The incisores, the canine, and the first four of the molares, are generally shed in the fifth, sixth, or seventh year, but they are commonly replaced by others in the seventh year, though sometimes not till the age of puberty. The shedding of these sixteen teeth is occasioned by the expansion in the gums for the new set which are at the bottom of the sockets, and by their growth press out the first. There not being any beneath the other grinders they remain, unless forced out by accident, and when lost they are hardly ever recovered.
There are four other teeth still, which are situated at the extremity of each jaw, but which every person does not have. These seldom appear till the age of puberty, and sometimes later; they are called dentes sapientiæ, or wisdom-teeth, and either appear one after another, or two at a time. The only reason of there being a variety in the number of teeth, which is from 28 to 32, is the irregularity of the wisdom-teeth. Women, it has been observed, have generally fewer teeth than men.
Some authors pretend that the teeth would continue to grow through life, if it were not for their continual attrition, occasioned by the food on which man subsists, and increase in size (as is the case in several animals) in proportion as he advances in age. But this opinion is contradicted by experience; for persons who live wholly on liquid nutriment have not teeth longer than those who live on solid food, and if any thing is capable of wearing the teeth, it is rather their mutual friction one against another than mastication. Besides, they possibly deceive themselves by confounding teeth with tusks, as to the growth of the former in certain animals. The tusks of the wild boar and elephant, for example, continue to grow during life, but it is highly improbable that the teeth of either, when once arrived at their natural size, should afterwards increase. Tusks have a greater affinity, by far, with horns than with teeth; but this is not the proper place to enter into their specific differences, nor shall any thing further be added, than that the first set of teeth in children is of a substance less solid than the subsequent one, and more loosely fixed in the jaws.