GENERAL CHARACTERS. 597 



bones have a spongy texture, due to cavities filled with air from 

 the nasal and Eustachian tubes. 



There is a well-developed sternum, generally with a keel, to 

 which the pectoral muscles are in part attached. The strong 

 coracoids reach and articulate ivith the sternum. In flying 

 birds, the clavicles are well developed, and are usually con- 

 nected by an inter clavicle, which is often fused to the top of the 

 breast bone. The fore-limb has not more than three digits, 

 the three metacarpals are fused (except in Archseopteryx), 



P'iG. 206. Position of Organs in a Bird. (After 

 SELENKA.) 



., Nostrils ; tr., trachea; cr., crop ; ^., heart ; st., sternum ; pr., 

 proventriculus ; -., gizzard ; c., caeca ; /., pygostyle ; pv., pelvis ; k., 

 kidney ; /., lung. 



and there are only two separate carpals, the others fusing 

 with the metacarpals, and thus forming a carpo-metacarpus. 

 The metacarpals and digits bear the primary feathers or 

 quills. 



The ilia of the pelvis are firmly fused to the complex sacrum ; 

 the acetabulum is incompletely ossified ; the pub es (or post-pubic 



