26 The Relation of the Pectoral Muscles of January, 
wing-line and the cleaving form of the conical body reduce as 
much as possible the resistance of the air in front. 
The cause of motion is the action of the wings upon the 
highly elastic air producing by reaction forward movement of the 
body. The source of this action is in the pectoral muscles, Its 
instruments are the wings. Wonderfully indeed are the wings 
adapted to their purpose. Of extreme lightness and of great 
rigidity and strength, their weight is but a slight hindrance and 
their leverage a vast advantage in producing motion. 
Concave beneath and convex above, with underlapping feathers, 
to the one side is presented a grasping surface almost impervious 
to air, and to the other a lattice-work structure through which 
air easily rushes, So in the depression of the wing all possible 
advantage is gained from the elastic resistance of the air, and in 
its elevation the least amount of force is lost. 
The muscles which furnish the propelling force to the wings 
are those of the breast, the pectoralis major and the pectoralis 
minor. The pectoralis major is a large, triangular muscle form- 
ing the principal part of the bulk of the breast. It arises from 
the ribs, from the outer portion of the ventral surface of the 
sternum, from the side of the keel of the sternum, from the fur- 
culum and the membrane connecting the furculum with the 
sternum and the coracoid. The fibers converge, the outer turn- 
ing under the inner and inserted by a tendon on the greater 
tuberosity of the humerus. In action this muscle depresses the 
wing and thus furnishes the great motive power of flight. 
The pectoralis minor is much smaller than the preceding, and 
beneath it; arising from the middle portion of the sternum and _ 
the membrane attaching the furculum to the sternum and the 
coracoid. Its fibers converging terminate in a tendon which, after 
passing through the end of the coracoid, is inserted on the inner 
side of the greater tuberosity of the humerus. This muscle, to- 
gether with the resisting force of the air, elevates the wing after 
it has been depressed. 
From the structure of the wing it is apparent that the work of 
this muscle is relatively small except, perhaps, in “ sailing,” as 
seen in the flight of swallows, where the wings must be held — 
tense and at a constant angle by this muscle. It would clearly 
appear, when we consider the law of muscular development, that __ 
in those species which fly most of these muscles would be rela- 
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