. Comparative anatomy and physiology. 386 COMPARATIVE ANATOMY AND PHYSIOLOGY. i Jj; ;.•;;;.• developed, as we know, in certain Birds; the skeleton of the fore-arm is specially modified (see page 352), and forms for the wing a firm anterior bar, comparable to the anterior nervure of the insect's wing ; this bar is moved by special mus- cles, which are at- tached near its base ; but all of which lie on the ventral or lower surface of the body, and thereby enable the centre of gravity of the bird to be lower than it would be were the extensor muscles of the arm placed, as in other Vertebrates, on
. Comparative anatomy and physiology. 386 COMPARATIVE ANATOMY AND PHYSIOLOGY. i Jj; ;.•;;;.• developed, as we know, in certain Birds; the skeleton of the fore-arm is specially modified (see page 352), and forms for the wing a firm anterior bar, comparable to the anterior nervure of the insect's wing ; this bar is moved by special mus- cles, which are at- tached near its base ; but all of which lie on the ventral or lower surface of the body, and thereby enable the centre of gravity of the bird to be lower than it would be were the extensor muscles of the arm placed, as in other Vertebrates, on the dorsal surface. A large surface of at- tachment for the pec- toral muscles is ob- tained by the great development in flying birds of the keel of the sternum (see page 347), and the extensor muscle works on a pulley. The greater portion of the wing is not formed by membrane or integument, but by the development of those integumentary structures which we call feathers. These feathers overlap one another in such a way that the wing is convex above and concave below, and that pressure from below forces the feathers more closely together. From this arrange- ment it is clear that in the up and down movement of the wing in the air, much greater effect is gained by. Fig. 16 i.—The Common Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Bell, F. J. (Francis Jeffrey), 1855-1924. London, Cassell
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