The Mouth.—The mouth is of great importance as an entrance for fresh air to the lungs when the nasal passages are for any reason impeded and as the resonant chamber from which proceeds the voice, man’s chief means of communication with his fellows. Its chief value may be said, however, to reside in the fact that it is the vestibule of the alimentary canal. It is an ovoid cavity lined with mucous membrane and is bounded in front by the lips, at the sides by the cheeks, below by the floor and tongue, and above by the hard palate anteriorly and by the soft palate posteriorly, the uvula depending from the latter like a curtain between the mouth and the pharynx. Shape is given to the mouth by the bones of the upper and lower jaw and its size is altered by the lowering and raising of the latter, which is quite freely movable.
Fig. 19.—The hyoid bone. (Toldt.)
At the back of the mouth, at the entrance to the pharynx, are the anterior and posterior pillars of the fauces, which contain muscular tissue, and between which on either side are thick masses of lymphoid tissue, the tonsils. The floor of the mouth is formed largely by the tongue, which completely fills the space within the lower teeth. Its base or root is directed backward and downward and is attached by muscles to the hyoid bone and the lower jaw, the [hyoid bone] being a horseshoe-shaped bone lying just below and as it were within the inferior maxillary. The base of the tongue is attached also to the epiglottis and at the sides to the soft palate by the anterior pillars. Except at its base and the posterior part of its under surface the tongue is free, but a fold of mucous membrane, the frenum, holds it somewhat in front. Thus it possesses great versatility of motion and serves as an auxiliary in articulation, mastication, and deglutition.
The Teeth.—Securely embedded in either jaw are the teeth, nature’s instrument for the first preparation of the food for digestion through tearing and grinding. The incisors, which are in front, have wide sharp edges for cutting the food. Next come the canine teeth with a sharp point for tearing it, while at the back are the molars with a broad flat top for grinding.
There are two sets of teeth: 1. the temporary or milk teeth, twenty in number—four incisors, two canines, and four molars in each jaw—which appear at from six months to two years, and 2. the permanent teeth, thirty-two in number—four incisors, two canines, known as eye teeth in the upper jaw and as stomach teeth in the lower jaw, four bicuspids, so called because they have two cusps where the molars have four or five, and six molars in each jaw—which come from the sixth to the twenty-first years. The first to appear are the two lower middle incisors, which come at the age of six months. The last to appear are the wisdom teeth, the farthest back of the molars, which come at the age of twenty-one years or thereabouts.
Each tooth consists of a crown or body above the gum, a neck, and a fang or root within the gum. The body is of dentine or ivory with a thin crust of enamel and contains the pulp, a vascular connective tissue containing many nerves. Beginning at the neck and covering the fang is a layer of cement or true bone.
The Sense of Taste.—The sense of taste lies chiefly in the taste buds as they are called which are filled with gustatory cells and are found in the papillæ of the tongue, principally in the circumvallate papillæ at the back of the tongue, which are few in number and arranged in a V-shape. There is also a certain power of taste in the tip and sides of the tongue but little in the upper surface or dorsum. Only five special tastes can be distinguished: bitter, sweet, acid, sour, and salt, but sometimes more than one can be distinguished at a time, as bitter and sweet. Every one can distinguish between different tastes but the power varies in different people and with different conditions. Certain tastes seem to be better distinguished in certain places, as sweet at the tip and bitter at the back of the tongue. Moreover, the sense of taste is very dependent upon the sense of smell, especially in the case of aromatic and savory substances, which one really does not taste but smell. If one held his nose and closed his eyes he would not know from the taste whether he was eating onion or apple. This leads to the habit of pinching the nose when taking nauseous medicines.
To be tasted a substance must be in solution. Friction against the tongue, lips or cheek increase the sense of taste. A temperature of 100° Fahrenheit favors taste, while both great heat and great cold impair it.
There are probably at least two nerves of taste, the lingual branch of the trifacial or fifth cranial and the gustatory branch of the glosso-pharyngeal.