In vertebrates, the upper and lower jaws protrude forward from the head and are covered by a hard, horny sheath. All birds, some reptiles, and a few primitive mammals have them. The skeleton of the upper beak of a bird consists of a pair of premaxillae, maxillae, and nasal bones. These surround a pair of external nostrils, and the internal nostril that leads to the oral cavity opens as a slit in the center of the palate. In parrots and crows, there is a unique hinge joint between the nasal bones and the frontal bone. Source: Heibonsha World Encyclopedia, 2nd Edition Information |
脊椎動物において,頭部の前方へ突出し,表面が固い角質の鞘(さや)に覆われた上下のあごのこと。すべての鳥類,若干の爬虫類,およびごく一部の原始哺乳類がこれをもっている。鳥類の上くちばしの骨格は各1対の前顎(ぜんがく)骨・上顎骨・鼻骨から成る。これらに囲まれて1対の外鼻孔があり,口腔に通ずる内鼻孔は口蓋(こうがい)の中央に裂け目として開く。インコ類やカラス類などでは鼻骨と前頭骨の間に独特のちょうつがい関節がある。
出典 株式会社平凡社世界大百科事典 第2版について 情報 |
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