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 with the sternum. Ifi flying 

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

 nected by a?i interclavicle, 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 Archsopteryx), 



-Position of Organs in li Bird. (After 

 Selenka.) 



n.. Nostrils ; tr., trachea; cr., crop ; h., heart ; St., sternum ; pr., 

 proveiitriculus ; ^., gizzard ; c, caica ; /., pygostyle ; /7',, 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 pubes (or post-pubic 



