212. The teeth, in composition, nutrition, and growth, are different from other bones of the body. They vary in number at different periods of life, and, unlike other bones, they are exposed to the immediate action of atmospheric air and foreign substances. The bones of the system, generally, when fractured, unite; but there is never a permanent union of a tooth when broken. ANATOMY OF THE TEETH. 213. The TEETH are attached to the upper and lower jaw-bone, by means of bony sockets, called _al´ve-o-lar_ processes. These give great solidity to the attachment of the teeth, and frequently render their extraction difficult. The gums, by their fibrous, fleshy structure, serve to fix the teeth more firmly in the jaw. _Observation._ When a _permanent_ tooth is extracted, these bony processes are gradually absorbed, so that in advanced age there remains only the jaw-bone covered by the lining membrane of the gum. This accounts for the narrow jaw and falling in of the lips in old age. Frequently, a piece of the alveolar process comes out with the tooth when extracted, and the dentist has then the credit of "breaking the jaw." No great injury results from the removal of the process in this manner. -=-=-=-=-=-=-=-=-=-=-=-= 212. What is said of the teeth? In what respect do they differ from other bones of the body? 213-218. _Give the anatomy of the teeth._ 213. What confines the teeth in the jaw-bone? What becomes of the socket when a tooth is removed? What effect has this absorption upon the jaw and lips? -=-=-=-=-=-=-=-=-=-=-=-= 214. The teeth are formed in the interior of the jaws, and within _dent´al cap´sules_, (membranous pouches,) which are enclosed within the substance of the bone, and present in their interior a fleshy bud, or granule, from the surface of which exudes the ivory, or the bony part of the tooth. In proportion as the tooth is formed, it rises in the socket, which is developed simultaneously with the tooth, and passes through the gum, and shows itself without. [Illustration: Fig. 54. 1, The body of the lower jaw. 2, Ramus, or branch of the jaw, to which the muscles that move it are attached. 3, 3, The processes which unite the lower jaw with the head. _i_, The middle and lateral incisor tooth of one side. _b_, The bicuspid teeth. _c_, The cuspids, or eye teeth. m, The three molar teeth. A, shows the relation of the permanent to the temporary teeth.] 215. The first set, which appears in infancy, is called _tem´po-ra-ry_, or milk teeth. They are twenty in number; ten in each jaw. Between six and fourteen years of age, the temporary teeth are removed, and the second set appears, called _per´ma-nent_ teeth. They number thirty-two, sixteen in each jaw. -=-=-=-=-=-=-=-=-=-=-=-= 214. Where and how are the teeth formed? Explain fig. 54. 215. What are the first set called? How many in each jaw? The second set? How many in number? -=-=-=-=-=-=-=-=-=-=-=-=