PECTORAL ARCH. 



91 



Fu(Cl), for its relations to the rest of the pectoral arch and to 

 the sternum). 



Amongst the Cursorial Birds, the Emeu (DromaBiis) and Casso- 

 wary (Casuarius) possess rudimentary clavicles : in the others 

 they are wanting. 



In Archseopteryx, the scapular region only of the pectoral 

 arch has been satisfactorily made out, and this resembles that of 

 existing Birds. 



Mammals. In Monotremes only amongst Mammals does the 

 coracoid extend ventrally to reach the sternum ; in all the others 

 it becomes reduced, and simply forms a prominent process on the 

 scapula (coracoid process), which becomes ossified from a 



FIG. 75. PECTORAL ARCH OF Ornithorhynchus paradoxus. 



Sf, sternum ; Ep, inter-clavicle ; Co, coracoid ; Co\ epicoracoid ; S, scapula ; Of, 



clavicle ; G, glenoid cavity. 



separate centre. Thus the scapula alone serves to support the 

 extremity ; it becomes at the same time greatly broadened out, 

 and gives rise on its outer side in connection with the highly 

 differentiated muscles of the limb to a strong ridge (spina 

 scapulae), which extends downwards to form the so-called 

 acromion. 



The distal end of the clavicle usually becomes connected with 

 the acromion, its proximal end articulating with the anterior edge 

 of the sternum. 



In those Mammals in which the fore-limhs are capable of very varied and 

 free movements, the clavicles are very strongly developed. In others, such 

 as the Carnivora and Ungulata, they may he entirely wanting or only 

 rudimentary, and in the latter case their relations to the scapula become 

 altered. 



