they have changed their places in the jaw. Upon this elemental truth is founded the art of orthodontia. Were the facts not as stated, it would do no good to alter the positions of teeth, since they would not retain their new positions after they had been moved into them. The fact that the jaws can be widened by spreading the teeth, taken in conjunction with the adaptibility of the "alveolar" process, make the remarkable results of the orthodontist possible. The size, shape and strength of the lower jaw, or mandible, depend in great part upon the work it has to do, and furthermore, the shape of the upper jaw is determined by that of the lower. The lower permanent teeth are erupted first, and by their repeated impactions upon their opponents in the upper jaw, aided by the constant restraining and forming action of the tongue and lips gradually force the upper teeth into their proper
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places and keep them there. Provided, that the lower jaw and the tongue and lips are strong and well developed, made so by sufficient chewing, especially from the years of two to six, in a child's life. If the child's education in chewing, however, has been neglected, the dentist can and does spread the jaw as already stated, so that it will have room enough for all the teeth. In other words, orthodontia does what nature would accomplish unaided were her simple laws of development properly observed.
A full set of teeth forms a beautiful arch, no stone of which should be missing. The shape and span of this arch are greatly determined by the size and position of the four permanent first molars, "six-year old molars," the largest and most important teeth in the head. If these teeth are properly disposed in the jaws, the regulation of the
