STRUCTURAL ADAPTATIONS 379 



The shoulder girdle — the series of bony rods charged with 

 the support of the wing — has undergone a few noteworthy 

 changes in adapting itself to different kinds of strains. The 

 horse-shoe shaped furcula, or merry-thought, formed by the 

 fusion of a pair of clavicles or " collar-bones," is either U- or 

 V-shaped, and the latter is still further modified into a Y-shape 

 by the outgrowth of a discoid, or sometimes of a styliform 

 element — the hypocleideum — at one time regarded as answer- 

 ing to the reptilian inter-clavicle, but now considered to have no 

 relation to this bone. Since the sternum or breast-bone also 

 participates directly in the special modifications due to flight it 



III. 39. — Shoulder-girdle and Sternum of an Eagle 



Sc. = Scapula. Ac. = Acrocoracoid. Cor. = Coracoid. F. = Furcula. 

 Car. = Carina. C.s. = Corpus Sterni. S.r. = sternal ribs. 



will be found convenient to deal with both girdle and breast- 

 bone at the same time. 



In birds of powerful and vigorous flight, such as Eagles 

 and Falcons, the coracoids are short, broad and thick, and the 

 furcula has the limbs wide apart, broad and strongly curved, 

 the convexity pointing forwards ; but in no case does the furcula 

 come directly into contact with the anterior end of the carina 

 or keel of the sternum, though the upper or free end of the 

 furcula is closely attached to the head of the coracoid by a 

 specially developed facet which is opposed to a similar facet on 

 the coracoid. 



