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Show 1899.] OSTEOLOGY O F T H E PYGOPODES. 1029 The exoccipital.-Externally it is bounded supero-internally by the supraoccipital, and supero-externally by that portion of the prootic cartilage which lodges the floccular fossa. Its internal border is < -shaped. The posterior limb is free and bounds the foramen magnum, the anterior runs along the outer border of the basioccipital. Its external border has fused with the prootic cartilage, the boundary between the two being indicated by a notch superiorly. The vagus foramen pierces it near its centre. Internally, it is largely concealed by the opisthotic, only its inner half being visible. The vagus foramen appears on this side at the base of the opisthotic, and indicates how much of the exoccipital is concealed by this bone. The supraoccipital is cleft in the middle line superiorly for more than half its length. Its ventri-lateral border is fused with the epiotic, but the distinction between the two bones can yet be made out. Its internal does not differ much from its external form. The prootic appears externally in the form of an oblong mass of cartilage separating the squamosal from the anchylosed opisthotic and exoccipital bones. Internally its size is seen to be considerable. Supero-dorsally it is notched to form the inferior border of the floccular fossa. Its outer half lies immediately in front of the squamosal, which bone it almost entirely shuts out from the inner surface of the cranium, only a small semicircular strip of about \ m m . being visible, and forming the floor of a groove separating the prootic from the parietal and alisphenoid bones. Its antero-ventral border is linguiform and imbedded in a mass of cartilage. Its inner lateral border is in part (superiorly) fused with the opisthotic and in part(inferiorly) imbedded in cartilage in common with the linguiform anterior end. The meatus internus is very deep. The epiotic is only ossified at its junction with the supraoccipital; the rest of the posterior vertical semicircular canal is yet cartilaginous, and forms the superior boundary of the floccular fossa. The opisthotic has almost completely fused with the prootic, but traces of the original suture still remain. It fuses posteriorly wdth the exoccipital. The vagus foramen passes between its autero-internal border and the exoccipital in front. There is no trace of the opisthotic visible externally. The basisphenoid is not visible externally, being uuderfloored by the parasphenoidal rostrum and its basitemporal plate, with which furthermore it has now completely fused. Below the pituitary fossa, and that portion of the basisphenoid immediately behind it, is a large air-sinus, and this forms the only indication of the division between the para- and basisphenoidal regions. The pituitary fossa forms a moderately deep pit. The dorsum selloe is yet membranous. In the adult the pituitary fossa seems to have become almost obliterated (p. 1025V Immediately behind the pituitary fossa the basisphenoid is marked by a deep < -shaped notch, dividing a median portion from a pair of lateral wings. The membrane stretched between the two wings forms the dorsum |