OCR Text |
Show 1899.] OSTEOLOGY OF THE PYGOPODES. 1025 The mesencephalic fossa agrees with that of the Tubinares, in that the groove for the sinus transversus branch of the vena cephalica posterior (pp. 1021-2) appears as a deep tunnel excavated out of the inner wall of the skull. It runs upwards, backwards, and downwards, following the curve of the anterior semicircular canal, finally piercing the wall of the supra-occipital tunnel wise, leaving the skull by a small aperture on either side of the foramen magnum, but much lower down than in the Impennes or Tubinares. The Pygopodes differ from the two last mentioned groups in that this cephalic vein enters the skull through the trigeminal foramen, and not by a separate aperture. The floor of the fossa bears a deep groove for the orbito-nasal nerve. The pituitary fossa differs conspicuously both from that of the Impennes and of the Tubinares by its peculiar shallowness. In the Colymbi it forms a moderately deep pit sloping gently backwards ; but in the Podicipides it is represented only by a very slight oblong depression, bounded on either side by a strong ridge forming a tunnel for the abducent nerve. This pituitary ridge, from the point where the nerve enters, is continued upwards and outwards, to terminate at the groove for the cerebral vein, already described. This second ridge forms the posterior boundary line of the mesencephalic fossa. There is a well-developed dorsum sellce and prepituitary ridge in the Colymbi, the latter flattened to form an optic platform. The pre-optic, as usual, passes on either side into the tentorial ridge. In the Podicipides there is no dorsum sellce, the internal carotid apertures opening directly on to the floor of the fossa. The optic foramen in the Colymbi is bounded in front by a vertical plate of bone, being that part of the interorbital septum bounding the interorbital fenestra posteriorly. This plate is wanting in the Podicipides. The cerebral fossce in the Divers and Grebes closely resemble one another, they both agree in that this region of the brain-cavity is greatly depressed dorso-ventrally, as in the smaller Tubinares. The tentorial ridge is sharply defined, particularly so in the Divers. The bony falx, which dips down between the pallial fissures of the brain, is not so strongly marked as in the Tubinares. There is only the faintest indication of the bony ridge marking the position of the Sylvian furrow such as is found in the skull of Diomedea, and this is entirely dorsal, and not lateral as in Diomedea. Again, the tentorial ridge of the Pygopodes and smaller Tubinares lies horizontally to the long axis of the skull; whilst in Diomedea, for instance, it is almost vertical in position. The olfactory fossce, like those of the Impennes, are extremely small, and pass insensibly into the cerebral fossae behind; wherein they stand strongly contrasted with the large tubular chambers of the Tubinares. They are divided in the middle line, forwards, by a small knife-like crista-galli. In the Tubinares, it will be remembered, this crista-galli is columnar. |