140 SKELETON OF THE DUCK TRIBE. 



penetrating the sternum to a greater extent tlian from 

 three to four inches. 



The breadth of the sternum, and the strong ridge or 

 keel that descends from the midline of its under surface, 

 relate to the increased extent of surface required for the 

 attachment of the "pectoral" muscles, which are the ac- 

 tive organs of flight. In the land-birds devoid of the 

 power of flight, such as the ostrich and apterjx, the keel 

 is wanting and the sternum is short. Its various propor- 

 tions, processes, notches, and perforations render it a very 

 characteristic bone in birds. 



In no order, founded upon modifications of the feet, is 

 the sternum more diversified in character than in the 

 palmipedes or web-footed order; for in none are the 

 powers of flight enjoyed in such different degrees, or ex- 

 ercised in such various ways, from the frigate-bird down 

 to the penguins, where the power of flight is abrogated, 

 and the rudimental wings used as fins. 



In the goose and duck tribes, as well as the swans 

 {anseres^ Linn.), the sternum is long and broad, and presents 

 two moderately wide and deep hind notches; the costal 

 processes are usually subquadrate ; the coracoid grooves 

 are continued into one another at the median line ; the 

 costal tract forms about half of the lateral margin in the 

 ducks and geese, and two-thirds or more in the swans ; 

 the interpectoral ridge extends from the prominent part 

 of the coracoid margin backwards, nearly parallel to the 

 lateral margin, to the inner side of the lateral grooves ; 

 the back part of the sternum between the grooves is quad- 

 rate, with the angles slightly produced in most; there is 

 a short manubrial process below the coracoid groove. 

 The form of the sternum, its long keel, and the backward 

 production of the long and slender ribs, give a boatlike 



