STRUCTURE OF BIRDS. 47 



In flight the centre of gravity is brought forward beneath 

 the origin of the wings ; but in standing it is carried in various 

 degrees backwards. As the legs are always articulated very 

 far behind, it becomes necessary that they should be much 

 bent, and that the toes should project anteriorly beyond the 

 vertical line drawn through the centre of gravity, in order to 

 give stability. When the feet are very short and placed far 

 behind, whether the neck be long or not, the bird is obliged to 

 stand and walk with the body nearly in an erect position ; but 

 when the bones of the leg are moderate or large, and esj)ecially 

 when the neck is long and flexile, the feet are brought forward 

 and the head backward, so that the body remains nearly in a 

 horizontal position. Birds generally rest upon one leg, the 

 other being drawn up towards the side ; and many species are 

 enabled to employ this attitude without fatigue by a peculiar 

 arrangement of the tarsal joint, there being a process from 

 the anterior part of the head of the metatarsal bone, which fits 

 into a corresponding cavity at the lower end of the tibia, from 

 w^hich it is not again withdrawn without an effort on the part 

 of the bird. Other birds are rendered secure during repose by 

 the weight of the body, which, bending the joints of the legs, 

 brings into a state of tension the long flexor tendons that go to 

 the extremities of the toes, which they thus cause to grasp their 

 perch. Some birds in reposing lie flat upon the ground, and this 

 posture is that adopted by most species during incubation. 



In walking, a bird throws its centre of gravity to one side, 

 raises and brings forward the leg of the other side, impels the 

 body forward by the foot that rests on the ground, shifts its 

 centre of gravity to the other side, and thus continues, the neck 

 all the" while undulating from side to side. These motions are 

 easily observed in hea^y birds, as Geese and Ducks. In birds 

 of a lighter form, as Pigeons and Crows, the alternate forward 

 and backward motion of the neck during walking is very con- 

 spicuous and often graceful, and is of course employed for 

 balancing the body. Those birds w^hich move by hopping or 

 leaping, throw the body forwards by bending all the joints of 

 the legs, and then by suddenly extending them give it the ne- 

 cessary impulse, which causes it to perform a curve in the air. 

 Some birds, as eagles, in thus leaping, use their wings also ; 



