88 THE STRUCTURE AND LIFE OF BIRDS chap. 



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 



