Page:EB1911 - Volume 26.djvu/529

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Next to the incisors comes the “canine tooth, ” the crown of which is somewhat peg-shaped, while. behind this are the two “ premolars ” or “ bicuspids, ” whose crowns are flattened from before backward and bear two cusps, the larger of which ninth month, or even later; then, after a few months, come the central and lateral upper incisors; again a few months rest and the lower lateral incisors appear, followed closely by the first molars. After another rest of four or five months come the canines, the eruption of which is a slow c°“"“1i“°i$°', C""i“° 2“d P"°“"°[a' “nd °l“"' process, while by about the end of the second year L°=¢fe1f°°iS°f “*P"°“'°l'“' i 's"“°1” 3'd "'°l” the second molars have appeared, and the milk dentition is complete. It will be seen from the above that ~ the milk teeth are cut in batches with resting intervals A between.

A        '  F/   As C. S. Tomes points out, we do not know what of -:-' causes the eruption of the teeth; the growth of the ~ '  ” f   '~ *ga roots is not of itself enough to account for it. It is i, ,, 5, '        possible, however, that blood-pressure may be the if  .. ig -» ' 'Y  Wi* determining cause. The first permanent tooth to be l

cut is the first molar, 'and this happens during or soon - M 5 ' 3 after the sixth year. It does not displace any of the if % Q)    ” 'env', milk teeth, but comes down behind the second milk Q(, A "   .' li   ""l§ molar. During the seventh year the central milk 1    T 3 '“l    incisors fall out and their place is taken by the per~ 2 it   1 ” crown, all the root having been absorbed, though not, as might be thought, owing to direct pressure of the succeeding tooth.
 * f, - A  manent ones; the shed teeth are mere shells of the

The lateral incisors succeed their milk predecessors at Lateral incisor rs: premolar:sz molar 3rd molar about eight years old, the first premolar takes the place Central incisor Canine and premolar and molar From Ambrose Birmingham, in Cunninghanfs Texl»Book of Anatomy. FIG. I.-The Permanent Teeth of the Right Side, Inner or Lingual Aspect.

The upper row shows the upper teeth, the lower row the lower teeth. The cingulum is distinct on the upper incisors and both camnes, the lingual cusp on the upper lateral incisor and the upper canine. is the external or labial cusp, while the smaller is the internal or lingual. As a rule there is a single root, though sometimes in the first upper premolar it is double. The three “ molars ” are placed behind the premolars, and the upper and lower sets can be easily distinguished because the upper have three roots-while the lower have only two. Of the three roots which the upper molars bear two are lateral or external and one mesial (see ig. 1), so that it is easy to tell the outer from the inner side of an upper molar. The front can, as a rule, be identified by the fact that the roots are generally bent a little backward at their tips, and this applies to other teeth than the upper molars. In the lower jaw, owing to the two fangs being anteroposterior, it is not possible to tell the lateral from the mesial surface of the molars by them, although the backward inclination of their tips shows the front from the back. When it is remembered that the upper teeth overlap the lower externally it is reasonable to expect that the lower molars would 'show some rounding due to wearing away of the edge of the crown on the outer side, and this is the case. The grinding surface of the crowns of the upper molars shows three or four cusps, while on that of the lower four or five are found.

Of the three molars the first is the largest, and the third, or wisdom tooth, the smallest, while the upper wisdom tooth is smaller than the lower.

In the “ milk teeth ” or temporary dentition of the child there are only twenty teeth, ten in each jaw and five in each segment. They are two incisors, one canine, and two so-called molars. These molars occupy the position which the permanent premolars later on take, and it is held by many that the adult molars really belong to the milk dentition, although they cannot appear until the jaw has grown backward sufficiently far to make room for them. The temporary teeth differ from the permanent in their smaller size, their whiter colour, the greater constriction of their necks, and in the fact that the roots of the molars are widely splayed.

The dates at which the milk teeth are cut are very variable. The lower central incisors come first between the sixth and of the first temporary molar about nine, the second premolar that of the second temporary molar about ten, the canine about eleven, while the second molar comes down behind the first about twelve, and so is known as the “ twelve-year-old tooth.” The third molar, or wisdom tooth, usually appears between eighteen and twenty, but may be much later, indeed it is sometimes never cut at all, and when it is, it often does not come down to a level with the other teeth. It is believed that man is gradually undergoing a suppression of his last molar teeth, which, if the process continue, will lead to our successors having a different dental formula from our own. It is interesting to notice that 1st molar

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Canine. Q end molar.

Lateral incisor '   Crow"

Central ipcisor.

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f  k d Ian

mo ar

-      .- 2nd molar

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Central incisor i d

Lateral in 'isor I. 2" mo ar

L Canine    crown


 * st molar

From Ambrose Birmingham, in Cunningharn's TerL~Book aj Anatomy. FIG. 2.*Th€ Milk Teeth of the Left Side. The masticating surfaces of the two upper molars are shown above. In the second row the upper teeth are viewed from the outer or labial side. In the third row the lower teeth are shown in a similar manner; and below are the masticating surfaces of the two lower molars. Ir1~the specimen from which the first upper molar was drawn the two outer or buccal cusps were not distinctly separated, as is often the case. in some of the lower races of mankind the last molar tooth is nearly as large as those in front of it, and this is the case in the anthropoid apes. A. Keith and D. Braden Kyle have pointed out that the second and third molar teeth are successively formed in the posterior wall of the maxillary antrum