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.]

[Ill.u.s.tration: FIG. 59.]

[Ill.u.s.tration: FIG. 60.]

[Ill.u.s.tration: FIG. 61.]

The evolution of the jaw is toward shortening in both directions. This shortening will continue so long as the jaw must be adjusted to a varying environment. The jaw of man having originally contained more teeth than at present, lack of adjustment to environment produces, from the shortening, degeneracy of the jaw and atavism of the teeth. While this may coincide with general advances of the individual, it indicates that he is not yet adjusted to his new environment. The shortening of the upper jaw causes supernumerary, cone-shaped teeth to erupt, in ma.s.s, at the extreme ends of the jaw, as shown in the following figures. Fig. 57 ill.u.s.trates a cone-shaped tooth between the two central incisors, forcing them out of position. Fig. 58 shows three supernumerary teeth--a cone-shaped tooth between the central laterals, and the cuspids out of position. The left permanent lateral is at the median line; another cone-shaped tooth remains in the vault, while the supernumerary left lateral is in place. As many as eight are at times to be observed in the anterior vault. Posteriorly these teeth are most often noticed in connection with the third molars, usually on a line with other teeth posterior to the last molar. Fig. 59 shows two supernumerary teeth in the anterior and two in the posterior part of the left arch; the molars have been extracted. Supernumerary teeth are not confined to these localities, but may be observed at any point in the dental arch (Figs. 60 and 61). The primitive cone-shaped tooth is rarely observed in the lower jaw. In twenty-six years' practice I have not seen a case. The mobility of the lower jaw prevents that mal-adjustment to environment present in the upper. The continual shortening, in both directions, of the jaw causes the third molars frequently to wedge in between the angle of the jaw and the second molars, so that eruption, if possible, is difficult.

[Ill.u.s.tration: FIG. 62.]

[Ill.u.s.tration: FIG. 63.]

[Ill.u.s.tration: FIG. 64.]

The third molar is often absent in the English-speaking and Scandinavian races. In 46 per cent. of 670 patients it was missing. Frequently its development is abortive. This tooth, in the struggle for existence, seems destined to disappear. It is more often absent from the upper than the lower jaw. When absent, or badly developed, the jaw is smaller and frequently teeth irregularities, nasal stenosis, hypertrophy of nasal bone and mucous membrane, adenoids and eye disorders coexist. Fig. 62 shows absence of the left third molar with irregularities of that side of the arch. In Fig. 63 both third molars are seen to be missing. Anteriorly, the lateral incisors are most often wanting; 14 per cent. of the laterals were wanting in 670 patients. In the progress of evolution man has lost one lateral upon each side of the mouth and the second lateral seems also destined to disappear. In Fig. 64 the left lateral incisor has disappeared; and in Fig. 65 both lateral incisors are absent. Not infrequently does it occur that centrals, cuspids, bicuspids, and even molars are absent, even their germs not being detectable. Fig. 66 shows three supernumeraries in the anterior part of the mouth and but two molars. The absence of the teeth indicates lack of development of germs, due either to heredity or defective maternal nutrition at the time of conception or during early pregnancy.

[Ill.u.s.tration: FIG. 65.]

[Ill.u.s.tration: FIG. 66.]

[Ill.u.s.tration: FIG. 67.]

Crescent-shaped, bitubercular, and tribucular as well as deformed teeth, tend to be cone-shaped. The malformation of these teeth results from precongenital trophic change in dentine development, dwarfing and notching the cutting and grinding edges of the second set of teeth, of which a familiar example is the so-called Hutchinson's teeth, usually referred to a syphilitic causation. Hutchinson's position has, however, been more strongly stated than his words justify, since he admits that in at least one-tenth of the cases this cause could be excluded.

Syphilis only plays the part of a diathetic state profoundly affecting the maternal const.i.tution at the time of dentine development; while these teeth may be due to secondary results of syphilis, they do not demonstrate syphilitic heredity.

In Fig. 67 are seen the teeth of an individual affected with const.i.tutional disease (referring to Fig. 51 it becomes evident that the defective lines represent the respective ages, 2-1/2, 4, and 5 years). The degree of pitting will depend, as a rule, on the severity of the const.i.tutional disorder. In the case just cited, however, although nutrition was but slightly disordered, each tooth shows a tendency to conate. Not infrequently cavities extend completely through the tooth. The cusps of the (permanent) first molars, calcifying at the first year, are usually attacked also, and arrested in development, producing the cone shape. These data, together with the dates of eruption of the temporary and permanent teeth, furnish an absolute basis for calculation as to malnutrition producing excessive or arrested development, not only of the teeth and jaws but all parts of the body.

[Ill.u.s.tration: FIG. 68.]

[Ill.u.s.tration: FIG. 69.]

[Ill.u.s.tration: FIGS. 70, 71.]

Fig. 68 shows a very degenerate jaw with cone-shaped, malformed bicuspids. The right lateral missing, the cuspids are erupting in the vault and the dental arch is a.s.suming a V-shape. The jaw shows, as a whole, marked arrest in development. Fig. 69 shows Hutchinson's teeth.

Were the first molars visible, they would present marked contraction of the outer surface with a malformed centre. Referring again to Fig. 51, it is observable that trophic changes affected the system at the age of birth. The outer surface exhibits a tendency to take the cone shape. Figs.

70, 71, 72, 73, and the molars in Fig. 66, exhibit malformations that a.s.sume the cone shape and the centre frequently a.s.sociated with this type of teeth. The coincidence in form between Hutchinson's and malformed teeth and those of the chameleon suggests that tropho-neurotic change produces atavistic teeth. Fig. 74 ill.u.s.trates the tendency of human bicuspids (when there is no antagonism) to rotate one-fourth round, thus again indicating an atavistic tendency toward the teeth of the chameleon. Fig. 75 exhibits extreme atavism; all teeth anterior to the molars are cone-shaped. The third molars are missing and would, probably, never erupt. In Fig. 76 appears more marked atavism. The upper and lower are both cone-shaped, and the superior first bicuspid exhibits tendency thereto. The right superior second bicuspid, second and third molars, the right inferior first and second bicuspids, with second and third molars are missing. The same condition, probably, exists on the left side. The s.p.a.ce in the upper jaw is due to the insufficient width of the teeth. Alternation of teeth in the upper and lower jaw is a reptilian feature.

[Ill.u.s.tration: FIG. 72.]

[Ill.u.s.tration: FIG. 73.]

[Ill.u.s.tration: FIG. 74.]

[Ill.u.s.tration: FIG. 75.]