OSTEOLOGY 25 



The ventral wall, floor, or basis cranii interna, may be regarded 

 as forming five fossae. 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, ot fossa cranii media, is circular 

 in outline and extenfls into the lateral fossae which lodge the ol- 

 factory lobes. Just back of the middle fossa is the sulla turcica, 

 upon which rests the pituitary body. The posterior fossa, ot 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, 

 aE of which may be seen distinct in the baby chick (Fig. 5, Part II, 

 No! i). 



Description. — In the center of the occipital bone is the foramen 

 magnum. Through this foramen the spinal cord extends into 

 the cranial cavity and connects with the medulla oblongata. The 

 occipital bone has a single condyle, which is located just below the 



4 



process of sternum. 43, Os fibtJa. 44, Zyphoid process of sternum. 45, In- 

 ternal notch. 46, Os patella. 47, Os tibia. 48, Os metatarsus. 49, Second 

 toe. so, Fourth toe. 51, Os metatarsale. 52. First toe. 53, Second toe. 54, 

 Second phalanx of second finger. S5> Os meta carpus. 56, Air opening in 

 humerus. 57, Hypocledium. 58, External notch. 59, Cervical vertebrae. 60. 

 Foramen oblongum. 



