The prehension of fluids is effected in two ways: sometimes the liquid is poured into the mouth, and is allowed to fall into it by its own weight; in other cases, the tongue is used after the fashion of a piston, being drawn within the mouth so as to exhaust the anterior part of that cavity, and fluids are thus forced to enter by atmospheric pressure.
2. Mastication is effected in the cavity of the mouth by means of 'the teeth. This cavity is bounded superiorly by the palate or roof of the mouth, and in other directions by the cheeks, lips, and tongue. Projecting into its interior, above and below, is an arched series of teeth, which are firmly fixed by roots into corresponding sockets in the upper and lower jawbones. The upper jaw (and consequently the dental arch imbedded in it) is immovable, or only movable with the entire head; but the lower jaw, with its teeth, is capable of moving upwards, downwards, backwards, forwards, and laterally, by means of the powerful muscles of mastication. It is by the varied movements of the lower teeth against the upper, through the action of these muscles, that the food is broken down or masticated. For information regarding the structure, etc., of the teeth, see TEETH; See also DENTITION.
The operation of mastication is very important, since the more the food is broken down the more easily will it mix with the saliva and other fluids which participate in the digestive process.
3. Insalivation is effected by the admixture of the secretions of the three pairs of salivary glands (the parotids, the submaxillaries, and the sublinguals) and of the buccal mucus with the triturated food. A brief description of these structures is given in the article GLAND. The common saliva, formed by the combined secretion of these various secreting organs, is a. colorless, turbid, viscid, inodorous, and tasteless fluid, which, after standing for some time, deposits a layer of pavement epithelium (see EPITHELIUM) and mucus corpuscles. In the normal state, its reaction is alkaline, but the degree of alkalinity varies, and is greatest during and after meals. Saliva does not contain more than five or six parts of solid constituents to 995 or 994 parts of water, the most impor tant ingredients being an organic matter termed plyaline, and' sulphocyanide of potas sium, neither of which substances occurs in any other solid or fluid of the body. The daily quantity of saliva secreted by an adult man is estimated at about 48 ounces, but determinations of this kind must be regarded only as approximations to the truth, since the activity of the salivary glands is dependent upon various influences and conditions.
Thus, movement of the lower jaw, as in masticating, speaking, or singing, increases the secretion; as also do acrid and aromatic substances, and dry hard food; while the use of moist and soft food is accompanied by a scanty secretion.
The uses of the saliva in reference to digestion are partly mechanical and partly chemical. The mechanical uses are almost too apparent to require notice. The moist ening of the dry food by the saliva serves the double purpose of adapting it for deglu tition and of separating the particles, and thus allowing them to be more freely acted on by the other digestive fluids; moreover, from its it lubricates the bolus of food, and thus facilitates deglutition; and it is probably also subservient to the sense of taste. The great chemical of the saliva is to convert the amylaceous (or starchy) portion of the food into glycose or grape sugar, and thus to promote its absorption.
4. Deglutition is the act by which the food is transferred from the mouth to the stomach. The pharynx, or cavity into which the mouth leads, takes so slight a part in the digestive process, that we need scarcely allude to any anatomical details connected with it. It is sufficient to observe that between it and the mouth is the pendulous or soft palate, is a movable muscular partition that separates the two cavities during mastication, As soon, however, as the latter act is accomplished, and the bolus is pressed backwards by the tongue, the soft palate is drawn upwards and backwards, so as to permit the passage of the food into the pharynx. The bolus or pellet of food having arrived near the esophagus or gullet (which is continuous inferiorly and posteri orly with the pharynx), is driven into it by the action of certain muscles, which almost surround the pharynx, and are termed its romfrictor muscles. All voluntary action ceases as soon as the food is pressed backwards by the tongue into the pharynx. It is impossible to recall the pellet, and it is necessarily carried on (without even our cogni zance) into the stomach. On receiving the food forced into its upper extremity by the action of the constrictor muscles of the pharynx, the esophagus is dilated (for it usually lies in a collapsed state, with its walls in contact, or nearly so); this contact of the pellet with its mucous membrane causes its muscular walls to contract, and the food is thus driven, by a series of these contractions, into the stomach. The act of deglutition is now completed.