Chap. VIII.] THE SKELETON. 169 



stem. The more anterior is the external xiphoid, process (e. x.p.\ 

 the more posterior the internal xiphoid process (i. x. p.). 



The plate of cartilage from which the sternum is formed arises 

 as two bands, which are segmented off from the ventral ends of 

 the ribs, and unite to form a common plate. 



(3.) In the Frog there are no ribs. The sternum is well ossi- 

 fied, and is in close connection with the pectoral arch. Behind 

 it is an expanded plate of cartilage, the xiphi-sternum (Fig. 54, 

 x. st.). Anterior to the pectoral arch is a median bony process, 

 the omostemum (o. st.), carrying a rounded cartilaginous plate. 



The Pectoral Arch. The shoulder-girdle of the frog comes 

 into close relation with the sternum and omo-sternum. Between 

 these sternal elements two bones pass outwards from each side 

 of the mid-ventral line. Of these the more posterior and stouter 

 is the coracoid (54, B. and C., o.) the more anterior and slenderer 

 is the clavicle (cl). Outside these, articulating with both, and 

 passing nearly vertically on each side, is a somewhat hourglass- 

 shaped bone, the scapula (sc.). Dorsal to this, and lying over the 

 transverse processes of the second, third, and fourth vertebrae, is 

 a flattened plate of bone and cartilage, the supra-scaputa (s. sc.). 

 At the point of junction of the coracoid and scapula is the 

 glenoidal cavity for the articulation of the fore-limb (gt. c.). In 

 Fig. 54, C., the parts are flattened out. In B. they have their 

 natural position. The inner edges of the coracoids remain 

 cartilaginous. Posterior to the clavicle (mainly a membrane 

 bone) lies a precoracoid bar of carti-lage. 



In the Babbit, while the ribs form an attachment with the 

 sternum which is absent in the frog, the attachment of the 

 shoulder-girdle is much less perfect. The main element is the 

 scapula or shoulder-blade (Fig. 54, D.). The coracoid is held 

 to be represented by a process (co. p.) of the scapula. Doubts 

 have, however, recently been thrown upon the validity of this 

 view. The small clavicle is the only part which has attach- 

 ment to the sternum, being connected by ligament on the one 

 hand with the acromion (ac.), and on the other with the sternum. 

 The form and relations of the scapula will be readily made out 



