The change of the temporary for the permanent or adult teeth commences, in the majority of instances, at about seven years of age; occasionally it occurs as early as five, and as late as eight years and a half.  The necessity which exists for this change, and the mode by which it is effected, are striking and beautiful; it is, however, not our object to enter fully upon its consideration here.

It has already been observed, that the infant is born with the rudiments of two sets of teeth in the jaw, although neither make their appearance till long after birth.  The time when, and the manner in which the first set appear has been pointed out.  Now although these admirably answer the purposes for which they were given up to the seventh year, after this period they fail to do so:  they are not sufficiently numerous,—­in their structure they are not strong or durable,—­nor is their power of mastication sufficiently great.

They are not sufficiently large or numerous.  If the mouth of a child at this age is examined, it will be seen, that a considerable interval has taken place between the teeth in consequence of the growth and expansion of the face; hence a larger set has become necessary to fill the arch.  But it may be asked, do not the teeth grow with the growth of the body? and if not, why is it so?  They do not, and for this reason:  the important office which these organs are destined to perform requires that they should be composed of a substance too dense and of too low an organization to allow of any subsequent growth and enlargement.  Thus the size of the teeth is determined and acquired before they make their appearance through the gums.  This being the case, it will be readily seen, that the teeth which would be of appropriate size in the mouth of the infant, would be quite inadequate to the enlarged dimensions of the adult; hence the necessity of a second set, exceeding in number, and size the teeth of the first.

That a necessity also exists at this age, that the weak and delicate teeth of childhood should be exchanged for a set stronger and more durable in their structure, more robust and more powerful, will be sufficiently apparent, if we only recollect the great change which has gradually been taking place in the nature of the food of the two epochs of childhood and adult age.

The second set, or permanent teeth, then, lying under the milk-teeth and hidden in the jaw, undergo in this situation their full developement, before they appear above the gum.  This occurrence commences about seven years of age, at which period the first set begin to fall out from their roots becoming absorbed, and no longer retaining their hold of the jaw; to be entirely replaced in the course of a few years by the permanent set, which thus succeeds them.  The first teeth of this set which make their appearance are the large double teeth, which emerge from the gum immediately behind the last of the temporary set.