The proper care of the mouth in relation to diet is an important subject which is frequently overlooked. In patients who are unable, from weakness or the prostration of fever, to use solid food or to cleanse the mouth themselves, lack of movement in the tongue and buccal muscles prevents proper cleansing of the teeth, and particles of food accumulate about the gums. In extreme weakness of the facial muscles the jaw drops and mouth breathing results. The air passing through the mouth evaporates the moisture present and gives rise to dryness of the tongue, which may become so extreme as to interfere with deglutition and articulation.

In such cases the mouth should be frequently moistened by the nurse and swabbed out by some antiseptic, such as a saturated solution of boric acid or diluted Listerine. This topic will be more fully treated under the heading Dietetic Treatment of Typhoid Fever.

Food which is allowed to collect in the cavities of decayed teeth favours the development of bacteria, which on being swallowed may become a cause of dyspepsia by exciting malfermentation in the stomach, especially of milk. Much bacterial filth accumulates beneath artificial teeth which are not frequently cleansed.

Imperfect or painful teeth, or swollen gums, interfere with the proper mastication of food, and when such conditions are present care should be exercised that all food eaten be soft and bland. This applies particularly to young children, very old persons, and the insane, who are incompetent to take care of their own mouths.

The digestion of non-nitrogenous food begins in the mouth, and depends upon the activity of the salivary ferment ptyalin, which converts starches into dextrin and maltose. In health this action is prompt and vigorous, and much of the starchy food is digested in the mouth, in its transit along the oesophagus, and also in the stomach, until the gastric juice becomes so acid as to check the process, for ptyalin is most active upon the alkaline side of the neutral point, and strong acidity inhibits its power. Chittenden has demonstrated that after neutralising saliva, ptyalin fermentation proceeds well in the presence of 0.005 Per cent of hydrochloric acid, but stops with 0.025 Per cent. Dufresne is responsible for the statement that ptyalin recovers its activity in the alkaline intestine, but that diastase is completely destroyed by the gastric juice.

In feeble and ill-nourished persons the salivary digestion becomes much impaired, and consequently their farinaceous food should be partially dextrinised artificially by diastase or by prolonged cooking.

An acid reaction in the mouth may be present because of fermentation, not because the saliva itself is acid. In rheumatism the saliva is very often acid. This reaction causes a sensation of dryness, lessens the taste for food, and gives rise to thirst. Saliva which is rich in cells and mucus is too viscid, and does not moisten the food properly during mastication. When salivation is present the ptyalin becomes too dilute to have any digestive action upon starchy foods. If swallowed, the too abundant saliva carries much air into the stomach; and if it is alkaline, it neutralises the gastric juice. A dry diet (see Dry Diet) will sometimes improve this condition.