204 THE STRUCTURE AND LIFE OF BIRDS cuap. 
only movements which it can produce.”! 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. 
Fic. 56.—Humerus of left wing. (a) lower; (4) upper side. 
E, elevator muscle attaches. The point of attachment varies, in distance from the 
shoulder and from the przaxial margin. ‘The action remains much the same. F, 
air foranien ; LD, latissimus dorsi attaches ; Gp, great pectoral or depressor ; P3, third 
pectoral. 
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 “przaxial,’ before care- 
fully finding your bearings. In addition to this it 
1 Animal Mechanism, p. 214. 
