The New Student's Reference Work/Teeth
Teeth are structures derived from the skin of the mouth, involving both the epidermis and the dermis. Only those of vertebrated animals need be considered. In fishes and snakes they usually are sharp processes without sockets, arising from bones of the jaws and the mouth-parts. In crocodiles and all mammals they are located in sockets. The teeth always show the habits and food of animals, and, therefore, are much used by naturalists in classification. They are of great importance to the students of fossil life, because they are usually well-preserved on account of their hardness. They have been of great service in tracing the evolution of animal life during geological periods. In the herb-eating animals there have been slow changes in the pattern of the crown of molar teeth, which can be read by students like hieroglyphics. The teeth of mammals contain living pulp on the inside provided with nerves and blood-vessels; this is surrounded by dentine or ivory, a bony substance produced from the dermis. The dentine is covered above the jaws by enamel, which is the hardest substance in the human body. The roots are covered with cement, which joins the enamel and sometimes extends into it. When the enamel is broken or worn through, the much softer dentine is exposed; this decays and leads to cavities in the teeth and to their destruction, if the cavities are not cleaned and filled. The typical number of teeth in mammals is 44, 22 on each jaw, and not 32, as most people suppose, because the latter number is found in man. Teeth of mammals are divided into incisors or cutting-teeth, canines or sharp tearing-teeth, premolars which appear earliest, and molars or grinding-teeth. There usually are two sets which appear in succession, the milk-teeth or temporary ones and the permanent set. The teeth are subject to great modifications. The great cutting-teeth of beavers and the tusks of elephants are modified incisors; the tusks of the walrus and those of the wild boar are enlarged canines. The molars or grinding-teeth have a pattern on the crown that is characteristic of the different groups of mammals.