THE PECTORAL ARCH. 



35 



continuous cartilage, on each side of the body, distinguish- 

 able only into regions and processes, and affording an articular 

 surface to the bones or cartilages of the limb. But ossifica- 

 tion usually sets up in the cartilage, in such a way as to give 

 rise to a dorsal bone, called the scapula, or shoulder-blade, 

 which meets, in the articular, glenoidal cavity for the hu' 

 merus, with a ventral ossification, termed the coracoid. 



By differences in the mode of ossification of the various 

 parts, and by other changes, that region of the primitively 



ec.r 



Tn.cr 



Flo. 12.— Side-view of the pectoral arch and steranro of a Lizard {Iguana tuberculata). — 

 Sc, scapula ; 5.SC, supra-scapula ; cr, coracoid ; gl^ glenoidal cavity ; St, sternum ; x.8i, 

 xiphisternum ; m.sc, mesoscapula ; p.cr, precoracoid; m.cr, mesocoracoid ; e.et\ epi- 

 coracoid ; d, clavicle ; i.c/, interclavicle. 



cartilaginous pectoral arch which lies above the glenoidal 

 cavity may be ultimately divided into a scapula and a supra- 

 scapula ; while that which lies on the ventral side may pre- 

 sent not only a coracoid, but a precoracoid and an epicora- 

 coid. 



In the great majority of the Vertebrata above fishes, the 

 coracoids are large, and articulate with the antero-external 

 margins of the primitively cartilaginous sternum, or breast- 

 bone. But, in most mammals, they do not reach the sternum, 

 and, becoming anohylosed with the scapula, they appear, in 

 adult life, as mere processes of that bone. 



Numerous Vertebrates possess a clavioula, or collar-bone, 

 which is connected with the pi'e-axial margin of the scapula 

 and coracoid; but takes no part in the formation of the 

 glenoid cavity, and is usually, if not always, a membrane 

 bone. In many Vertebrata, the inner ends of the clavicles 



