xvi. 7 SKELETON OF WINGS 447 



side of the thigh, and its tendon runs beneath the patella on to the 

 lateral surface of the lower leg, where it is attached to the upper end 

 of the muscles that flex the toes. This arrangement provides a single 

 string crossing hip, knee, and ankle and allows the weight of the body 

 to flex the toes as the joints bend. 



The second mechanism for maintaining the bird on its perch is a 

 locking device that holds the toes flexed. The under-surface of the 

 flexor tendon is ridged at the metatarso-phalangeal joint, where the 

 weight of the body presses it against a branch. The upper side of 

 the tendon sheath is also ribbed and as the bird settles on its perch 

 the two sets of ridges interlock. 



The feet show a wide variety of adaptations for special habitats (Fig. 

 260). In the cursorial and walking birds there are often long digits in 

 front and behind to give a long base for balance, but the number may 

 be reduced — to two in running birds, such as the ostrich. Hopping is 

 used by small birds on the ground and in the trees and produces quick 

 movement. It is expensive because of the large displacements of the 

 centre of gravity, and for long distances or large animals walking is 

 more efficient. Many different groups of birds have acquired webbed 

 feet for swimming. In birds exposed to cold the digits may be enclosed 

 in a coat of feathers. Birds of prey develop long raptorial talons. 

 Throughout the great group of perching birds one digit is directed 

 backwards, allowing firm grasp of a branch. In climbing birds the 

 fourth digit is often directed backwards as well as the first, so that the 

 foot forms a sort of pincer, with long curved claws. 



7. Skeleton of the wings 



The wing is designed to have a minimum moment of inertia about 

 an axis parallel to the sagittal plane and passing through the shoulder 

 joint. Movements are produced by muscles lying either outside the 

 arm or in its proximal part, with long tendons. The wing feathers are 

 carried along the post-axial border of the humerus, ulna, and hand, 

 and the shape of the wing depends on the position in which the feathers 

 are held by their muscles, as well as on membranes, the pre- and post- 

 patagia, developed where the limb joins the body. The active move- 

 ments of flight are produced mainly by the pectoral muscles; the joints 

 and muscles of the wing itself serve to spread the wing and to adjust its 

 shape during each beat. The humerus is short and broad with a large 

 head and an expanded surface for attachment of the pectoral muscles. 

 Radius and ulna are both large, especially the latter. There are only 

 two free proximal carpals and the remainder of the wrist is formed of 



