364 



into the abdomen, by means of membranous sacs, and into the skeleton, 

 by means of canals which establish a communication between these abdo- 

 minal and osseous aerial tubes and the pulmonary organ ; so that the whole 

 body, distended by air rarified by a considerable degree of heat, since it 

 is ten degrees above that of other warm-blooded animals, clothed in fea- 

 thers almost as light as the air itself, requires but a moderate degree of 

 force to support itself in that medium. On the other hand, when the 

 wings are expanded, they present to the fluid a very extended surface ; 

 the pectoral muscles which set them in motion, are besides sufficient- 

 ly strong to strike the air with a power, and to repeat the stroke 

 with a rapidity and continuousness of which no other animal would 

 be capable. We know how powerful* the muscles of the wings are, 

 even in the tame fowl, which make so very little use of them. Last- 

 ly, the contractility of these very powerful muscles, is greater in birds 

 than in any other animal, no one possesses so much strength in so small 

 a compass. What quadruped of the same weight as an eagle, could 

 strike with his foot so violent a blow as that bird, when to stun his prey 

 or to defend himself, he gives repealed blows with his pinion ? This mus- 

 cular energy is, no doubt, connected with the extensive respiratory or- 

 gans, with the highly stimulating qualities of a blood that is warmer, 

 more oxidized, more concrescible, in a word, more arterialized, than that 

 of any other animal. 



Let us now inquire how birds, endowed with an organization so favour- 

 able to flying, perform that action. A bird begins by ascending into the 

 air, either by rising at once from the ground, or by allowing himself to 

 fall from a height. If, on the ground, and if his wings are too large to 

 be freely spread, he has a difficulty in rising ; in that case he goes to an 

 elevated spot and throws himself from it, that he may have sufficient 

 room to extend his wings and strike, in the air, the first stroke that is to 

 raise him. The wings expand horizontally, the humerus which forms 

 their principal part, standing off from the body ; they then descend ra- 

 pidly, and, as the air resists the sudden effort which tends to depress it, 

 the body of the bird is elevated by a kind of elastic re-action, correspond- 

 ing to the leap of man, and to the swimming of fishes; the impulse be- 

 ing given, the bird closes his wings, contracts his dimensions, as- much 

 as possible, that the impulse may be almost entirely employed in raising 

 his body, and may not be counteracted by the resistance of the air. The 

 resistance of the air, but particularly the v/eight of the bird, would soon 

 overcome the velocity that has been obtained, and he would drop, if, by 

 again striking the air, he did not again rise. If the bird strikes a second 



* Birds have three pectoral muscles : the third, or lesser pectoral, is destined to 

 draw the Immerus, towards the body, the great pectoral, which is attached to their 

 enormous sternum, and alone exceeds in weight all the other muscles of the 

 bird together; the middle pectoral, whose tendon turns over a kind of pulley, 

 and is attached to the head of the humerus which it raises ; by means of this 

 mechanism, nature has placed an elevator muscle at the lower part of the body, 

 so as to increase the weight of this part of the bird, which, without this kind 

 of ballast, might have been upset in the air. By these and other peculiarities in 

 the organization of flying animals, the centre of gravity is always below the in- 

 sertion of the wings, and near the point on which the body is, during flight, sus- 

 pended. The positions, also, assumed by the head and feet, are often calculated 

 to facilitate flight, and give to the wings every assistance in continuing progress- 

 ive motion. Author's Note. 



