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.
