GENERAL CHARACTERS OF BIRDS. 



595 



-the articular — which works on the quadrate. Many of 

 the skull 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, 

 with a separate centre of ossification, to which the pectoral 

 muscles are in part attached. The strong coracoids reach 

 and articulate with the sternum. In flying birds the clavicles 

 a7'e usually well developed, and connected by an interclavicle, 



>» /*> t : ■** n - 



c g. pr. 



Fig. 258. — Position of organs in a bird. — After Selenka. 



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

 proventriculus ;&., gizzard ; c, cffica ;/., pygostyle ; pv. , pelvis ; 

 %., kidney; /., lung. 



which may be connected with the apex of the sternum. The 

 fore-limb has not more than three digits, the three metacarpals 

 are fused (except in Archaeopteryx), and there are only two 

 separate carpals, the others fusing with the metacarpals, and 

 thus forming a carpo-metacarpus. 



The ilia of the pelvis may be firmly fused to the complex 

 sacrum; the acetabulum is incompletely ossified; the pubes 

 are directed backwards parallel to the ischia. There is no 

 pubic symphysis except in the African ostrich (Struthio), and 

 no ischiac symphysis except in the American ostrich (Rhea). 



