. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. The Skeleton 589 Premaxi I la ry ^--Ti-j Maxillary- -rj Lacrimal—r Nasa velum that shuts off the posterior openings of the nostrils (choanae) from the surrounding water, thus enabling the reptile to breathe through the choanal passage-way with the tip of the snout out of water, while the mouth is kept submerged until the struggling prey within, held fast by the conical teeth, is drowned (Fig. 350). (h) Birds.—In birds the most characteristic feature of the
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. The Skeleton 589 Premaxi I la ry ^--Ti-j Maxillary- -rj Lacrimal—r Nasa velum that shuts off the posterior openings of the nostrils (choanae) from the surrounding water, thus enabling the reptile to breathe through the choanal passage-way with the tip of the snout out of water, while the mouth is kept submerged until the struggling prey within, held fast by the conical teeth, is drowned (Fig. 350). (h) Birds.—In birds the most characteristic feature of the skull, like that of the entire skeleton, is the elimination of every bit of bone that can be spared (Fig. 505). Compared with reptiles, from which birds arose, a typical avian skull is more spherical and roomier. The relatively enormous size of the eyes, together with the well-developed interorbital septum, tends to restrict the brain to the pos- terior part, while the ear-capsules on either side are more deeply em- bedded than in reptilian skulls. Notwithstanding the fact that many typical bones, both replacing and investing, are distinguishable in the embryo, they become welded to- gether in the adult by the early closure of the sutures into an almost continuous capsule of bone, which is remarkably strong and protective in spite of its lightness and the paucity of bony material employed. The zygomatic arch is reduced to a slender bar, the postfrontals disap- pear, and the premaxillaries, mamil- laries, palatines, and pterygoids are so narrowed that they fail to join into a hard palate. The lightness of the skull is furthermore enhanced by the pneumaticity of the bones, characteristic of the entire skeleton, and also by the substitution of a horny beak in place of heavy teeth set in the jaws. Only in the interorbital septum and in the ethmoid region does cartilage persist. Remains of the investing parasphenoid fuse with the replacing basi- sphenoid to form a unique, forwa
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Keywords: ., bookcentury1900, booksubjectanatomycomparative, booksubjectverte
