THE SKELETON OF THE FROG. THE STERNUM. 163 



stout rod of bone constricted in the middle, and forming the 

 dorsal half of the glenoid cavity. 



The coracoid portion consists of three parts, the cora- 

 coid, precoracoid and clavicle. 



The largest and most posterior of these is the coracoid 

 (fig. 30, A, 4) which like the scapula, is contracted in the 

 middle and expanded at the ends, especially at the ventral 

 end. It forms a large part of the glenoid cavity. The 

 ventral ends of the coracoids which meet one another in the 

 middle line are unossified, and form narrow strips of calcified 

 cartilage, the epicoracoids (fig. 30, A, 5) ; these are often 

 regarded as sternal elements. 



The precoracoid forms a narrow strip of cartilage lying 

 in front of the coracoid, from which it is separated by the 

 wide coracoid foramen (fig. 30, A, 9). The dorsal end is 

 continuous with an area of unossified cartilage which separates 

 the coracoid and scapula and forms part of the glenoid cavity. 



The clavicle is a narrow membrane bone closely attached 

 to the anterior surface of the precoracoid, its dorsal end is ex- 

 panded. 



THE STERNUM. 



The sternum consists of four parts arranged in two groups ; 

 two parts to each group. The anterior members are the epi- 

 sternum and omosternum. 



The episternum (fig. 30, A, 10) is a thin almost circular 

 plate of cartilage much of which remains hyaline. 



The omosternum (fig. 30, A, 11) is a slender bony rod 

 widest at its posterior end : it connects the episternum with 

 the ventral ends of the precoracoids. 



The sternum proper is a short rod of cartilage sheathed 

 in bone ; it is contracted in the middle and expanded at each 

 end. It bears attached to its posterior end a broad some- 

 what bilobed plate of partially calcified cartilage, the xiphi- 

 sternum (fig. 30, A, 13). 



112 



