Page:First course in biology (IA firstcourseinbio00bailrich).pdf/567

 jaw. At the age of twelve or thirteen years all the permanent set have appeared except the four wisdom teeth, which appear between the ages of seventeen and twenty-five. The second set not only replaces the twenty of the first set, but to fill the larger jaws twelve molars are added, three at the back in each half jaw, making thirty-two teeth in the second set (Exp. 7). The teeth in each quarter of the mouth, named in order from the front, are: two incisors, one canine, two premolars, three molars. in right half of lower jaw. . Care of the Teeth.—The best way to care for the teeth is to keep the digestion perfect. Perfect digestion tends to preserve the teeth, and sound teeth tend to keep the digestion perfect. The teeth should be washed regularly. Prepared chalk is the best dentifrice. Do not rub across, but from gums to teeth, to prevent rubbing the gums loose from the teeth. An unclean brush may harbor germs. Toothpicks and dental floss are useful. If one eats only soft food, in which the mill and the cooking stove have left no work for the teeth, the teeth will decay; for it seems to be a law of nature that useless organs are removed. The pressure from chewing