22 



A BOOK OF BIRDS 



a sort of cage fixed on to the front of the sternum or breast-bone. 

 This bears, as everybody knows, a rough resemblance to the hull 

 of a ship, but with an extremely deep keel. A reference to the figure 

 here should make this clear. The deep keel and the broad, flat 

 plate of the breast-bone serve for the attachment of the breast-muscles, 

 which in the bird are of enormous size, equalling or exceeding in 

 weight all other muscles of the body. 



These muscles, which constitute the large mass of flesh familiar 

 to every one as the " breast-meat " of a bird as served up at table, 

 are arranged in two layers. The outermost runs forward, to be 

 inserted into a shelf of bone which projects from the upper surface 

 of the humerus, or upper arm ; while the lower runs beneath it, along 



the coracoid, and finally 

 passing into a round ten- 

 don, runs through a pulley 

 formed by the meeting of 

 the coracoid, blade-bone, 

 and merry-thought, and into 

 the head of the humerus. 

 These two muscles play the 

 most important part in 

 raising the body and keep- 

 ing it in motion, for these, 

 by their contraction, bring 

 about the downward v/ing- 

 beat. About the other muscles which aid in this work, and 

 those which raise the wings at the end of the stroke, we need not 

 worry here. How intimately the keel is associated with flight may 

 be seen by an examination of birds which fly but little. In them the 

 keel is always shallow, while in those which have lost the power of 

 flight altogether it is reduced to a mere ridge of bone, as in the Owl 

 Parrot (Stringops), or has vanished altogether, as in the flightless 

 Ostrich tribe. 



The wing itself is no less profoundly modified, as we pointed out 

 in an earlier part of this chapter. Suffice it to say here, that of the 

 original five fingers, but three remain. Of these the thumb and 

 third finger are reduced, and little more than stumps ; while the second 



Fig. 8.— Trunk of a Bird, showing Bones 

 of Shoulder and Hip^girdles. 



