18 LOCOMOTION. 



these organs so frequently, and therefore do not become so 

 readily fatigued: and, in fact, all birds remarkable for rapid 

 and long sustained flight have large wings, while those that have 

 short or moderate wings, compared with the volume of their 

 body, fly less swiftly and require rest more frequently. 



To rise vertically, it is necessary that the wings of the bird 

 should be entii'ely horizontal, but this is not ordinarily the case; 

 in general they are inclmed from front to rear so as to impart to 

 the animal an obliquely ascending movement ; sometimes even 

 this inclination is such, that, to mount nearly vertically, into the 

 atmosphere, the bird is obliged to fly against the wind. The 

 length of the remiges influences the facility with which he can 

 rise in a calm air; birds that have the anterior remiges longest, 

 and most resisting at theii' extremity, fly more obliquely, than 

 those in which the wings are truncated at the end.] 



19. The feathers of the tail also assist in flJght, but in a dif- 

 ferent way ; the bird makes use of them as a rudder to direct 

 its course. The number of the feathers which perform this 

 oflice is ordinarily twelve, and they are called rectrices, and the 

 name of coverts of the tail, is given to those feathers which 

 cover their base. 



[We have seen that, during flight, the centre of gravity of the 

 bird should be near the shoulders : in order that he may preserve 

 his balance on his legs, which are placed near the posterior part 

 of the trunk, these organs must be flexed considerably forward, 

 and the toes must be sufficiently long, to be in advance of 

 the point where the vertical line should fall that passes through 

 the centre of gi'avity, or the centre of gravity must be carried 

 behind, so as to be above the base of support. (See First Book of 

 Natural History, Pac/e 92.) This explains the utility of the great 

 flexion of the thigh and the obliquity of the tarsus on the leg ; 

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

 the head behind, the equilibrium is thus established without, the 

 body being thrown much out of the horizontal position ; but 

 when the neck is short and toes of moderate length, the animal is 

 obliged, while standing or walkmg, to assume an almost vertical 

 position. It is for the purpose of more easily preserving their 

 equilibrium, that birds generally place their heads under their 

 wing, while they sleep, perched on one leg. In most of these 

 animals this position is rendered singularly commodious by a 

 peculiarity in the structure of the knee ; in man and most ani- 

 mals, the extremities bend under the weight of the body the 



19. How do the feathers ot the tail assist in flight? What are the 

 rectrices ? What is tlieir number ? 



