tooth

(tith), pl. /eeth. Any one of a set of small, bone-like structures of the jaws for masticating the food. There are two sets of teeth, the temporary (milk or deciduous) teeth, which are lost in childhood, and the permanent t., which begin in the seventh year to displace the temporary teeth and last until old age. There are 20 temporary teeth, 10 in each jaw, as follows: 4 incisors, 2 canines, and 4 molars. There are 32 permanent teeth, 16 in each jaw, as follows: 4 incisors, 2 canines, 4 bicuspids, and 6 molars. Each tooth has three parts—a crown, or body, above the gum and covered with enamel; a acck, between the crown and root; a rool, or fang, within the alveolus, and covered with cement. The incisors are cutting teeth, and have a crown shaped like a wedge. Each has one long and conic fang. The cuspids are tearing teeth. Each has a long and conic crown and a single fang. The cuspid a in the upper jaw are called the eye-teeth, and those in the lower, the stomach-teeth. The crowns of the bicuspids have two cusps. Each bicuspid has a grooved, conic, single fang. Molars, or grinders, have broad, square crowns. The upper molars have four cusps and the lower molars five. The lower molars have two fangs; the upper, three fangs. The third molar has only one fang. This third molar is known as the dens sapient i a, or “ wisdom tooth.” A tooth consists of a solid portion and a pulp-cavity. The solid portion consists of dentin, or ivory, forming most of the tooth, and resembling bone; enamel, the hardest of organized bodies, covering the crown; and cement, covering the fang. Cement, or crusta petrosa, is true bone. The pulp cavity within the crown is continuous with a canal in the fang which opens at the root-apex for the pa age of nerves and ve els. This cavity contains the pulp, a soft, sensitive substance containing many ve els and nerves, and composed of cells and connective ti ue. Development: The temporary teeth are formed in the primitive dental groove by an involution of epithelium about the ninth or eleventh week. The enamel comes from a collection of epithelial cells, the enamel-germ. This enamel-germ descends until it meets a vascular growth coming up from the connective ti ue, and called the papilla. Over the papilla the enamel germ forms a cup. A membrane inclosing the enamel-germ extends upon the papilla, and cuts the enamel off from the epithelial structure. This is the dentinal sac. The cells are after a time calcified. Odontoblasts in the papilla form the dentin. The cement comes from the wall of the dentinal sac. The permanent teeth, which replace the milk-teeth, develop, like them, from a secondary dentinal groove, recurring, after their formation, behind the germs of the milk-teeth, and inclosed in sacs. The molars, or permanent teeth superadded, are formed by backward extension of the enamel germ of the tooth just in front. Eruption, or culling of the teeth, is due to growth of the fang, the germ being absorbed by pre ure. The order of eruption of milk-teeth: Lower central incisors, six to eight months; upper central incisors, six to eight months; 1016 TOPOTHERMESTHESIOMETER lateral incisors, seven to nine months; upper and lower first molars, fourteen to sixteen months; canines, seventeen to eighteen months; second molars, eighteen to twenty-four months. Permanent teeth: First molars, five to seven years; first central incisors, seven to eight years; four lateral incisors, eight to nine years; four first bicuspids, ten to eleven and a half years; four second bicuspids, eleven to twelve years; first canines, twelve to fourteen years; four second molars, twelve to fifteen years; four third molars, seventeen to twenty-two years. Various other dental structures in the animal and vegetable organisms are also called teeth. See further under tecth. t.-ache, pain in a tooth; odontalgia. t.-key, a dentist’s instrument formerly used for extracting teeth by torsion. t.-pulp, the connective, nervous, and other ti ues which fill the pulp-cavity of a tooth. t.-rash. See strophulus. virgin t., a horse’s tooth not yet worn down by use.