THE CLASS BIRDS. 



289 



tation ; hence the utility of the extremely flexed thigh and 

 the obliquity of the tarsus on the leg. When the foot is 

 large and broad, and the neck is so flexible as to carry the 

 head well backwards, the equilibrium may be thus maintained 

 without abandoning the horizontal position (Fig. 229) ; but 

 when the neck is short, the head large, and the toes of mode- 

 rate length, the bird is forced to maintain an almost vertical 

 position (Fig. 230). To maintain their equilibrium, birds 

 place the head under the wing whilst they sleep perched on 

 one foot (Fig. 240). The mechanism by which they stand 

 with such ease and for such a length of time on one foot, is 

 this : the lower extremity of the femur presents a hollow, in 

 which is lodged, during the extension of the limb, the top of 

 the tibia, which cannot quit this cavity but by a muscular 

 effort. The foot once spread out, remains so, requiring no 

 further muscular effort, and consequently giving rise to no 

 fatigue. 



Most birds perch, and it is easier for them to spread their 



Fig. 229. The Ibis. 



Fig. 230. Manchot. 



wings and take flight when perched on a branch of a tree or 

 rocky edge, than when resting on the level ground. In order 

 to perch with safety, they embrace the bough closely with 

 their toes ; and if this required an incessant muscular effort, it 

 could not be supported for any length of time. A mechanical 

 contrivance enables the bird to dispense with this even whilst 

 asleep. The flexor muscles of the toes pass over the articula- 

 u 



