Identifier: dentalcosmos3118whit (find matches)
Title: Dental cosmos
Year: 1889 (1880s)
Authors: White, J. D McQuillen, J. H. (John Hugh), 1826-1879 Ziegler, George Jacob, b. 1821 White, James William, 1826-1891 Kirk, Edward C. (Edward Cameron), 1856-1933 Anthony, Lovick Pierce, 1877-
Subjects: Dentistry
Publisher: Philadelphia : S. S. White Dental Manufacturing Co
Contributing Library: The College of Physicians of Philadelphia Historical Medical Library
Digitizing Sponsor: The College of Physicians of Philadelphia and the National Endowment for the Humanities

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for instance, the line of this force is almostexactly the line of the palatal roots, which have been well calledthe t4 props of the teeth. Plates Q and E.—The Movements of Occlusion. Plates Q and E are made from casts of natural teeth, which aremounted on the jaws to further show the relations of the teethbefore described, and more especially the movements which occurin occlusion and mastication. The superior jaw being fixed and the inferior movable, it is obviousthat if the latter were pivoted exactly opposite the line of occlusionits movements in occlusion would be extremely simple. Preciselylike the swinging of a door on its hinges, every portion of it woulddescribe an arc of a circle. In the jaw, however, the hinge is placedat a point an inch to an inch and an eighth above this line, thebody of the jaw carrying the teeth being suspended at nearly aright angle to the ramus by which it is hinged to the glenoid cavity.This disposition of the parts modifies the movement. Instead of
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612 THE DENTAL COSMOS. swinging in the arc of a circle, the body of the lower jaw is pro-jected forward as it closes, and falls back as it opens,—a movementwhich is exactly adapted to bring into play in mastication the advan-tages of the arrangement of the teeth before described. Henceresults the shear action of the incising teeth. The effectiveness ofthis movement in the cutting of food as compared with that whichwould result if the jaw moved in the arc of a circle is well illustratedby the different effects produced by pressing a sharp knife-bladeagainst a piece of wood, and by drawing it across the wood withthe same pressure. This shear action is observable in the bicuspids,in that the labial edges of their cusps are cutting-edges, and it alsoassists the molars in the discharge of their office. Another movement of the jaw—rotatory or oscillatory from sideto side—is due to the form of the hinge. The condyle forms a ball-and-socket joint with the glenoid cavity, giving the jaw

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