The major power stroke of the wing
during
flight originates from the large pectoralis
muscle located superficially in the
breast region. The wing is elevated by the supracoracoideus muscle located
between the pectoralis and the sternum. The supracoracoideus, although
adjacent
and parallel to the pectoralis, is able to cause an opposite movement
(raising
instead of lowering the wing) because its tendon is inserted onto the
opposite
side of the humerus to the pectoralis tendon. In poultry, the
pectoralis
muscles are the largest muscles of the body and they comprise
approximately 8%
of the body weight. The small muscles located within the wing are
concerned
with controlling the shape and degree of rotation of the wing during
flight.
The insertion of the flight muscles in the
breast region should be examined critically. At first sight, it might
appear
that most of the expanded sternum in poultry is used for the attachment
of the
pectoralis. In fact, the only part available for the pectoralis is the
zone
around the supracoracoideus muscle. The bulk of the pectoralis
originates from
the furcula and from a membrane stretched between the furcula and the
coracoid.
The main tendon of the muscle is continued intramuscularly as a layer
of
connective tissue that divides the muscle into two heads which probably
have
different functions during flight.