GENERAL CHARACTERS OF BIRDS. 



627 



the basisphenoid. There is an interorbital septum formed 

 from presphenoid and mesethmoid. The otic bones fuse with 

 adjacent bones and with one another about the same time. 

 In modern birds there are no teeth, but the jaws are covered 

 by horny sheaths. The premaxillcz are large, a?id form most 

 of the beak. There is a complete infra-temporal arcade 

 formed bv a delicate jugal and quadrato-jugal reaching back 

 to the quadrate. The supra-temporal arcade is usually 



n. 



pv 



St. 



FIG. 308. Position of organs in a bird. After Selenka. 



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

 proventriculus ; ^. , gizzard ; c. . ca?ca ;/., pygostyle ; pv., pelvis ; 

 -., kidney ; /. , lung. 



incomplete, but in some cases a process of the squamosal joins 

 a postorbital process of the frontal. The lower jaiv consists 

 on each side of five membrane bones and a cartilage bone 

 the articular which works on the quadrate. Many of the 

 skull bones have a spongv texture, due to cavities filled with 

 air from the nasal and Eustacliian tubes. 



There is a well-developed sternum, generally witJi 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 



