BIRDS. O 



greatly enlarged, but its surface is still further 

 increased by its medial portion rising into a high 

 perpendicular keel or ridge, the two faces of 

 which, from their direction, afford a point of re- 

 sistance, or purchase, of peculiar advantage. 



To resist the tendency of the shoulders to be 

 drawn together by the powerful muscular action 

 exerted during flight, there is inserted between the 

 two bones (coracoids) to which the shoulder-blades 

 are attached, a singular bone of an arched form, 

 well known as the merry -thought. In the com- 

 mon fowl, which flies but little and weakly, this 

 bone is feeble, but in birds of vigorous flight, 

 as the Hawks, the Swallows, and the Hum- 

 ming birds, it is very strong and elastic. On 

 the other hand, where the bird never rises upon 

 the wing, as in the case of the Ostrich, Emu, 

 and kindred birds, this bone is reduced to a mere 

 rudiment. 



The posterior extremities also differ materi- 

 ally in structure from those of quadrupeds. 

 The general number of toes is four, which is 

 never exceeded ; but a few birds have but three, 

 and the Ostrich but two. For the most part, 

 three toes are directed forwards, and one, answer- 

 ing to the great toe, backwards. The climbing 

 birds, as the Parrots and Woodpeckers, have the 

 outer toe also reversed ; while the Swifts have all 

 the four turned forwards. The feet are much 

 more lengthened than in the Mammalia generally, 



