374 CONFORMATION OF THE LEGS. 



permanent contraction of these organs, which renders a stationary 

 position so fatiguing. But in the Stork, and other long-shanked 

 Birds, it is very different ; the I jwer extremity of the femur 

 presents a hollow, in which is imbedded, during the extension 

 of the limb, a projection of the tibia, which can escape from it 

 only by a muscular effort; the foot once straightened con- 

 sequently remains extended, without the animal requiring to 

 contract the muscles, and without any fatigue arising from it. 



337. It is always more difficult to a Bird to take flight, 

 when it is on the ground, than when it can dart from an elevated 

 point : we have already seen the reason of this ; and every one 

 knows that most of these animals perch more frequently than 

 they settle on the ground. To maintain themselves in equili- 

 brium on a branch, they must grasp it closely with their claws ; 

 and if it had been necessary to employ for this a considerable mus- 

 cular force, such a position would soon have become fatiguing. 

 But here another very simple mechanism renders all effort 

 nearly useless, and enables the Bird to grasp the branch which 

 supports it. even when it sleeps. The flexor muscles of the claws 

 pass over the articulations of the knee and heel in such a manner, 

 that when the latter bend, they necessarily press on the tendons 

 of the muscles, and make them bend the toes ; the weight of the 

 body, pressing down the thighs and legs, necessarily produces 

 therefore this movement; and it results from this, that the 

 animal grasps, without effort, the branch on which it is perched, 

 and there maintains itself fixed. 



338. Great differences exist in the conformation of the legs, 

 according to the kind of life to which the Birds are destined. 

 Thus Birds that have the power of walking with great rapidity, 

 have very long and powerful legs, and the foot comparatively 

 small ; in the Cassowary and the Ostrich (Fig. 179), whose speed 

 is as great as that of the horse, this arrangement is very 

 remarkable; and it is observed also in the Secretary, which 

 walks rapidly while pursuing the Serpents, that constitute its 

 principal nourishment. In the Eagle, the Falcon, the Vulture 

 (Fig. 177), these members are equally strong but short ; and 

 the claws are armed with large, sharp, hooked talons, by the aid 



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