THE BIRDS 407 



sistance to the wind and they are capable of sustained high speeds. 

 Many of the principles of airplane construction have been patterned 

 after the bodies of birds. 



Birds have a keel-shaped sternum, or breastbone, which accommodates 

 the large amount of muscle necessary for flight. The space on either 

 side of the keel is filled with large muscles which are used on the down 

 beat of the wings where most of the power is needed. The muscles 

 which lift the wings upward are also on the underside of the wings, but 

 these muscles, by means of a long tendon which circles over the shoulder, 

 are able to raise the wings. As a result of this unusual arrangement 

 there is little flesh on the back of a bird. Flightless birds, such as the 

 ostrich and emu, have vestigial wings and have lost the keel on the 

 sternum. 



The birds have a highly developed cerebellum, which is the part of 

 the brain that regulates the many unconscious movements associated 

 with equilibrium and muscle coordination. This is another important 

 adjustment to flying. Consider a bird flying toward a cliff at high speed. 

 It will seem as if he surely is going to dash himself to pieces against it, 

 but suddenly he throws his wings in reverse, spreads his tail, comes to a 

 dead stop, and settles on a slight projection. All this involves a high 

 degree of coordination of the various muscles of the body, made possible 

 by the highly developed cerebellum. 



Birds have air sacs that run from their lungs out into the flight 

 muscles and also extend into many of their bones. The airfilled bones 

 are very light, and this helps the birds by reducing the weight which 

 they must lift when flying. Also, the air sacs in the muscles function in 

 respiration while a bird is flying. When not flying, birds expand their 

 chest for inhalation ; but while flying, the chest bones must be held 

 rigid as a support for the chest muscles. The alternate contraction and 

 relaxation of these muscles, however, exert a bellows-like action on the 

 air sacs between them and air is pumped in and out of the lungs with 

 each beat of the wings. In our less efficient lungs at least one third of 

 the air, called residual air, remains in the lungs and air passages even 

 with a maximum exhalation. When we inhale, the pure air coming in 

 mixes with the foul air already in the lungs. As a result, the bird's lungs 

 are far more efficient than human lungs. It has been estimated that 

 man would need lungs ten times the present size to supply the oxygen 

 needed to enable him to fly under his own power. 



Birds have a syrinx which is located at the point where the trachea 

 divides to form the bronchi in the chest. This structure is found only 

 in birds and is not homologous with the voice mechanism in the larynx 

 of other vertebrates. The bird produces its sound by means of vibrat- 



