An obvious advantage attending the vitality of the teeth is, that it enables them to accommodate themselves to the growth of the jaw and the rest of the system at the different periods of life. In early infancy, when the human being is designed to live exclusively on his mother’s milk, which of course requires no mastication and consequently no teeth, the latter are still imperfectly formed and entirely hidden in the jaw: it is only at the end of some months that the front or cutting teeth begin to appear; and the whole set of _milk_, _deciduous_, or _falling-out_ teeth, twenty in number, is not completed till about or after the third year. In the course of three or four years more, however, growth has advanced so far that the first set of teeth no longer fill the jaw; and they soon begin to be displaced by the second or permanent set, the gradual development of which commences at that period of life, and is not finished till the appearance of the last four grinders or _wisdom-teeth_, about the age of maturity. It is a curious fact that the infant is born with the rudiments of both sets of teeth in the jaw at the same time, although neither makes its appearance till long after birth. The permanent teeth lie in a line under the milk-teeth, and it is from their growth causing the gradual absorption of the roots of the first teeth that the latter no longer retain their hold of the jaw, but drop out as soon as the others are ready to protrude. In the preceding wood-cut, the situation of the permanent teeth before they emerge from the jaw is rudely represented at A, where the outer surface of the jaw-bone has been removed on purpose, to shew the appearance of the roots. But nothing of this kind is to be found in the adult jaw, the parts marked A being inserted in the plate merely to illustrate what was once the position which the permanent teeth occupied. The changes in the condition of the teeth, it may be remarked in passing, indicate clearly what species of food nature has intended for us at different ages. In early infancy, when no teeth exist, the mother’s milk is the only nutriment required; and in proportion as the teeth begin to appear, a small addition of soft farinaceous food prepared with milk may be made with propriety and gradually increased. But it is impossible to look at the small jaw, moderate muscle, and imperfect teeth of early life, without perceiving that only the mildest kinds and forms of animal food are yet admissible, and that the diet ought to consist essentially of soft and unirritating materials. It is not till the permanent teeth have appeared, that a full proportion of the ordinary kinds of butcher-meat becomes either beneficial or safe.