70 THE FLIGHT OF BIRDS 



slopes more outward in strong than in weak flyers, 

 and, as a rule, more in big birds than in small. At 

 the shoulder -joint the coracoid is met by two other 

 bones, the clavicle or merrythought (which varies 

 much in make and strength), and the scapula or 

 shoulder-blade, and the three bones together form a 

 first-rate pivot, so that the strong wing-beats do not 

 shatter the bird's framework. 



2U 



Fig. 20. 



Clavicle of (1) Tern ; (2) Eagle. Drawn to scale. 



Muscles and Quality of Muscle. 



From the keel and from the outer part of the 

 sternum (thus covering up the small Elevator 

 muscle), and also from the clavicle, springs the Great 

 Pectoral — the Depressor muscle ; it attaches by a 

 short tendon to the front part of the under-surface 

 of the humerus (upper-arm bone), and, attaching 

 where it does, its pull tends to lower the front part 

 of the wing relatively to the back. And springing 

 as it does not only from the breastbone but also from 

 the clavicle, it pulls the wing, not downward and 

 backward, but simply downward. The underlying 

 Elevator sends out a long tendon that passes 



