Part 13 (1/2)

The enamel organs and dentine bulb for the permanent teeth form just before birth (Fig. 51) in like manner with the temporary set. They form just above the temporary set on the upper and below on the lower jaw. The permanent molars begin to calcify at the twenty-fifth week of foetal life.

The permanent incisors do not calcify until a year after birth. Any deviation in size or contour of the permanent teeth from the normal must hence be due to defect in nutrition in the dentine bulb, between the fifteenth and twenty-fifth week of foetal life. Any deviation in calcification (except the cusps of the first permanent molars) must occur after birth. At the third year twenty-four teeth are fairly well calcified. At the fifth year the second permanent molars, and at the eighth year the third molars or wisdom teeth, begin to calcify.

The following table gives the age of eruption of permanent teeth:

First Permanent Molars Circa 6 years.

Upper and Lower Central Incisors ” 7 years.

Upper and Lower Lateral ” ” 8 years.

First Bicuspids ” 9 years.

Second Bicuspids ” 10 years.

Cuspids ” 11 years.

Second Permanent Molars ” 12 years.

Third Permanent Molars ” 17 to 24 years.

Man, at this present stage of evolution, has twenty teeth in his temporary and thirty-two in his permanent set. Any deviation in number is the result of embryonic change occurring between the sixth and fifteenth week, for the temporary teeth, and the fifteenth week and birth for the permanent.

The germs of teeth which erupt late in life, and are called third sets, of necessity appear ere birth and are completely formed at the beginning of the second year, although they remain protected in the jaw until eruption.

More than twenty teeth in the temporary set, or thirty-two in the permanent set, is hence an atavistic abnormality. From the maxillary and dental standpoint man reached his highest development when well-developed jaws held twenty temporary and thirty-two permanent teeth. Decrease in the numbers of teeth meant, from the dental standpoint, degeneracy, albeit it might mark advance in man's evolution as a complete being. In the New Mexican Lower Eocene occur monkeys like the lemurarius and limnotherium, each the type of a distinct family. The lemurarius, most nearly allied to the lemurs, is the most generalised monkey yet found. It had forty-four teeth in continuous series, above and below. The limnotherium, while related to the lemurs, had some affinities with the American marmosets.

These solved the problem of the origin of the extra teeth (known as supernumeraries) that sometimes occur in man, and demonstrated that man, during his evolution from the lowest monkey, lost twelve teeth. These supernumerary teeth a.s.sume two forms; either they resemble the adjoining teeth or are cone-shaped. While they are rarely exactly counterparts, every tooth can be duplicated, as the following ill.u.s.trations show.

Fig. 52 ill.u.s.trates fairly well-formed duplicate central incisors, the normal incisors being outside the dental arch. They are crowded laterally by the large roots of the supernumerary incisors.

[Ill.u.s.tration: FIG. 52.]

[Ill.u.s.tration: FIG. 53.]

[Ill.u.s.tration: FIG. 54.]

Fig. 53 shows an extra right lateral in a temporary set in the upper jaw.

Fig. 54 an extra right lateral in the permanent set. Fig. 55 ill.u.s.trates normally developed supernumerary cuspids which are all grouped together upon the right side, the bicuspid being also duplicated on each side; indeed, all but the molars are duplicate. Fig. 56 shows supernumerary third molars, easily demarcated from the normal molars. The teeth which fail to approximate their normal neighbours a.s.sume the cone shape of the primitive tooth.

[Ill.u.s.tration: FIG. 55.]

[Ill.u.s.tration: FIG. 56.]

The fact that the cone-shaped tooth, as a rule perfect in construction, is found everywhere in the jaw, but especially in the anterior and posterior part of the mouth, is of much value in outlining tooth and jaw evolution, especially in the degeneracy phase. The upper jaw, being an integral part of the skull and fixed, is, of necessity, influenced by brain and skull growth; hence degeneracy is more detectable in it than in the lower.

[Ill.u.s.tration: FIG. 57.]

[Ill.u.s.tration: FIG. 58.]