THE SKELETON 71 



Phylogenetically, the oldest elements of the pectoral girdle 

 are the scapula and coracoid, and of the pelvic girdle the 

 ischium and pubis ; for though in certain Fishes the clavicle and 

 the ilium are indicated, they are only fully developed from 

 the Amphibia upwards. 



Fig. 51 is the ventral view of the pectoral girdle of a 

 tailed Amphibian. It shows that the clavicles (cl.} are directed 

 forwards (i.e. towards the head), and that the coracoids (co.) 

 overlap each other ventrally. The edges of the latter, which 

 are connected by fibrous tissue, only loosely overlie the small 

 so-called " sternum " (s). The connection between the coracoids 



FIG. 51. PECTORAL GIRDLE OF A TAILED AMPHIBIAN, FROM THE 



VENTRAL SIDE. 

 cl., clavicle ; co., coracoid ; ar., shoulder-joint ; st., so-called "sternum." 



and the sternum becomes much closer in Eeptiles and Birds, and 

 persists in the lowest Mammals. The withdrawal from this 

 connection seen in the higher Mammalia is proportionate to the 

 greater development of the antero-ventral element of the pectoral 

 girdle, the clavicle. Through the mediation of this bone the 

 scapula finds a new support upon the sternum, and thus the limb, 

 being the farther removed from the trunk, attains far greater 

 freedom of movement. 



The expanded coracoid of the lower Vertebrata is, in Man, 

 represented by an apparent process of the upper edge of the 

 scapula, called the processus coracoideus (co., Fig. 52). This serves 

 as a point of origin and attachment for certain ligaments and 



