. Outlines of zoology. Zoology. 6i6 BIRDS. windpipe may for a time breathe. The whole system of air-containing cavities is continuous, except in the case of the skull bones, whose spaces receive air from the nasal and Eustachian tubes. In view of these facts, it used to be supposed that a bird with heated air in the sacs and spaces was comparable to a balloon. But this is fallacious. The air must indeed lessen the specific gravity of the bird, but a few mouthfuls of food are sufficient to counteract the lightening. Moreover, in many small birds of powerful flight, all the large bones, or all e
. Outlines of zoology. Zoology. 6i6 BIRDS. windpipe may for a time breathe. The whole system of air-containing cavities is continuous, except in the case of the skull bones, whose spaces receive air from the nasal and Eustachian tubes. In view of these facts, it used to be supposed that a bird with heated air in the sacs and spaces was comparable to a balloon. But this is fallacious. The air must indeed lessen the specific gravity of the bird, but a few mouthfuls of food are sufficient to counteract the lightening. Moreover, in many small birds of powerful flight, all the large bones, or all except the humerus, contain marrow, and are there- forenot "pneumatic"; and thehornbill, which has no great power of flight, is one of the most pneumatic of birds. It is certain that in ordinary flight the lightest of birds has to keep itself from falling by constant effort. The bird is not comparable to a bal- loon, but to a flying machine; "it has to be not a buoyant cork, but a buoyant ; In short, the air-sacs increase the bird's respiratory content, secure more perfect aeration of the lungs, and probably aid in regulating the body temperature. Ruskin has compared the flight of a bird to the sailing of a boat. " In a boat the air strikes the sail; in a bird the sail strikes the air ; in a boat the force is lateral, and in a bird downwards; and it has its sail on both ; But, as he says, the sail of a boat serves only to carry it onwards, while wings have not only to waft the bird onwards, but to keep it up. To carry the weight of the bird the wings strike vertically, to carry the bird onwards they strike obliquely; sometimes the direc- tion of the stroke is more vertical, and then the bird mounts upwards ; sometimes it is more oblique, and then the bird speeds onwards; usually. Fig. 271. —Position of wings in pigeon at maximum elevation. —From Please note that these images are extracted from scanned page images that may ha
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
