vy 7 ~~ oe ee 
972 THE DIGESTIVE SYSTEM. 
their fellows of the lower jaw, which throws the upper canines and the succeeding 
teeth into a position behind (distal to) that of the same named teeth of the lower 
set. But as the lower molars are 
larger in their antero-posterior 
diameter than those of the upper 
row—and this remark applies par- 
ticularly to the wisdom teeth— 
the two dental arches terminate 
behind at approxunately the same 
point. | 
The upper dental arch is said to form. 
an elliptical, the lower a parabolic curve _ 
(Figs. 652 and 653). The line formed by 
the grinding surfaces of the upper teeth, 
as seen on profile view (Fig. 655), i 
usually somewhat convex, owing largely 
to the failure of the wisdom tooth to 
descend into line with the others. 
Similarly the line of the lower teeth 
is as a rule concave. 
In both jaws the crowns of the front 
teeth are higher (longer) than those of 
the molars. 
Fic. 655.—To show the relation of the upper to the lower Period of Eruption of the Per- 
teeth when the mouth is closed. The manner in which a manent Teeth.— Although there 1s 
tooth of one row usually strikes against two teeth of the egnsiderable variety in the dates i 
opposite row, and the resulting interlocking of the teeth, on 5 a: : 
Peto The noted. : at which the various permanent 
teeth appear above the gums, the 
order of eruption is practically constant in different individuals, and is as 
follows :—Before any of the temporary teeth are lost the first permanent molars 
appear behind the 2nd milk molars. Next the central milk incisors fall out, and 
their places are taken by the permanent teeth of the same name; then follow the 
remaining teeth in the following order: Lateral incisors, Ist premolars, 2nd 
premolars, canines, 2nd molars, and 3rd molars. It will be observed that the 
eruption of the canine is delayed until the two premolars, which succeed it in the 
row, are cut, so that it breaks the otherwise regular order of eruption. The Ist — 
molar is sometimes popularly known, owing to the date of its eruption, as the “six- 9 
year-old tooth,” and the 2nd molar as “ the twelve- year-old tooth.” ' 
The dates at which the eruption usually takes place may be simply stated as 
follows for the lower teeth ; those of the upper jaw appear a little later :-— . 
Ist molars appear soon after the 6th year. 
Central incisors appear soon after the 7th year. 
Lateral of * ¥ ‘Sth 
Ist premolar = x 9th 
2nd Ae 4 * 10th 
Canine ,, Be . Gh. 
2nd molar % os 2 che 
BIRLA i from the IAdive 5A 160) 
THE MILK TEETH. 
The deciduous, temporary, or milk teeth (dentes decidui) are twenty in number, 
ten above and ten below, or five in each half of each jaw—namely, two incisors, one 
canine, and two molars. They may be distinguished from the permanent teeth 
by their smaller size, their well-marked and constricted necks, and, in the ease of 
the molars, by the wide divergence of their roots (Fig. 656). Otherwise they 
correspond so closely to the same named teeth of the permanent set, that they 
require no separate description, except in the case of the molars. The first upper 
molar has but three cusps on its crown—two external and one internal; the 
first lower molar has four—two external and two internal, and the crowns of both 
