. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. THE SKULL 69 cartilaginous skull (chondrocranium) itself, which is thus more or less completely replaced by an osteocranimn. In general the higher the systematic position of the animal, the less extensive are the cartilaginous constituents and the more important the bony. I. The Visceral The primarily cartilaginous visceral arches encircle the anterior section of the alimentary canal, lying embedded in the inner part of the walls of the throat (Figs. 52 and 53) and usually becoming ossified latter. They are
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. THE SKULL 69 cartilaginous skull (chondrocranium) itself, which is thus more or less completely replaced by an osteocranimn. In general the higher the systematic position of the animal, the less extensive are the cartilaginous constituents and the more important the bony. I. The Visceral The primarily cartilaginous visceral arches encircle the anterior section of the alimentary canal, lying embedded in the inner part of the walls of the throat (Figs. 52 and 53) and usually becoming ossified latter. They are always present in a greater number (up to as many as nine) in forms which possess gills than in higher types (Amniota), in which they gradually be- come reduced, and may undergo a change of function, certain of them in some cases taking on definite relations to the auditory organ and larynx. The most anterior arch, serving as a support for the walls of the mouth and receiving its nerve supply from the trigeminal, arises first, and is distin- guished from the other or post-oral arches as the mandibular arch. The post-oral arches only function as gill- bearers in the Anamnia: even the first of these, the hyoid, which is supplied by the facial nerve, becomes modified from those lying behind it: the latter, or branchial arches proper, are supplied by the glossopharyngeal and vagus. All the visceral arches must originally, how- ever, have borne gills. Primarily unsegmented, the indi- vidual post-oral arches may become broken up into as many as four pieces, of which the uppermost becomes inserted under the base of the skull, while the lowermost is connected with its fellow by a median basal piece (Fig. 52). The mandibular arch also undergoes segmentation, and becomes divided into a short proximal piece, the quadrate, and a long distal mandibular or Meckel's cartilage (Fig. 53). The quadrate grows out ant-feriorly into a process, the palatoqimdrate or palatofterygoid, which usually be
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
