A manual of the anatomy of vertebrated animals / by Thomas H. Huxley.

Date:
1871
Catalogue details

Licence: Public Domain Mark

Credit: A manual of the anatomy of vertebrated animals / by Thomas H. Huxley. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    read a.s.c, anterior, and p.s.c, postei'ior semicircular canal; x, the parasphenoid ; y, the basisphenoid ; Vo, the vomer; F., the pituitary fossa. sphenoidal and orbitosphenoidal regions commonly, but not always, remain unossified. In most osseous fislies, the base of the skull in front of the basisphenoid is greatly compressed from side to side, and forms an interorbital septum {I. Or.). The anterior moiety of the cranial cavity is consequently reduced to a comparatively narrow passage above the septum (Fig. 45). In the Siluroid and Cyprinoid fishes, however, this septum is not formed, and the cranial cavity is of nearly equal size throughout, or gradually diminishes forwards. The ethmoidal cartilage usually remains unossified, but sometimes, as in the Pike, ossification may take place in it. (Fig. 44, 3, 3.) The antorbital, or lateral ethmoidal, processes of the primordial cranium ossify, and give rise to the prefrontal bones (Prf.). The postorbital processes also ossify as postfrontals {Ptf.). The ui)per and posterior part of the primordial cranium exhibits five processes—one postero-median, two posterolateral, and two postero-external. The postero- median ossifies as part of the supra-occipital {8.0.) The posterolateral ossifies as part of the epiotic (Ep.O.), which lies upon the summit of the superior vertical semicircular canal. The postero-external closely corresponds with the squamosal of the higher Vertebrata in position; but, as a cartilage bone, it corresponds with an ossification of the capsule of the ear, called pterotic in the higher Vertebrata. Not unfrequently, as in the Cod, for example, the opisthotic [Op.O.) is a distinct bone, and enters into the formation of the postero-external process. The pro-otic {Pr.O.) is always a well-developed bone, and occupies its regular place, in front of the anterior vertical semicircular canal, and behind the exit of the trigeminal nerve. In addition to these cartilage bones, the brain-case of osseous fishes is additionally defended by numerous mem- brane bones. These are, on the roof of the skull— 1. The parietal bones {Pa.), which sometimes meet in a