DENTITION, PERIOD OF (Lat. dentitio, the process of teething, from dens, a tooth). In man and most mammals, there are two distinct sets of teeth; one set which appears shortly after birth, and which are termed the milk or deciduous teeth; and a second set, which, after a few years, replaces these, and which are termed permanent teeth.
In the human subject, the milk-teeth are twenty in number, each jaw containing (from before backwards) four incisors, two canines, and four molars; while the perma nent teeth are thirty-two in number, each jaw containing four incisors, two canines, four premolars or bicuspids, and six molars. Anatomists are in the habit of briefly expressing the number of the different kinds of teeth in any mammal by what they term a dental formula. The permanent teeth in man are represented by the formula, — 2' c 1 — 1' — p 2 — 2 3 — 3 - = 32, where the letters i, c, p, m, stand for incisors, canines, premolars, and molars, and where the two terms in each numerator and in each denominator represent the number of each particular kind of tooth in each half of the upper and lower jaw respectively. As these formula= are of common use in most works on zoology and comparative anatomy, we add another example—that of the permanent . — 3 teeth of the hog, whose formula is z c 1 _1 p 4-4 8-3 = 44: which signifies 3 — 3 1 — 1 4 — 4 3 — 3 that there are on each side of both upper and lower jaws three incisors, one canine, four premolars, and three molars, making in all 44 teeth.
For a general description of the form and uses of these different kinds of teeth, we refer the reader to the article DIGESTION, ORGANS AND PROCESS of where their special usts are noticed in reference to the digestive function; while the history of their struc ture. etc., is given in the article TEETH.
The following is the usual order and period of appearance of the milk-teeth: The four central incisors usually appear through the gums about the seventh month after birth, those of the lower jaw showing themselves first. The lateral incisors next appear between the 7th and 10th months; the anterior molars show themselves about the 13th month, and are soon followed by the canines, which usually appear between the 14th and 21st months. The posterior molars are the last and most uncertain in their
time of protusion, which may range from the 18th month to the end of the 3d year. Except in the case of the incisors, there is no definite law as to Whether the upper or lower teeth first apper.
About the middle or end of the 7th year, the jaw-bones have become sufficiently elongated to permit the appearance of the first true molar; and about the same time, the central incisors are replaced by the corresponding permanent teeth. The advance of the permanent teeth towards the surface of the gum causes the absorption of the roots of the temporary teeth, and thus facilitates their shedding; the crown falling off, and room for the permanent tooth behind it to come forward and supply its place.
In the replacement of the first by the second set of teeth, the following order is observed: The middle incisors are first shed and renewed (usually when the child is about eight years of age), and then the lateral incisors (perhaps a year later). The anterior molars of the first set are then replaced by the anterior premolars (this usually happens about the 11th year); and about a year afterwards the posterior deciduous molars are replaced by the second premolars. The persistent canines take the place of the deciduous ones in the 12th year; these being the la.4t of the milk-teeth to be exchanged. The second molars appear between the ages of twelve and a half and fourteen years; and the third molars, or denies sapientice (wisdom teeth), seldom appear till three or four years subsequently, and often much later.
The factory laws in England render it very important that we should be able to determine the ages of nine and thirteen in children, because before a child is nine years old, it is illegal to employ it in factory-work; and until it is thirteen, it may only be employed for a limited number of hours a day. Mr. Saunders, a well-known dentist, has shown in his pamphlet. the Teeth a Test of Age, considered with Reference to Factory Children, that the teeth afford a far better test of age at this period of life, than the standard of height which has been adopted by the legislature for this purpose.