INTRODUCTION xxxvii 



bird as served 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 



Fig. 7.- — Trunk of a bird, showing bones of shoulder and hip- 

 girdles. 



beneath it, along the coracoid, and finally passing into 

 a round tendon, rims 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 keeping it in motion, for these, by their 

 contraction, bring about the downward wing-beat. 

 The other muscles which aid in this work, and those 

 which raise the wings at the end of the stroke, need 

 not be studied here. How intimately the keel is as- 

 sociated 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 bones, as in the Owl Parrot (Stringops), or has 

 vanished altogether, as in the flightless Ostrich tribe. 



