THE FLIGHT OF BIRDS 301 



front wing-membrane stretched taut. There is another tendon 

 (not shown in the figure) which performs the same office for 

 the hind wing-membrane. Besides these muscles there are many 

 others which adjust the parts of the wing in various ways. The 

 thumb, for example, is well off in this respect. 



One or two other points in the structure of the wing deserve 

 notice here. The long tendon of one of the tensor muscles (T. i ) 

 runs along the edge of the front wing-membrane to be attached 

 to the hand. Unlike tendons in general it is elastic, and is 

 stretched to three times its normal length when the wing is 

 extended. This helps to keep the membrane taut, and the 

 hand in line with the forearm. And when the bird folds up its 

 wing the elasticity of this tendon comes into play and gives con- 

 siderable assistance. 



The large rowing-feathers are divided into two sets primary 

 quills attached to the hand, and secondary quills borne by the 

 forearm. There is also a tuft of feathers on the thumb, making 

 up the bastard -wing. When the wing is extended the quills 

 spread out to give an expanded surface without muscular effort. 

 This is effected by the agency of an elastic band or ligament 

 which stretches from hand to arm-pit, and is pierced by the 

 bases of all the secondary and some of the primary quills. Its 

 work is aided by a second similar band which runs under the 

 quills nearer their attachment to the wing. A still more beauti- 

 ful mechanical arrangement affects the secondary quills only, 

 bringing it about that during the down-stroke of the wing they 

 are pressed firmly together so that no air can pass between them, 

 while during the up-stroke they are slightly separated so as to 

 allow of such passage, thereby diminishing the amount of work. 

 Dissection reveals the cause of this. From the back edge of 

 the muscle (6.2) which bends the hand on the forearm a minute 

 tendon passes to each secondary quill. When the wing is ex- 

 tended these tendons pull the overlapping quills together so that 

 an unbroken surface is constituted. But when the up-stroke is 

 made the hand is bent to some extent upon the forearm, the 

 little tendons relax, and the quills being loosened, permit air to 

 pass between them. 



The stumpy tail with its radiating steering-feathers is well 

 provided with muscles, by which it can be raised, lowered, and 

 otherwise adjusted so that it may be able to perform its work 



