THE FLIGHT OF BIRDS. 211 



bird as a screw of which, the air formed the nut. AVe do 

 not think that we need refute such a theory. It is too evi- 

 dent that the alternating type which belongs to every muscular 

 movement cannot tend to produce the propulsive action of a 

 screw ; for while we admit that the wing revolves on an axis, 

 this rotation is confined to the fraction of a turn, and is fol- 

 lowed by rotation in the opposite direction, which in a screw 

 would entirely destroy the effect produced by the previous 

 movement. And yet the English writer to whose ideas we 

 refer has been so fully convinced of the truth of his theory 

 that he lias wished to extend it to the whole animal kingdom. 

 He proposes to refer locomotion in all its forms, whether 

 terrestrial, aquatic, or aerial to the movements of a screw 

 propeller. Let us only seek in the anatomical structure of 

 the organs of flight the information which it can afford us ; 

 that is to say, that which refers to the forces which the bird 

 can develop iu flight, and the direction in which these forces 

 are exerted. 



Comparative anatomy shows us in the wing of birds an 

 analogue of the fore limb of mammals. The wing when 

 reduced to its skeleton, presents, as in the human arm, the 

 humerus, the two bones of the fore-arm, and a rudimentary 

 hand, in which we still fiud metacarpal bones and phalanges. 

 The muscles also present many analogies with those of the 

 anterior limb of man ; some parts of these bear such a resem- 

 blance both in appearance and in function, that they have 

 been designated by the same name. 



In the wing of the bird, the most strongly developed muscles 

 are those whose office it is to extend or bend the hand upon 

 the fore-arm, the fore-arm on the humerus, and also to move 

 the humerus, that is say, the whole arm, round the articula- 

 tion of the shoulder. 



In the greater number of birds, especially of the larger 

 lauds, the wing seems to remain always extended during flight. 

 Thus, the extensor muscles of the different portions of this 

 organ would serve to give this organ the position necessary 

 for rendering flight possible, and for maintaining it in this posi- 

 tion ; as to motive work, it would be executed by other muscles, 

 much stronger than the preceding namely, the pectorals. 



