The American textbook of prosthetic dentistry ... / edited by Charles R. Turner.

Date:
1907
Catalogue details

Licence: Public Domain Mark

Credit: The American textbook of prosthetic dentistry ... / edited by Charles R. Turner. Source: Wellcome Collection.

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    mandible. The genio-hyoid and the anterior belly of the digastric are inserted into its inner surface near the median line; the mylo-hyoid being attached to the ridge of that name. They operate from the hyoid bone, which must first be fixed by the omo-hyoid, sterno-hyoid, and thyro-hyoid muscles. The platysma myoides is attached below to the sternum and clavicle as its fixed base. Fig. 10.—Median sagittal section of head, showing relation of structures in oral cavity when the mouth is closed. (Cryer.) Under ordinary conditions when the mouth is closed (Fig. 10), the muscles are in repose and there is an absence of any active contraction on their part. The mandible is maintained in position partly by the tonicity of the levators, their length, however, in an ordinary state being too great to keep the jaws in contact. This is assisted by the influence of a certain negative intra-oral air-pressure amounting to ‘about 2 to,4 mm. of mercury (Donders) which “is formed by the con- traction of the muscles of the tongue and by the sinking of the mandible. The closed space arises from the closure of the lips, and the placing of
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