320 STERNAL MUSCLES. 



considerable breadth, and curved in its section as 

 well as in the principal line of its crest, the bird is 

 one of the most powerful wing, and whether its chief 

 exercise be in long flight, or very rapid flight of 

 shorter continuance, depends a good deal on the 

 number, form, and arrangement of the feathers. 

 Generally speaking, however, birds which do not 

 require the violent rush of the accipitres, have the 

 sternum straighter and narrower, though a little 

 longer in proportion. Sterna which are very broad, 

 with the crest low, always indicate bad fliers. Here, 

 however, the adaptation of the sternum for flight, 

 begins to be combined with that for swimming ; and, 

 as those two habits occur in many proportions with 

 regard to each other, it is not very easy to frame 

 such a general description of the bone as will apply 

 to all or even half the varieties. When birds of this 

 description come to have the two motions of the 

 wings, the flying one in the air, and the swimming 

 one under water, the form of the external part of the 

 wing is adapted to the one or the other, according to 

 the habit, while the form of the sternum itself remains 

 nearly the same. 



STERNAL MUSCLES. 



In order that the action of the wings of birds 

 and the influence which the different modifications of 

 the sternal bones have in that action, may be more 

 clearly understood, it is necessary to say a little more 

 respecting the muscles by which the wings are moved 

 than the mere enumeration which was given in a 

 former chapter. This may have somewhat the appear- 

 ance of repetition ; but, in treating of complex struc- 

 tures, so much of repetition as shall keep the several 

 parts constantly in the reader's view is not only 



