The occipital bone is originally composed of four pieces: the basilar, below, (a,fig.126,) the two condyloid, laterally, ( b, b,) and the expanded spinous process, or supra-occipital piece above (c). These fulfil the usual functions of the occipital bone, protecting the cerebellum and medulla oblongata, and form ing the medium of connection between the cranial and cervical vertebra.
The head is articulated to the spine by means of a single hemispherical tubercle (x, jig. 126,) which plays in a corresponding cavity of the atlas. In most birds the tubercle is formed exclusively by the basilar piece of the occipital bone, but in the Ostrich and Penguin the condyloid portions also contribute to its formation, which is an approximation to the structure of the occipital condyle in the Chelonian reptiles. In all birds, however, the articulation is such as to allow of a much greater extent and freedom of motion to the head than exists in the Mammalia.
The temporal bone consists of the petrous por tion, the squamous portion, (d, d,fig.126, 127) and the tympanic bone, or os quadratum (e.) The petrous bone includes the complex parts of the internal ear, and is soon anchylosed to the condyloid portions of the occipital bone, which fulfil the functions of the mastoid pro cesses. The squamous, or, as it may be termed, the zygomatic portion of the temporal bone (d) remains for a longer time separate; it forms the lateral boundary of the cranial cavity, as in quadrupeds, and the tympanic element is move ably articulated to its inferior part.
The parietal bones (f, ,f, fig. 127) retain their separated condition till after the union of the occipital pieces, they then unite and protect the posterior part of the cerebral hemispheres.
The sphenoid bone is composed of a basilar portion, (g,fig. 126,) two orbital plates, (h,fig. 127,) forming the floor and part of the septum of the orbits, and which rapidly anchylose with the preceding; two cranial portions, or alw ma jores, (g, fig. 127, 128,) which remain longer separate, and form the posterior part of the or bits, and two pterygoid portions (‘ interarticular ' or omoid ' bones), (i, i, fig. 126,) which, in birds, abut against the tympanic or quadrate bones. The great aim of the sphenoid join the parietal, and separate the temporal from the frontal bones.
The frontal bone (k, fig. 127) continues for a longer period than the parietal to be sepa rated into two lateral halves by the continuation of the sagittal suture through its whole length.
The ant-orbital processes (13, Jig. 127) are elongated and pointed, ex tending for wards to join the lachrymal bones, (o, o, fig. 127,) con siderably be yond the ori gins of the nasal bones, and are separated from each other by these and by a process of the ethmoid bone, The post-orbi tal processes are most de veloped in the Parrots and Maccaws, in the latter of which they join the lachrymal bones, and complete the bony circumference of the orbits, (fig. 128.) In the Emeu they remain for a long time distinct bones, as in the reptiles. The frontal bone thus forms the whole of the superior, and, more or less, of the outer boundary of the orbits, and protects the anterior part of the cerebrum. It supports the horn-like prominences which are seen upon the heads of the Cassowary, Pintado, and Cu rassow, the bony bases of which commence by distinct ossifications.° A small part of the ethmoidal bone (/, fig. 127) is seen, in the Ostrich, on the ex terior of the cranium lodged between the ant-orbital processes and nasal bones (n, n.) The ethmoid separates, as usual, the orbits from the cavity of the nose, and forms a great part of the inter-orbital septum where this exists, as in the parrots, (m, fig. 128.) In the mature bird the whole of the prece ding bones, with the exception of the tym panic elements of the temporal bone, are usually found anchylosed into one piece.
The internal surface of the cranium exhibits a well-marked transverse ridge, which divides the cavity into two principal depressions. In the anterior division the hemispheres of the cerebrum are lodged, the rest of the brain is contained in the posterior division. The relative proportion of these divisions varies in the different orders; in the Insessores and Ac cipitres the anterior superior depression is the largest; in the Rasores, the posterior inferior depression equals, and in some species, ex ceeds the former in size. The orbits form two slight projections in the anterior fossa of the cranium, which is partially divided longitu dinally by a ridge corresponding to the inter space of the cerebral hemispheres. This is developed in the Gallinaceous birds into a thin falciform osseous crest, which is especi ally remarkable in the Partridge, Turkey, and Capercailzie. It is also well developed in the Parrot tribe. The sella turcica in all birds is a deep round cell, lodging the pituitary gland, as in the Mammalia.