MECHANISM OF FLIGHT IN BIRDS. 149 



double, as in Pavo bicalcaratns, to be a superadded weapon 

 to the metatarse. As the toes in the tridactyle emeu, 

 cassowary, and bustard, have respectively three phalanges, 

 four phalanges, and five phalanges, we recognize them as 

 answering to the second, third, and fourth in other birds ; 

 the toes in the didactyle ostrich have respectively four 

 and five phalanges, and what is here truly suggestive, the 

 outermost, which is much the smallest and shortest toe, 

 has the greater number of joints, viz: five, thus retaining 

 its ornithic type, as the fourth, or outermost, toe. 



The entire form of the body, and consequently that of 

 its bony framework, in a bird, has special reference to 

 the power of flight. The trunk is an oval with the large 

 end forwards. The vertebral column of this part is short 

 and almost inflexible, so that the muscles act to great ad- ; 

 vantage ; the spine of the neck being long and flexible, ; 

 the centre of gravity is readily changed from above the ; 

 feet — as when standing or walking — to between and be- \ 

 neath the wings during flight; when suspended in the air 

 the bird's body naturally falls into that position, which 

 throws the centre of gravity beneath the wiugs. The axis I 

 of motion being situated in a different place in the line of '. 

 the body when walking from that which is used when flying, : 

 the discrepancy requires to be compensated by some means 

 in all birds, in order to enable them to perform flight 

 with ease. Eaptorial birds take a horizontal position 

 when suspended in the air, and the compensating power | 

 consists in their taking a more or less erect position when \ 

 at rest. Another class, including the woodpeckers, wag- ! 

 tails, &c., take an oblique position in the air ; with these 

 the compensating power consists in their cleaving and 

 passing through the air at an angle coincident v/ith the 

 position of the body, and performing flight by a series of 



18^ 



