The Audubon Societies 399 



or better two tripod- shaped girdles from which the wing-bones and leg-bones 

 could be suspended, we would have a very simple diagram of a bird's skeleton, 

 especially if we added a few curving lines for ribs just back of the front or 

 flight girdle, and a big breastbone. Try making this diagram, again, and then 

 think what the skeleton it pictures must do or be made to do in order that the 

 bird can fly. We have seen that it must be as light as possible, and that 

 it is so by means of air-spaces, and also tiny holes through which air 

 can pass. 



At the same time, however, it must be very strong to bear the strain of the 

 rapidly beating heart, which is the pump or motor of this flying-machine, as 

 well as the pull of the muscles and their weight. It must be particularly rigid 

 in the wings or sails, as we may call them, and at the same time very flexible 

 about the powerful lungs or bellows that supply air to the machine to keep the 

 motor going. Perhaps only a comparative anatomist could explain to you 

 clearly how this is accomplished, but, if you will look at the breastbone of a 

 chicken, you will see that it is large in two ways and really equals two sets of 

 bones on account of the thin middle piece that is welded to the broad, flat piece. 

 It can thus carry and bear the strain of large flight muscles. It is a remarkable 

 bone, and the next time you eye the 'white meat' of a chicken at table spend a 

 few moments looking at the bone that holds the meat in place. This large 

 bone, however, must be joined to other bones in order to be of much use, and 

 so we must look at the flight-girdle to which it is joined, and learn how the 

 three bones of which that is made are braced together, and how the rigid wing- 

 bones are attached to it, and how the small rib-bones, with the curious little 

 interlock between each which makes them flexible and still strong, fit on to the 

 backbone. 



The leg-girdle too is strongly braced and in places entirely joined together, 

 so that it can bear the weight of the bird when the bird walks or hops or runs. 

 The neck-bones are very flexible, much more so than those of most if not 

 all other vertebrates, and at the same time they are strongly locked together, 

 so that, no matter how fast a bird is flying, it can not only hold its head in 

 position, but also move it about readily and safely. 



And lastly, the bones of a bird's skeleton are remarkably thin in places 

 where they can be so, as in the skull, for example, or in the middle piece of the 

 breastbone. The reason, of course, is to make the skeleton or framework of the 

 flying-machine as light as possible, and by reducing every separate bone to 

 the smallest size that will still insure safety and strength, as compact as possible. 



All the bones of a bird's skeleton tell us the same story, that is the story of 

 a successful flying-machine, and when we have learned this story completely, 

 we might turn to the breathing-apparatus, the blood or circulatory apparatus, 

 the nerve-apparatus and the reproductive apparatus, and find the same 

 story repeated. 



The story of the reasons why the skeleton of a fish or of a horse or of a 



