302 



ANIMAL MOVEMENT 



in directing the course of flight. It also possesses elastic liga- 

 ments. 



Active Flight. — For a bird, in the absence of wind, to rise 

 from the ground involves very considerable muscular effort. If 

 the legs are fairly long the start is greatly helped by a standing 

 or running jump, but in any case the wings have to be moved 

 rapidly up and down until the bird is fairly under weigh. As will 

 be explained later, the support afforded by the air is dependent 

 upon the speed attained. Let us now suppose that the bird is 

 flying along, its wings having acquired sufficient "purchase" upon 

 the air for propulsive purposes. It will be seen from fig. 835 

 that each wing may now be considered a lever of the third 



order, with the motive power 

 acting between the fulcrum and 

 the weicrht. The fulcrum here 



o 



is not a firm fixed point, as in 

 a crowbar used to raise a block 

 of stone, but is constituted by 

 the resistance of the air, and 

 may be conveniently considered 

 as located near the end of the 

 wing. A similar yielding ful- 

 crum is presented by the water to the blade of an oar used to 

 propel a boat. The weight to be moved is that of the bird, and 

 the power is supplied by the great pectoral muscle. That this 

 is attached very near to the shoulder means great expenditure 

 of force, with corresponding gain in speed, a matter which is of 

 the greatest importance in this case. The most effective part 

 of the wing is clearly that which is furthest from the body, for 

 this moves most rapidly, and therefore gets the best grip upon 

 the air. The near section, though less useful in this respect, is 

 of greater value as a parachute to retard descent between the 

 down-strokes, and the wing-membranes are of considerable use 

 in this respect. It must also be remembered that the wing is 

 not flat, but concave below and convex above, which gives a 

 better hold on the air during the down-stroke and makes the up- 

 stroke more easy. The same ends are furthered by the arrange- 

 ments for adjusting the quills, of which an account has already 

 been given (p. 301). 



The action of the wings has so far been treated as if they 



Fig. 835. — Diagram of Wing- Action (end view) 



M and M^, Great pectoral muscle relaxed and contracted 

 respectively. 



