THE BOOK OF MIGRATORY BIRDS 73 



the contraction of the middle pectorals and their congeners 

 is absolutely necessary, since without it the wings would 

 fall by their own weight, and the action of the great 

 pectorals would be useless. Besides, in the depression of 

 the wings, the fixed point of the middle pectorals, where 

 the respective tendons attach themselves to the humerus, 

 being removed, the sudden contraction of these pectorals 

 must necessarily facilitate the ascension of the trunk until 

 the humerus is stopped by the cessation of action in 

 the great pectorals. It may easily be conceived why the 

 projecting muscles of the trunk and the depression of the 

 wings are stronger than the elevators; it is because the 

 former cause the trunk to start, and by this means depress 

 the wings, notwithstanding the resistance of the latter. 

 These, being unable to prevent the humerus from de- 

 scending, become fixed there, and draw up the trunk, thus 

 assisting the action of the great pectorals, and also par- 

 ticipating in projecting the trunk both forwards and above. 

 "Thus, that the bird may raise and direct itself in the 

 air, all the muscles must contract themselves in the follow- 

 ing manner : The clavicle and the omoplate being fixed 

 by the trapezium, the rhomboid, the upper part of the 

 great dorsal, the costo-scapular, and the short clavicular, 

 and the wing being partly unfolded, brought forward, and 

 raised by the action of the middle pectoral of the internal 

 sub-clavian, the elevators of the humerus, of the coraco- 

 brachial, and of the extensors of the anterior membrane 

 of the wing, the bird springs into the air, completely 

 expanding its wings. At the same time, the great pec- 

 torals, the primary agents of the wings, of which the point 

 is fixed in the humerus by the insertion of their respective 

 tendons, contract suddenly, and, in consequence of the 

 resistance which the air opposes to the movement of the 

 wings, carry all their power to the sternum. By this inter- 

 vention they cause the trunk to rise, and the wings, whose 

 immediate depression is resisted by the atmospheric air, 

 as we have just said, are nevertheless depressed by these 

 indirect means. 



