Teeth

hard bodies developed usually in the jaws of animals and designed primarily for seizing, tearing, or grinding food. The tusks of the boar serve for defense. The tusks of the elephant are used to tear up roots. The tusks of the walrus are used to dig up clams. The beaver uses his front teeth to cut down trees. Worms, Snails, and even insects have so-called spiny or hardened structures, but not genuine teeth, growing from sockets. Turtles lack teeth. Birds are without teeth, though fossil remains show that earlier birds had teeth not unlike those of serpents. The anteater and the pangolin have no teeth. The armadillo has no front teeth. Otherwise quadrupeds have teeth. Although true teeth seem to grow from the jaw, they are not bone, but are modifications of the skin, like hair, horn, and claw. They have been compared to the scales of fishes.

Teeth are classified, according to position and shape, into front teeth or incisors, canine or pointed teeth, premolars, and molars or grinders. Man is furnished with two sets of teeth. The first set, known as milk teeth consists of twenty teeth,-four incisors, two canines, and four premolars in each jaw. The permanent set adds twelve molars,-double teeth or grinders. A child begins ordinarily to cut teeth at six months of age, incisors first, and has a full set at the age of two. It begins to exchange them for the second set at the age of six. The fangs of a milk tooth are absorbed, so that the body of the tooth is pushed off painlessly by the permanent tooth. The last four molars or wisdom teeth come at an uncertain date-from sixteen to twenty-five. The number and kind of teeth is expressed by a dental formula, the numerals above the line referring to the teeth in the upper jaw. The formula for man is as follows:

I 2-2/2-2 C 1-1/1-1 P M 2-2/2-2 M 3-3/3-3 = 32

That is to say, man has four incisors, two canine, four premolar, and six molar teeth in each jaw,--in all, thirty-two.

The formula for an opossum,

I 5-5/4-4 C 1-1/1-1 P M 3-3/3-3 M 4-4/4-4

Indicates that it has ten incisors in the upper jaw and eight in the lower. The hare has incisors and molars only. The ox and the sheep have no front teeth in the upper jaw. A tooth channeled or folded to permit a flow of poison is called a fang. The roots are known also as fangs.

A tooth consists of a hardened outer portion of dentine and an inner cavity filled with pulp, nerves, and blood vessels. Ivory is a particularly fine, hard variety of dentine. The wearing surface of the tooth, called the crown, is covered usually by a hard layer of dentine known as enamel.

Toothache may arise from inflammation of the fang or from a decay of the dentine and exposure of the nerve. In the latter case, a bit of cotton dipped in chloroform or oil of cloves may be pushed into the cavity to allay the pain.

For fear of injuring the dentine, metallic toothpicks should not be used. The best toothpicks on the market are made from the white birch of Maine, the orange-wood of Portugal, and the reeds of Japan. The care and repair of the teeth forms the distinct profession of dentistry.

See Dentistry; Squirrel; Ivory; Beaver; Boar