674 BIRDS. 
offers relatively little resistance. The attachment of the wings high 
up on the thorax, the high position of such light organs as lungs and 
air-sacs, the low position of the heavy muscles, the sternum, and the 
digestive organs, the consequently low centre of gravity, are also 
structural facts of importance. But it must be remembered. that the 
frictional resistance of the air is slight. 
(4) The muscles of flight.—The pectoralis major brings the wing 
downward, forward, and backward, keeping the bird up and carrying 
it onward. As it has 
most work to do, it is 
by far the largest. The 
pectoralis minor razses 
the wing for the next 
stroke. There are others 
of minor importance. 
On an average these 
muscles weigh about one- 
sixth of the whole bird, 
nearly one-half in some 
pigeons. Buffon noted 
that eagles disappeared - 
from sight in about three 
minutes, and a common 
rate of flight is about fifty 
feet per second. In mi- 
gration many birds fly at 
a rate of over 100 miles 
an hour. 
(c) The skeleton.—The 
rigidity of the dorsal part 
of the backbone, due to 
fusion of vertebrze, is of 
advantage in affording a 
firm fulcrum for the wing- 
a strokes, while the arched 
; clavicles (meeting in an 
Fic. 374.—Pectoral girdle and sternum _interclavicle and often 
of Bewick’s swan. fused in front to the 
A part of carina removed shows peculiar loop of sternum) and the strong 
trachea (¢r.); cl., clavicle; cor., coracoid; sc,, Coracoids (which articu- 
scapula ; g/., glenoid cavity for head of humerus; late with the sternum) 
%., parts of sternal ribs. are adapted to resist the 
inward pressure of the 
down-stroke. As the keel of the breast-bone serves in part for the in- 
sertion of the two chief muscles, its size bears some proportion to the 
strength of flight. It is absent in the running birds, such as the 
ostriches, and has degenerated in the New Zealand parrot (S¢rzngops), 
which has ceased to fly and taken to burrowing. 
(ad) Air-sacs and air-spaces.—The lungs of birds open into a number 
of air-sacs, which have a larger cubic content than the lungs, and in 
many cases these air-sacs are continued into the bones, among the 
