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EMBRYOLOGY .
The See also:lip is marked off from the See also:rest of the mouth region by a " lip groove," which, in the See also:case of the See also:lower See also:jaw, grows obliquely down-See also:- WARD
- WARD, ADOLPHUS WILLIAM (1837- )
- WARD, ARTEMUS
- WARD, EDWARD MATTHEW (1816-1879)
- WARD, ELIZABETH STUART PHELPS (1844-1911)
- WARD, JAMES (1769--1859)
- WARD, JAMES (1843– )
- WARD, JOHN QUINCY ADAMS (1830-1910)
- WARD, LESTER FRANK (1841– )
- WARD, MARY AUGUSTA [MRS HUMPHRY WARD]
- WARD, WILLIAM (1766-1826)
- WARD, WILLIAM GEORGE (1812-1882)
ward and backward, and the See also:mass of ectodermal cells bounding itpenetrates for some distance into the surrounding mesoderm below the bottom of the groove. This is known as the " tooth See also:band."
On the under See also:surface of this oblique tooth band (still taking the lower jaw), and See also:close to its edge, appear ten thickenings, below each of which the mesoderm rises up into a " dental papilla," and so moulds the thickening into a cap for itself—the " See also:enamel See also:organ." The superficial cells of the dental papilla become the " odontoblasts " and manufacture the dentine, while those cells of the cap (enamel organ) which are on its See also:concave surface and therefore nearest the dental papilla are called " ameloblasts," and See also:form the enamel. The cutting or grinding See also:part of the tooth is first formed, and the See also:crown gradually closes See also:round the dental papilla, so that at last, when the See also:root is formed, the central part of the papilla remains as the pulp cavity surrounded by dentine except at the See also:apex of the root. The roots, however, are formed slowly, and as a See also:rule are not See also:complete until some See also:- TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
- TIME, MEASUREMENT OF
- TIME, STANDARD
time after the tooth is cut. The mesoblastic connective See also:tissue surrounding the developing tooth becomes condensed into a fibrous bag which is called the tooth-See also:sac, and round this the lower jaw grows to form the alveolus. The crusta petrosa which covers the root is See also:developed from the tooth-sac. It will therefore be seen that, of the various structures which make up a tooth, the enamel is derived from the ectoderm, while the dentine, pulp and crusta petrosa or See also:cement are mesodermal.
So far only the See also:milk dentition of the lower jaw has been accounted for.
Returning to the tooth band, it was noticed that the enamel See also:organs were formed not at the extreme edge but a little way from it. From the extreme edge, which, it will be remembered, points inward toward the See also:tongue, the permanent tooth germs are derived, and it is therefore clear that the permanent See also:teeth must come up on the lingual See also:side of their milk predecessors.
For further details and literature see Dental See also:Anatomy, by C. S. Tomes, See also:London, 1904; and Development of the Human See also:Body, by J. P. McMurrich, London, 1906.
End of Article: EMBRYOLOGY
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