248 PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



tliat to sustain tlie bird against gTavitation the wing needs only to 

 oscillate in a vertical plane, in consequence of the predominance of the 

 resistance of the air acting from below over that acting conversely. 



Before discussing the value of this hypothesis, it is necessary to 

 ascertain whether the wing of a bird really oscillates only in a vertical 

 plane. We thus find presented to us, as at the beginning of our studies 

 on the flight of insects, questions which experiment alone can answer. 

 The problem nevertheless presents itself here under definite conditions. 

 From its much larger body than that of the insect, and from our better 

 knowledge of its anatomical conformation, the bird offers to us the 

 means of study and experiment of another description. I shall exhibit, 

 as far as possible, the nature of the muscular force of the bird, and the 

 influence which tlie particular arrangement of its muscles and the form 

 of its wings exerts upon flight. The methods of myography renders 

 the analysis of the diflereut forms of motion produced by wings so easy 

 that it has been of great assistance in these researches. 



Comparative anatomy exhibits the analogue of the anterior limbs of 

 mammals in the bird's wing. Keduced to its skeleton the wing presents, 

 like the human arm, the humerus, the two bones of the fore-arm, and a 

 rudimentary hand in which the metacarpals and phalanges are to be 

 found. The muscles also ofier numerous analogies with those of the 

 anterior limbs of man. Among others, some have such an analogy of 

 aspect and function that they can be designated by the same names. 

 In short, in the case of birds, the muscles which attain the greatest 

 development are those which act in extending or flexing the hand on 

 the fore-arm, the fore-arm on the humerus, and, finally, of moving the 

 humerus, that is to say, the whole arm upon the articulation of the 

 shoulder. 



Among most birds, especially of the large kinds, the wing appears to 

 remain always extended during flight. So that the extensor muscles of 

 the diflereut parts of the wing serve to give that organ the posture 

 necessary for flight, and to maintain it in that position, while the motive 

 work is executed by other and much stronger muscles than the pre- 

 ceding, viz, the pectorals. All the anterior portion of the thorax of 

 birds is occuj^ied by powerful masses of muscle, and above all by a gTeat 

 muscle which, from its being attached to the sternum, the sides, and 

 the humerus, is evidently the analogue of the great pectoral muscle of 

 man and other mammalia. Its ofdce is clearly to draw down the wing- 

 quickly and strongly, and to take sufficient hold on the air to sustain as 

 well as to move the vrhole body of the bird. The middle pectoral is 

 found below the great pectoral; without an analogue in other kinds of 

 animals, this muscle serves to raise up the wing. Lastly, the small 

 pectoral passes externally from the sternum to the humerus ; it is an 

 accessory of the great pectoral. It is well known that the strength of a 

 muscle is in proportion to its volume, and as the pectoral muscles of a 

 bird are about one-sixth of its total weight, we readily comprehend 

 that it is upon these poMcrful organs that the principal duty in the act 

 of flight devolves. Borelli endeavored to deduce from the volume of 

 these muscles the force which the bird employs in flying, and concluded 

 that this equals 10,000 times the weight of the body. 1 will not stop to 

 refute the error of Borelli, which so many others have called in ques- 

 tion, while they endeavored to substitute for the figures of the Italian 

 physiologist estimates which will scarcely be more easy to prove. 

 The great discrepancy which exists between the diflereut estimates of the 

 muscular power of birds shows that the attempts at measurement were 

 l^re mature. If we would make a true estimate to-day of the x)ower 



