2o 4 THE STRUCTURE AND LIFE OF BIRDS chap. 



only movements which it can produce." 1 Almost 

 everything beyond the mere up and down motion he 

 attributes to the resistance of the air. It is true that 

 some important movements can be set down wholly 

 or in part to this. Still much is left for the muscles 

 to do. 



Before their position and their working can be 



a. b. 



Fig. 56.— Humerus of left wing. («) lower ; (b) upper side. 

 E, elevator muscle attaches. The point of attachment varies, in distance from the 

 shoulder and from the pra^axial margin. The action remains much the same. F, 

 air foramen ; ld, latissimus dorsi attaches ; gp, great pectoral or depressor ; P3, third 

 pectoial. 



understood, some further account must be given of the 

 humerus. Its position, even when at rest, is abnormal ; 

 it has received a twist at the joint which has left it 

 set differently from the humerus of any other animal, 

 so that you must not use the terms " above " and 

 "below," "postaxial" and " praeaxial," before care- 

 fully finding your bearings. In addition to this it 

 1 Animal Mechanism, p. 214, 



