142 TEXT-BOOK OF ZOOLOGY 



sequently, the centre of gravity of a flying bird invariably comes to lie 

 in the lower part of its body, so that the body, hanging in the shoulder- 

 joints, as it were, is always in a condition of stable equilibrium (something 



like a ship's lamp, which is kept vertical, 

 however much the ship pitches or rolls). 



Although the wings deviate in form con- 

 siderably from the fore-limbs of mammals 

 (why?), they are nevertheless constructed 

 on exactly the same plan (see Part I, 

 p. 12). In the wing of a bird, as in the 

 fore-limb of the mammal, we distinguish 

 three portions, viz., the upper arm (hu- 

 merus), the lower arm (radius and ulna), 

 and the hand. In the hand we may, in- 

 WING FOLDED TOGETHER. deed, still distinguish the carpus, metacar- 

 (For explanation of figures, see illus- pus, and digital bones ; but in the adult 



on, p. 140.) 



with each other, and in part rudimentary. The three digits which 

 remain correspond to the thumb and the second and third fingers of the 

 human hand. When at rest, and during all other movements of the 

 bird, except flight, the wings are laid close against the body in the form 

 of a Z. (Why ? Compare, however, the illustration of the penguin.) 



4. Hind-limbs. All birds are occasionally obliged to descend upon 

 the earth or upon solid objects. (Why?) Even those whose life is 

 almost entirely passed in the water (penguins) must come on shore for 

 the purpose of breeding. Hence the hind-limbs cannot likeivise be 

 developed into organs of flight (compare, on the other hand, with fishes), 

 but must take on the form of legs, enabling the bird to progress along 

 the ground, or, at least (swallows, etc.), to obtain a hold on solid objects. 

 In many birds the legs are, of course, adapted by special arrangement 

 for other activities besides (compare scansorial, natatorial, raptorial 

 birds, etc.) ; the structure of these organs, however, must in all birds be 

 essentially the same, since all are employed in one and the same kind of 

 work (walking or running, and holding on to solid objects). As in 

 mammals, the leg consists of three distinct portions : the thigh (femur), 

 which is always concealed within the body ; the leg proper (tibia and 

 fibula), the latter usually very rudimentary) ; and the foot. The bones 

 of the tarsus are not separate in the adult bird, coalescing, even during 

 development within the egg, partly with the tibia, partly with the 

 metatarsal bones. The latter likewise coalesce, and, in combination 

 with the lower tar sal bones, form a long tubular bone, the tarso-meta- 

 tarsus. (Locate the knee and ankle joints in the bird's leg.) All birds 



