PLATE A.

PLATE B.

These three teeth form a series of intermediate steps between a most ancient molar and the modern molar of the human type. The second tooth is halfway between the first and third. The second molar, seen from above, has exactly the same cusps as the first, so it is not difficult to recognise that each cusp has been directly derived from its fellow. The third tooth of the series (Fig. 7) has lost one of its cusps; it has lost a cusp of the triangle. It is now a tooth where only half the triangle is left on the anterior side and with a very long heel. That tooth has exactly the same pattern as the lower human molar tooth (Fig. 8), the only difference is that the heel is somewhat more prolonged. These teeth belong to one of the oldest fossil monkeys, anaptomorphus. Human lower molars, not very exceptionally, instead of four cusps, have five. The fifth cusp always appears in the middle of the heel, or between the posterior lingual and the posterior buccal. This occurs in monkeys and other animals, but no record exists of the ancient anterior lingual reappearing. The human lower molar, with its low, quadritubercular crown, has hence evolved by addition of cusps and by gradual modelling from a high-crowned, simple, pointed tooth.

Human teeth are of excellent service in the initial determination of degeneracy in the child. For this purpose the teeth should be studied from the first evidence of their development until they are all in place, which occurs normally, in most cases, by the twenty-second year.

Teeth-enamel is formed from the epiblast, and dentine, cementum, pulp (except as to nerve tissue) from the mesoblast. The enamel organs of the first set appear during the seventh week of fœtal life; the dentine bulb during the ninth week. At this period the tooth obtains its periphery. This models the enamel cap which fits over the dentine like a glove. When imperfections in hand or fingers exist these deformities are distinctly observed upon the glove, and in precisely the same manner are observed the different shapes and sizes of the incisors, cuspids, and molars. Calcification of the teeth begins at the seventeenth week of fœtal life. The illustration (Fig. [50]) shows the progress of calcification and development of the temporary set of teeth. Examination will show that any defect in nutrition, from conception to birth (due to inherited states or maternal impressions), has been registered upon the teeth. The state of the constitution and the locality register the date of such defects. Thus if the tooth, as a whole, be larger or smaller than normal, or abnormally irregular, taint is undoubtedly inherited from one or both parents. If, on the other hand, there be defect at any part on the crowns of the teeth, and the contour be perfect, the date of malnutrition can be easily determined from this chart. More or less than the normal number of teeth, abnormally placed, demonstrates the existence of inherited defect, since the germs must have been deposited at the period mentioned. No absolute rule can be laid down as to date of the eruption of the teeth. The teeth of the temporary set erupt nearly as follows:

After Birth. Time of Eruption.
Lower Central Incisors 7 months 1 to 10 weeks.
Upper"" 9 months 4 to 6 weeks.
Upper and Lower Lateral 12 months 4 to 6 weeks.
First Molars 14 months 1 to 2 months.
Cuspids 18 months 2 to 3 months.
Second Molars 26 months 3 to 5 months.