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Encyclopedia Britannica - Main :: TAV-THE |
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TEETH (O.E. Eel); plural of tooth, O.E. top) , the modified papillae or elevations of the mucous membrane of the mouth, impregnated with lime salts. Each tooth has a biting part or crown covered by enamel, a neck where the gum surrounds it, and one or more roots or fangs fitting into sockets (alveoli) in the jaw bone. For surgery of the teeth see DENTISTRY. There are thirty-two permanent teeth in man, sixteen in the upper and sixteen in the lower jaw; they are also arranged in symmetrical sets of eight teeth on each side. The two teeth on each side of the mid-line in front are " incisors " and have chisel-shaped crowns. The mesial or central incisor of the upper jaw is broader than any of the others, consequently it bites against the central and lateral incisors of the lower jaw, and the same want of exact adaptation continues throughout the series , so that every tooth in the upper jaw except the last molar bites against its corresponding tooth of the lower jaw and the tooth behind that.ninth month, or even later; then, after a few months, come the central and lateral upper incisors; again a few months' and the lower lateral incisors appear, followed closely by first molars. After another rest of four or five months come the canines, the eruption of which is a slow process, while by about the end of the second year 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 between. As C. S. Tomes points out, we do not know what causes the eruption of the teeth; the growth of the roots is not of itself enough to account for it. It is possible, however, that blood-pressure may be the determining cause. The first permanent tooth to be cut is the first molar, and this happens during or soon after the sixth
The lateral incisors succeed their milk predecessors at about eight years old, the first premolar takes the place of the first temporary molar about nine, the second pre-molar that of the second temporary molar about ten, Fro. r.The Permanent Teeth of the Right Side, Inner or the canine about eleven, while the second molar comes Lingual Aspect. down behind the first about twelve, and so is known as The upper row shows the upper teeth, the lower row the lower teeth. The the "twelve-year-old tooth." The third molar, or wisdom cingulum is distinct on the upper incisors and both canines, the lingual tooth, usually appears between eighteen and twenty, but cusp on the upper lateral incisor and the upper canine. 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
1st molar 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 whichis 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 fig. r), 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 gene-rally 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
Canine Lateral incisor 1 A Central incisor 1 in molar From Ambrose Birmingham, in Cunningham's Text-Book of Anatomy. 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. In the specimen from which the first upper molar was drawn
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
and molar Central incisor Canine sand premolar 2nd molar From Ambrose Birmingham, in Cunningham's Text-Book of Anatomy. Lateral incisor 1st premolar rest the .Central incisor Canine and premolar Lateral incisor zst premolar ist molar 3rd molar F I 11 11 1st molar 2nd molar crown Central incisor Lateral incisor Canine and their crowns look backward. It is owing to the gradual growth backward of this antrum and the maxilla that they are rotated round a quarter of a circle and so at last look down- ward (see A. Keith, British Journal of Dental Science, vol. xlv., June 16, 1902). Extra teeth are occasionally met with in the incisor, premolar and molar regions; their significance will be better realized after the embryology and comparative anatomy of the subject have been sketched. For an accurate and detailed description of man's teeth see A Manual
End of Article: TEETH (O.E. Eel); plural of tooth, O.E. top) If you wish, you can link directly to this article.
<a href="http://jcsm.org/StudyCenter/Encyclopedia/TAV_THE/TEETH_OE_Eel_plural_of_tooth_O.html"> TEETH (O.E. Eel); plural of tooth, O.E. top) </a> |
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