THE CLASS BIEDS. 285 



inferiorly ; finally, in some owls they are united inferiorly by 

 cartilage, whilst in the greater number of birds their bony 

 union is complete. In many instances they form a crest at 

 this point of union, and seek a direct support from the 

 sternum. 



The anterior extremities of birds are employed only as 

 wings ; they must not be confounded with the so-called wings 

 of bats, which we have already seen to be of an entirely dif- 

 ferent nature : they are formed of stiff feathers or quills, 

 which require to be fixed only at their base, and the hand in 

 consequence ceases to present any appearance of fingers. 



Fig. 224. Bones of the Shoulder and Sternum.* 



432. The large quills of the wings are called rtmiges, 

 and it is more on their length and strength than on the 

 extent of the bones, that the power of flight depends. Each 

 time the bird prepares for flight he raises the arm and its 

 plumage unfolded ; then he enfolds it by extending the arm, 

 at the same time suddenly depressing it: the air which 

 is struck forms the point of support and resistance to a down- 

 ward movement ; upon it he rises like a projectile, and the 

 impulsion once given to the body is maintained and directed 

 by the same instruments and movements : the bird would 

 soon fall to the earth by the force of gravity, but before the 

 speed acquired by the first effort is exhausted, a second and a 

 third take place, continuing the living projectile in its course. 

 Whilst the bird is being thus suspended in the air, it 



* *, the sternum ; e, notch of the sternum ; co, origin of the sternal ribs ; 

 6, crest ; f, fpurchette; <?, coracoid clavicle; o, scapula; t, fibrous mem- 

 brane extending from the fourchette to the sternum. 



