. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Ports of Entry 779 Sinus internasal septunv coiled concha mouth cavity. Mammals show traces of Jacobson's organ embryonically, but in most instances it is a degenerate structure, its best manifestation being in monotremes, marsupials, insectivores, and rodents. It is entirely absent in whales, bats, and Old World monkeys. The history of Jacobson's organ in man is wholly intra-uterine. Arising at the beginning of the third month as small outpocketings of ep
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Ports of Entry 779 Sinus internasal septunv coiled concha mouth cavity. Mammals show traces of Jacobson's organ embryonically, but in most instances it is a degenerate structure, its best manifestation being in monotremes, marsupials, insectivores, and rodents. It is entirely absent in whales, bats, and Old World monkeys. The history of Jacobson's organ in man is wholly intra-uterine. Arising at the beginning of the third month as small outpocketings of epithelium in the lower part of the nasal septum, it forms a slender blind sac on either side, reaching its greatest elaboration at about the fifth month. Before birth it becomes entirely reduced. This struc- ture, which Kingsley refers to as "a kind of an olfactory organ," probably serves as an accessory olfactory apparatus for testing odorous substances held in the mouth. The enlargement of surfaces for the exposure of epithelium within the nasal chambers is brought about in three ways: by the folding of the mucous membrane; by sinuses in communication with the main nasal cavities; and by conchae, or skeletal shelflike extensions of the nasal walls. The method of membranous duplication is common among fishes. The mucous lining of the nasal pits of the dogfish, for example, bears a distant resemblance to the leaves of a book because of its numerous folds. Such a device would not be effective out of water, for the folds would tend to adhere together when not kept separate by immersion in an aqueous medium. Reptiles have a single unrolled concha, or projection from the ectethmoidal wall of the nasal chamber on either side, which is slight in turtles, but of considerable size in crocodiles and alligators. The nasal chamber of birds is compressed, in accordance with the general policy of compact- ness that characterizes avian anatomy. Its sur- faces are compensatingly incr
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