POSITION OP THE LEGS. 373 



336. We have said that, during flight, the centre of gravity 

 of the body of the Bird ought to be under the shoulders; but in 

 order that it may retain its equilibrium on its feet, which are 

 situated behind the trunk, these organs must be capable of 

 being brought sufficiently forward, and the claws must be 

 long enough to project beyond the point, where a vertical line 

 would fall, passing through the centre of gravity ; or else this 

 centre must itself be further back, so as to be above the base of 

 support. This explains the reason of the great curvature of the 

 thigh, and the obliquity of the tarsus on the leg ; when the 

 foot is large, and the neck can be so bent back as to carry back 

 the head, the equilibrium is thus established without the body 

 being much removed from the horizontal position (Fig. 188); 

 but when the neck is short, the head large, and the claws of 



Fio. 188 IBIS. FIG. 189. PENGUIN. 



moderate length, the animal is obliged to assume during rest or 

 walking, a nearly vertical posture (Fig. 189). It is to preserve 

 their equilibrium more easily, that Birds generally place their 

 heads under their wings while they sleep perched on a single 

 foot. In many of these animals, this position is rendered 

 singularly convenient, by a peculiarity in the construction of the 

 articulation of the knee. In Man and most other Animals, the 

 members bend under the weight of the body, as soon as their 

 extensor muscles cease to contract: and it is the necessity of the 



