The Anatomy of Birds ij 



Frigate-bird, which is perhaps the most expert bird of flight, the area of wing 

 muscle is proportionately smaller than in any other bird, while the wishbone is 

 united at its apex with the breast-bone and soldered to the coracoids at the other 

 end, thus forming a rigid support for the wing. The shoulder blade is slender and 

 as a rule more or less pointed, the Penguins being exceptional in having a broadly 



I expanded scapula, while the Woodpeckers are characterized by having the end 

 bent downwards. Attached to the coracoids in front is the wishbone, which 

 represents the clavicles or collar bones of other animals. This is usually "U"- 

 or " V "-shaped, with the apex near the keel of the sternum or even united with it, 

 as in the Stcganopodes. In some Parrots and Toucans the upper portions 

 only of the clavicles remain attached to the coracoids, and in the struthious 

 birds, save Emeu, clavicles are entirely lacking. In birds of 

 prey the clavicles are broadly "U "-shaped and heavily built, 

 serving to brace the wings apart, but in the majority of birds 

 they are of little structural importance, and in such ad- 

 mirable flyers as the Hummers are practically of no use. 



The breast-bone (Fig. 12), sternum, bears a direct relation 

 to habits, and to a less extent is valuable in classification. 

 While the terms Ratitae and Carinatae, keelless and keeled, are 

 convenient in forming a key, and the corresponding condi- 

 tions were formerly held of primary importance in classifying 

 birds, they have been abandoned by the best anatomists, 

 as they do not express the truth. The development of the 

 keel of the sternum bears a direct relation to the extent to 

 which a bird moves its wings, whether in flight or swimming. 

 Birds which do not fly have the keel of the sternum small or 

 absent, according as the power of flight has been lost, geologi- 

 cally speaking, for a longer or shorter period of time. 



The members of the Auk family fly somewhat heavily, 

 owing to the small size of their wings, but as these are used for flying under 

 water as well as above it, the breast muscles and sternum are large. For the 

 same reason the Penguins, which do not fly at all, have a large sternum, since 

 they swim entirely with their wings, presenting in this respect an analogy to the 

 eared seals, which swim with their fore limbs. 



The keel of the sternum is very much reduced and the body of the sternum 

 greatly shortened in birds which sail, this mode of flight involving the expenditure 

 of comparatively little muscular energy. The Albatross has a small breast-bone, 

 and the Frigate-bird smaller still, and these birds are those which fly with the 

 fewest movements of the wing. On the other hand, birds that fly by strokes of 

 the wings have large breast muscles and a correspondingly large sternum, these 

 reaching their maximum in the Hummingbirds, whose skeleton when brought up 

 to the size of a Pigeon is seen to be very powerfully built. The Pigeon, by the 

 way, exhibits the development of the sternal keel for powerful flight. 



The front part of the sternum bears the coracoids ; the ribs are attached to its 

 sides, while the body of it supports the viscera. In all water birds the breast-bone, 



FIG. 1 2. Sternum 

 of a Guinea Hen, 

 seen from in front. 

 (After Gegenbaur.) 



crs, crest; c, coracoid 

 bone. 



