipation, results. In the absence of all irritating food, less juice is secreted and constipation follows. In selecting a dietary, enough waste or indigestible matter needs be provided, to insure a moderate flow of intestinal juice, otherwise the bowels will not be kept open.

One who understands the process of digestion thoroughly is often able, when occasion requires, to regulate his diet to nicety. For instance, in cases of hyper-acidity of the stomach, if this be caused by over-activity of the gastric glands, the larger use of fats, in some form, which tend to depress such secretion, would be indicated. As a rule, the ingestion of a food principle incites the flow of the fluids that digest it; and, if too much of any fluid be secreted, other principles might be substituted, until such time as the digestive equilibrium can be maintained.

We have said that bread and tea were not a good dietetic combination. Butter, however, is an aid to the digestion of bread. Fats are digested in the intestine, where they stimulate the secretion of the pancreatic juice. Here also may be digested whatever starch - bread is largely starch - has escaped the ptyalin of the saliva; and the presence of the fat (butter) insures sufficient activity of the pancreas to digest the starch.

Starch and fat in the form of pastry is quite another matter, though even here thorough mastication is a precautionary measure. In pastry the starch granulas are enveloped in fat, which is not affected by the ptyalin. The fat keeps the ptyalin from the starch and the latter is not changed in any manner, until it comes into contact with the amylopsin of the pancreatic juice. At this point the fat, by stimulating this secretion, aids in the digestion of the pastry as a whole.