I. Physiological Relations

The process of digestion begins with the reception of food, more or less prepared by cooking. During its brief stay in the mouth, the food is triturated and mixed with mucus and saliva, and its starchy constituents are partly converted into sugar.

1. Food forms no part of the subject of the present work, and it will be sufficient to remind the student that the chief proximate principles of a proper diet are proteids, amyloids, fat,

Baits, and water. The relative proportions of these constituents vary greatly in different kinds of food.

2. The sensory nerves of the mouth (the glosso-pharyngeal, and the lingual and other "branches of the trigeminus) receive and transmit to the cerebrum and medulla the impressions of taste, as they are commonly called, whether sweet (the pleasant taste referable toamylolytic action), hitter, salt, sour, hot, burning, warm, pungent, acrid, or nauseous; and the many kinds of aromatic flavours, which are chiefly, however, odours. In the medulla the gustatory impressions fall into a special centre, whence they are reflected (1) to the stomach, the functions of which they modify, as we shall see; and (2) to the salivary and mucous glands of the mouth, which they also influence, chiefly through the chorda tympani. Through the same efferent nerve come other impulses: from the cerebrum, as the result of the sight, taste, smell, or even idea of food; from the stomach, conveyed by the vagus; and, doubtless, from many other sensitive parts, especially in the abdomen.

3. The flow of saliva and mucus is the result of the nervous impulses which have just been traced, and which stimulate the protoplasm of the epithelial cells, and actively dilate the vessels. The saliva is secreted at the commencement of digestion, is intimately mixed with the food, and imparts to the bolus a faintly alkaline reaction which has an important effect on secretion in the stomach.

4. It is well to distinguish from the ordinary secretions of the mouth, the excretions which are also thrown out by the glands. Although these are but little appreciated in health, they are familiar as the source of certain unpleasant tastes in the mouth and odours of the breath, after particular kinds of food and drink, such as wines, and many drugs.

5. The muscular acts of mastication and swallowing are guided by the afferent impressions and by the will.

II. Pharmacodynamics

We come now to inquire, according to the plan which we have sketched, whether we possess any means of influencing the normal functions of the mouth, and if so, how far such powers can be usefully applied.

1. Food

Food. We have absolute control over our food. We can withhold it altogether; we can alter its quantity and its quality as we please. Especially as regards the mouth, we may modify the proportion of amyloids in the diet, affect their condition by cookery, or convert them wholly or partially into sugar before administration. Malt extracts consist chiefly of dextrin and maltose, made from malted grain and flour.

The control which we thus possess over food is the foundation of the vast subject of dietetics.

2. The sensory apparatus in the mouth can he variously influenced. The variety of natural tastes and flavours of which we may avail ourselves is endless; artificial products are hardly less numerous. The art of cookery is much concerned with the proper use of these; so is the growth of wines; and the many natural and artificial condiments act chiefly upon the palate, such as mustard, pickles, and sauces. Beyond the culinary art, an immense number of medicinal agents are contained in the materia medica which may be used in therapeutics proper, to act upon the tongue and palate, and thus upon the nervous centres and viscera. These may be arranged as follows: (1) The great group of warm aromatic oils, including Cloves, Allspice, Peppermint, Rosemary, Lavender, Nutmeg, and many others, each with its own peculiar flavour; (2) bitters, such as Calumba, Quassia, Quinia, etc.; (3) aromatic bitters, of which Gentian, Orange, and Cascarilla are examples; (4) the spirituous group, including Spirits, Wines, Chloroform, and Ether; (5) pungent substances proper, such as Mustard, Horseradish, and Pyrethrum; (6) sweet substances, including Sugar, Liquorice, Glycerine, etc.; and (7) acid or sour substances, such as the Mineral Acids, Acid Fruits, and Acid Tartrate of Potash, to which we shall presently return.

The value of aromatics, bitters, and the other stimulants of the nerves of the mouth, lies in the fact that whilst they increase relish or the enjoyment of eating, and thus the appetite and the amount of food consumed, they provide for the digestion of this increased quantity of nourishment by stimulating the secretion of the digestive fluids in the mouth, and, as we shall see in the next section, in the stomach also.

The effect of these substances on the palate also affords us means of covering the tastes of nauseous medicines, of which we constantly avail ourselves. On the other hand, we may employ the unpleasant taste or flavour of certain drugs, such as Valerian and Assafoetida, to produce through the afferent nerves a powerful influence on the sensorium which we may sometimes have occasion to employ.

3. Salivary And Mucous Glands

Salivary And Mucous Glands. Substances and measures which increase the flow of saliva are called sialagogues ( II Pharmacodynamics 1 saliva, and II Pharmacodynamics 2 to cause to flow), and include the greater number of the stimulants of the sensory apparatus just classified. Of these the most important sialagogues are unquestionably diluted acids, including the Diluted Mineral Acids, Carbonic Acid in effervescence, Vegetable Acids and their salts, wines (which are all acid to a degree), and acid fruits and juices, of which Lemon may serve as a type. The familiar effect of acid drinks in relieving thirst cannot, however, be entirely explained by their influence on the nerves of taste. Here the student is introduced to a great physiological law, which we shall frequently have occasion to notice, that acid substances stimulate alkaline secretions, and alkaline substances stimulate acid secretions. The action is probably a local one, the acid or alkali, as the case may be, being quickly absorbed, and reaching the protoplasm of the glands direct.