The ventral wall, floor, or basis cranii interna, may be regarded as forming five fossæ. These are: one anterior, one middle, one posterior, and two lateral. The _anterior fossa_ supports the frontal and olfactory parts of the cerebrum. It lies at a higher level than the middle fossa. The _middle fossa_, or _fossa cranii media_, is circular in outline and extends into the lateral fossæ which lodge the olfactory lobes. Just back of the middle fossa is the sulla turcica, upon which rests the pituitary body. The _posterior fossa_, or _fossa cranii posterior_, lodges the medulla oblongata. =Bones of the Cranium= (Figs. 4, 6, 7, and 8).—The bones of the cranium fuse early in the chick’s life. The sutures uniting the bones can usually be seen in the fetus or in the baby chick soon after it emerges from the shell. The major portion of the cranial bones become entirely fused. Each cranial and each facial bone ossifies from a distinct center or centers. The cavity for the cerebrum is much larger than the cavity for the cerebellum. The cranial cavities in birds are relatively larger than in mammals. The bones are designated as in mammals. There are three single bones, the occipital, the ethmoid, and the sphenoid. Those in pairs are the frontal, the parietal, and the temporal. =The Occipital.= _Location._—The occipital bone or os occipitale, is situated at the posterior part of the cranium, of which it forms the posterior wall. This part is called the base of the cranium. The occipital bone articulates with or touches inferiorly, the sphenoid, laterally, the temporal, and superiorly, the parietal. _Development._—The occipital bone is developed from four centers of ossification; the dorsal, or os occipitale superius, two lateral, or ossa occipitales laterales, and the ventral, or os occipitale inferius, all of which may be seen distinct in the baby chick (Fig. 5, Part II, No. 1). [Illustration: FIG. 5.—Bones of the head and vertebra. Part I.—1, The cranium. 2, The skull cap. 3, The ethmoid bone. 4, The inferior maxilla. 5, The palatine bone. 6, The quadrato-jugal. 7, The jugal. 8, The superior maxilla. 9, The premaxilla. 10, The horny covering for the premaxilla. 11, The horny covering for the os dentale. 12, The os quadratum. 13, The nasal bone. 14, The pterygoid. Part II.—1, The os occipitale (1, dorsal; 2, two lateral; 3, ventral portions and 4, foramen magnum). 2, The fourteen cervical vertebræ. (1, atlas; 2, axis). 3, The seven dorsal vertebræ. 4, The fourteen lumbo-sacral vertebræ. 5, The seven coccygeal vertebræ. 6, The pelvis (1, ilium; 2, ischium; 3, pubis; 4, cotyloid cavity or acetabulum). 7, The vertebral column complete from the baby chick. 8, The parietal bone. 9, The temporal bone. 10, The frontal bone. ]