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  Section: Anatomy of Vertebrate Animals » The Classification and the Osteology of Birds
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The quadrate bone


The quadrate bone is almost always movable upon the skull, articulating with the prootic, alisphenoid, and squamosal, by a single, or double, head. The distal head articulates with the mandible below, the quadrato-jugal on the outer, and the pterygoid on the inner, side. Hence, when the ethmonasal joint is developed, any forward movement of the distal end of the quadrate, such as must take place when the mandible is depressed by the digastric muscle, causes the maxillo-jugal bar to thrust the premaxilla upward and forward; the palatine and pterygoid bones, at the same time, sliding forward upon the rostrum of the basisphenoid. Thus it comes about that the upper jaw of such a bird as a Parrot rises, when, in opening the mouth, the mandible is depressed. Each ramus of the mandible consists primitively of six pieces, as in other Sauropsida, but the dentary pieces of each side are, as in the Chelonia, very early united, if indeed they are not ossified from one centre. Very often, a fontanelle remains between the dentary and the other elements, as in Crocodilia; and the dentary long remains readily separable from the rest; or, as in the Goatsuckers, is united with the others only by fibrous tissue, so that it is movable. The angle of the mandible may be truncated or produced backward into a long curved process, as in Fowls (Fig. 83), Ducks, and Geese.

The hyoid is composed of basal elements, the anterior of which, usually composed of two portions, lies in the tongue; and of two short, anterior, and two long, posterior, cornua, which are never united with the periotic region of the skull, and commonly remain quite free. In some of the Woodpeckers, however, the long posterior cornua are immensely elongated, and curved upward and backward over the skull (the frontal bones being grooved to receive them), and their free ends are inserted between the ascending and maxillary processes of the right premaxilla.


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