286 BIOLOGY FOR BEGINNERS 



muscle tissue equal in some cases to one-third the whole weight 

 of the bird. To connect these muscles with the wing bones, a 

 very remarkable set of tendons pass over the shoulder joints like 

 ropes over pulleys and transmit the motion to the wing, much as 

 our fingers are closed by muscles located in the forearm. 



Shape of Wing. The attachment of the feathers to the wing is 

 no less perfectly adapted for its purpose. The longest feathers 

 (primaries) are attached to the fingers where their leverage will 

 be greatest. Back of them come the secondaries, which brace 

 them at the base and cover the spaces between their quills. These 

 in turn are further supported by other rows, both above and below. 

 The outline of the wing as a whole, with its concave under surface, 

 thick forward edge, and thin flexible rear edge and tip, has just 

 the form which man has recently discovered best for his aeroplane, 

 and is beginning feebly to imitate. 



Flight. In ordinary flight the wing stroke resembles horizontal 

 figure eight down and back, up and forward. The soaring of 

 birds, like the hawk, where they seem to fly without any motion 

 at all, is not understood. It may be due to slight wing motion, 

 to balancing, or to utilization of wind currents, but so far, man 

 has not satisfactorily explained, much less imitated it. 



When man flies in the aeroplane, of which we are so proud. 

 he flies not like the bird, with beating wings, but rather like the 

 locust or beetle with stiff planes and a propeller behind. Thus 

 far we have no engine powerful enough to swing a vibrating 

 wing machine, large enough to carry a man in flight like a 

 bird. 



Muscles. The " white meat " of a chicken is the mass of breast 

 muscles used in flight and the large breast bone with its projecting 

 ridge is familiar to all of us. This ridge gives additional room to 

 attach the powerful muscles. The outer layer of the white meat 

 separates easily from an inside portion, this latter being very 

 tender. The explanation is that the outer, larger, and tougher 

 muscle was the one used in pulling the wing, down and backward 

 in the " stroke " of flying, while the inner and more tender muscle 

 acts by way of a tendon over the shoulder to raise the wing for 



