88 THE STRUCTURE AND LIFE OF BIRDS cuap. 
breast muscles must make it move less easily. Owing 
to the thicker coating of feathers it is difficult to see the 
movements clearly. When a large bird, a goose or 
a crane, utters a loud cry, is the best opportunity. 
Then, if he is standing, his breast may easily be seen 
to move forward and upward. When the muscles 
relax, the breast will sink and the air will be expelled, 
but the latter process will be greatly assisted by the 
contraction of other muscles—viz., those that lie over 
the abdomen and connect the pelvis and the breast. 
The action of these will be to drive the air out of the 
great hinder air-sacks. The chest is loosely hinged 
on to the back by muscles near the shoulder joint, so 
that very little exertion on the part of the abdominal 
muscles will be required. Take a dead bird and see 
how easily the hinder end of the breast works up and 
down. Thus the abdominal muscles in a bird play a 
most important part in breathing, in a man they play 
a very small one. 
But most birds breathe most actively during flight, 
and then a different system must be adopted: John 
Hunter, the celebrated anatomist, held that birds did 
not inhale and exhale during flight, but merely used 
the air which they had stored in their air-sacks. This 
view appears absurd in the face of the fact that they 
will sometimes fly hundreds of miles without alighting. 
But he was led to adopt it by what is a very real 
difficulty—namely, that the movement of the breast in 
breathing would seriously derange the machinery of 
flight. The socket in which the wing works is formed 
mainly by the coracoid, which is buttressed by the 
clavicle. Both bones are almost rigidly fixed to the 
