162 THE VERTEBRATE SKELETON. 



The columellar chain, which has been already described 

 (p. 157), should be mentioned with the hyoid as it. is homo- 

 logous to the hyomandibular of fishes. 



The sternum, of the Frog, though regarded as part of the 

 axial skeleton, is so intimately connected with the pectoral 

 girdle, that it will be described with the appendicular 

 skeleton. 



2. THE APPENDICULAR SKELETON. 



This consists of the skeleton of the two pairs of limbs and 

 their respective girdles. It is at first entirely cartilaginous 

 but the cartilage becomes later on mainly replaced by bone. 

 The only bone developed in connection with the appendi- 

 cular skeleton, which has no cartilaginous predecessor, is the 

 clavicle. 



A. THE PECTORAL GIRDLE. 



This consists originally of two half rings of cartilage en- 

 circling the sides of the body a short way behind the head. 

 These two halves meet one another in the ventral middle line, 

 and separate the anterior elements of the sternum from the 

 posterior ones. 



Each half -ring bears on the middle of its outer and pos- 

 terior surface a prominent cup, the glenoid cavity, with 

 which the proximal arm-bone articulates. This cup divides 

 the half-arch into a dorsal scapular and a ventral coracoid 

 portion. 



The scapular portion consists of two parts, the supra- 

 scapula and the scapula. 



The suprascapula (fig. 30, A, 2) is a wide, thin plate 

 attached by its ventral and narrowest border to the scapula. 

 Its proximal and anterior half is imperfectly ossified, its whole 

 border or sometimes only its dorsal and posterior borders consist 

 of unaltered hyaline cartilage, while the rest of it is composed 

 of calcified cartilage. The scapula (fig. 30, A, 3) is a fairly 



