OSTEOLOGY 23 



The air reservoirs in bones are most capacious in the best flyers. 

 In the non-flyers more of the bones retain their red marrow. 



The bones supplied with air spaces are relatively larger than 

 in mammals, and are provided with small transverse osseous columns 

 which cross in different directions and from side to side. These cross 

 beams give stability to the thin wall of the bone. The membranes 

 lining these cavities are very vascular. 



THE AXIAL SKELETON 

 THE SKULL 



The skull is divided into the cranial and facial portions. In 

 these parts we find present 31 bones: one occipital, two parietal, two 

 frontal, one ethmoid, one sphenoid, and two temporal; all of which 

 constitute the cranial group; two premaxillary, two maxillae, two 

 nasal, two lacrimal, two palatine, two pterygoid, two zygomatic, 

 one vomeral, the two jugal, and two quadrato-jugal, which constitute 

 the facial group; two quadrati and one inferior maxillary, which 

 constitute the inferior jaw group. 



The peculiarities of the skull are the long os incisivum and the 

 single condyle located on the occipital bone just below the foramen 

 magnum. The condyle articulates with the atlas. 



The head of the bird is small in proportion to the size of the 

 body, and in front it is conical in shape. 



THE CRANIUM 



The cranial cavity, or cavum cranii, incloses the brain with its 

 membranes and vessels. 



The dorsal wall, or roof, is formed by the frontal and the parietal 

 bones. In the median line of the cerebral portion is the internal 

 parietal crest. The roof of the cerebellar portion is marked centrally 

 by a groove. 



The posterior wall of the cerebellum is formed by the occipital 

 bone. 



The lateral wall is formed chiefly by the temporal bone. It is 

 marked by a ridge which divides the cavity into the cerebral and 

 cerebellar compartments. The cerebral portion is marked by a 

 depression which receives the optic lobes. A crest divides this 

 cavity from the optic portion. The walls are marked by digitations 

 and vascular grooves. 



