THE FLIGHT OF BIRDS. 493 



bone keel to which these muscles are attached, the lightening 

 of the body by means of balloon-like air-sacs and smaller air- 

 spaces, the boat-like shape of the body, and some other 

 structural peculiarities, are more or less directly associated 

 with the power of flight. Let us consider these peculiarities. 



The feathers used in flight are borne by the arm and by 

 the fingers. In those which are most important, the vane of 

 the feather is large, and its lateral barbs are firmly bound 

 together by little barbules, so that the vane, though very light, 

 is able to beat the air as an oar strikes the water. But the 

 base of each feather is embedded in the skin in such a way 

 that it can be independently moved by muscles, and, accord- 

 ing to Marey, each feather has in flight a motion of its own. 

 The feathers come close together during the down-stroke, 

 they are separated and placed on edge during the elevation 

 of the wing. 



The most external muscle of the breast depresses the 

 wing, making the stroke which carries the bird forward and 

 keeps it from sinking. As it has most work to do, it is by 

 far the largest muscle. Internal to it lies a second muscle, 

 which raises the wing for the next stroke, and there are 

 other smaller muscles of less importance. When we con- 

 sider the number of strokes which a bird may make in a 

 minute, or the distance that it can travel on the strength of 

 one stroke, we realise how powerful these wing muscles 

 are. 



The keel of the breast-bone serves as a base of insertion 

 for the two important muscles, and its size bears some pro- 

 portion to the strength of flight. It is naturally absent in 

 the running-birds, such as the ostrich and the emu ; it has 

 degenerated in the New Zealand parrot Stringops, which has 

 ceased to fly and has taken to burrowing. In connection 

 with the skeleton, we should also notice that the well- 

 developed collar-bones or clavicles (which are often joined 

 in front to the breast-bone), and the strong coracoids which 

 articulate with the breast-bone, are admirably adapted to 

 resist the inward pressure caused by the stroke of the wing. 



As to the air-sacs and air-spaces, we must avoid falling 

 into the old error of supposing, that the bird is a sort of 

 balloon which can hardly help rising. For there is no 

 doubt that even the lightest of birds has to keep itself from 



