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The greatest single factor which separates birds from other vertebrate animals, and gives them their specific character, is their ability to fly. Apart from a few animals like the flying foxes, flying fish, and certain exotic frogs (which cannot be said truly to fly, but rather glide with the aid of specially developed membranes), birds and bats are the only existing vertebrates which.have, by natural evolution, developed true organs of flight. Both have evolved from primitive lizards, the best flying member of which group was the pterodactyl, which had a large membranous wing similar in appearance to that of a bat and, like that wing, involving the feet muscles in its operation. The bird's wing, on the other hand, has developed highly specialized features : the membrane of skin is replaced by a complex system of feathers, which interlock to form, as we shall see, a very efficient organ for flying. In addition, the bird's wing is harnessed only to the " arm " bones and muscles, and not connected in any way with the legs. This latter is an important modification, since it enables the bird to keep its feet quite free from the flying mechanism and to use them as independent units.
Let us see first how an individual feather is constructed, and from that go on to consider how a bird's wing is built up from such feathers, and how their arrangement leads to the production of an efficient mechanism for flight. Feathers are composed of keratin, which has a chemical composition common to horn, hooves, hair, wool and all similar body growths. That is why, when feathers or wool or horn, are burned, they all smell alike ; the smell of a blacksmith's shop. A single flight feather consists of a stout main shaft roughly circular in cross section near the root, but flattening and thinning as it approaches the tip. From the central shaft radiate smaller shafts, called " radii " or " barbs " which in their turn support very fine structures Continue reading "a bird in the air"...
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