vii FLIGHT 207 



lies below it. But the humerus turns as on a pivot 

 at the joint, and if it be pulled inward it will be 

 raised to an upright position, if only the pull comes 

 from a point not below the pivot. A tendon, therefore, 

 must be made to pass through a pulley fixed at a 

 point high enough, and must attach to the inner side of 

 bone some little way out from the joint. By such means 

 a mast may be raised, if it be prevented from slipping 

 at its base. A pulley on a level with the shoulder- 

 joint is available. There are two bones, as explained 

 above, which meet to form the socket, the coracoid 

 and the scapula or shoulder blade. On their inner 

 side the top of the clavicle or merrythought meets 

 them, and the three together form a tunnel of bone. 

 Through this passes the tendon and fastens on to the 

 back of the humerus near its prseaxial margin. 



The upstroke is at first in a backward direction, 

 mainly owing to the action of the wind due to the bird's 

 own velocity ; such a wind must necessarily drive the 

 wing backward and at the same time lift it. 1 The move- 

 ment to rearward, which is often difficult to detect in 

 photography, is clear in diagrams obtained by Professor 

 Marey by various methods. 2 Towards the end of the 

 upstroke the wing is moved forward, till it stands above 

 the pulley and the pivot, mainly by the action of the 

 Elevator muscle. The way to see the working of 

 these muscles is to take a dead bird and cut away the 



1 The third pectoral muscle, springing from the outer and 

 lower part of the coracoid and from the sternum close to the 

 ribs, assists— its chief office being to draw the wing backward. 

 It attaches to the upper side of the humerus, just over the air 

 foramen. 



2 See p. 221. 



