114 ANATOMY OF VERTEBRATES. 



is unfavourable for walking. Swans partially expand tlieir wings 

 to the wind while swimming, and thus move along the water by 

 means of sails as well as oars. The act of diving is performed 

 by the feet striking the water backward and upward, assisted by 

 the compression of the air-cells : the habitual divers (Penguins, 

 Awks) move in and through the water by the rapid and forcible 

 action of wings, shortened and shaped like paddles, and beating 

 the water as in flight. 



Flight, the chief and characteristic mode of locomotion in birds, 

 results principally from the construction and form of the anterior 

 extremities. The form of the body has reference thereto, the 

 trunk being an oval with the large end forward : being also short 

 and inflexible, the muscles act with advantage, and the centre of 

 gravity is more easily changed from above the feet as in the 

 stationary position, to between the wings as during flight. The 

 long and flexible neck compensates for the rigidity of the trunk, 

 and alters the poise according to the required mode of progression, 

 by simply projecting the head forward, or drawing it back. The 

 head of the bird is generally small, and the beak pointed, which 

 is a commodious form for dividing the air. The position of the 

 great pectoral muscles tends to keep the centre of gravity at the 

 inferior part of the trunk. The power which birds enjoy of 

 raising and supporting themselves in the air is aided by the 

 lightness of the body. The large and usually air-filled cavities 

 in the bones diminish their weight without taking away from their 

 strength, — a hollow cylinder being stronger than a solid one of 

 the same weight and length. But the specific levity principally 

 depends on the great air-cells which occupy almost every part of 

 the body. The air which birds inspire distends these cells, and is 

 rarified by the heat of the body. Lastly, the feathers, and espe- 

 cially the quills, from their lightness and elastic firmness, contri- 

 bute powerfully to the act of flying by the great extent which 

 they give to the wings, the breadth of which is further increased 

 by the expanded integument situated in the bend of the arm and 

 in the axilla. 



When a bird commences its flight it springs into the air, either 

 leaping from the ground, or precipitating itself from some elevated 

 point. During this action it raises the humerus, and therewith the 

 entire wing, as yet unfolded ; it next spreads the wings horizon- 

 tally by an extension or abduction of the fore-arm and hand : the 

 greatest extent of surface of the wing being acquired, it is 

 rapidly and forcibly depressed : the resistance of the air thus sud- 

 denly struck occasions a reaction on the body of the bird, which is 



