(2) The facial portion.
(3) The mandible.
(4) The hyoid.
(1) The Cranial portion.
This is a rounded box expanded dorsally and posteriorly, but tapering antero-ventrally. In the young skull the divisional lines between the several bones can be easily seen, but in the adult they are quite obliterated.
(a) The dorsal surface is rounded, expanded in front and behind, but encroached upon in the middle by the cavities of the orbits. There is a prominent divisional line in front, separating it from the facial part of the skull. It is formed mainly by the frontal (fig. 59, A, 6) and parietal bones, but the frontals diverge a little anteriorly and enclose between them the ends of the nasal processes (fig. 59, A, 4) of the premaxillae. Just in front of the orbit the outer margins of the frontals are either notched or pierced by a pair of foramina.
(b) At the posterior end of the cranium the most prominent feature is the large, almost circular foramen magnum, through which the spinal cord and brain communicate; this in young birds is seen to be bounded by four distinct bones, dorsally by the supra-occipital, ventrally by the basi-occipital, and laterally by the exoccipitals.
The basi-occipital forms the main part of a prominent convex knob, the occipital condyle, with which the atlas articulates. The occipital condyle is slightly notched above, and the ventral surface of the cranium is deeply pitted just in front of it; the exoccipitals also contribute slightly to its formation. Slightly in front of and ventral to the foramen magnum is a small foramen through which the hypoglossal nerve leaves the cranial cavity.
The supra-occipital is separated from the parietal by a suture line along which run a pair of prominent ridges, the lambdoidal crests (fig. 60, B, 30). There are often a pair of prominent vacuities in the supra-occipital dorsal to the foramen magnum. The epi-otics and opisthotics become completely fused with the bones of the occipital segment at a very early stage.
(c) The ventral surface of the cranium is wide behind, where it is formed by a broad transverse membrane bone, the basitemporal (fig. 60, A, 23), the sides of which are fused with the auditory capsules. Slightly in front of and an eighth of an inch external to the hypoglossal foramen the cranial wall is pierced by a pair of foramina through which the tenth or pneumogastric nerves leave (fig. 60, A, X). At the sides of the basitemporal are a pair of depressions, the tympanic recesses, in each of which are three holes. Straight lines joining these holes would form an isosceles triangle with its apex directed forwards. Of the two holes at the base of the triangle, the one nearer the middle line and leading into the cranial cavity, is for the exit of the ninth or glossopharyngeal nerve (fig. 60, A, IX), it lies just in front of the pneumogastric foramen. The more external leads into the tympanic cavity, while the more anterior at the apex of the triangle is the posterior opening of the carotid canal (fig. 60, A, 25), which traverses the base of the cranium, and during life lodges the carotid artery.