In birds there is a double circulation. The heart consists of two moieties, or lobes, known as the auricle and ventricle. It is conical in form, and occupies the anterior part of the thorax, its apex passing between the lobes of the liver; but there is little perceptible distinction between auricles and ventricles. Their blood is richer in globules than that of the Mammalia, being more thoroughly permeated by air; the respiratory function is also more energetic, from the same cause--in fact, they consume a larger quantity of oxygen, and produce a proportionately greater degree of heat; for while their lungs are small, and placed in the upper part of the thorax only, where they are confined on each side to a cavity, bounded above by the ribs, and below by an imperfect diaphragm, they are perforated by tubes, which communicate with membranous cells, distributed over the thoracic and abdominal cavities, between the muscles, and beneath the skin,--often in all parts of the body. What distinguishes the bird, in fact, is not the wing; for certain of the Mammalia, as the Bat, and even some fishes, as the Gusard and Exocoetus, can traverse the air by expanding their wings. In birds the diaphragm which arrests the air in the Mammalia is scarcely perceptible, so the external air penetrates into every part of the body by the respiratory tubes, which ramify the whole cellular tissue, the interior of the bones, and even the feathers, and between the muscles. Their bodies, dilated by the air inhaled, lose a proportionate amount of weight; balloon-like, they float in the air, and, from their peculiar forms, they can swim, so to speak, in any direction in the gaseous element. [Illustration: Fig. 45.--Skeleton of the Swan.] Wings alone, then, would fail to support the bird in space. The position renders a double system of breathing necessary. Vital heat in animals is always in proportion to their respiration, for the oxygen of the air, which penetrates every cell and cavity of their bones, feathers, and body, warming and giving increased activity to their circulation, and specific lightness to their bodies, from its rich organisation enables birds to live in the coldest atmospheric regions. In Fig. 46 the respiratory organs of a Pigeon are represented. The trachea, or windpipe, is composed of many bony rings, varying in different species. In the Falcons it is slightly flattened, and tapers in a small degree; but in many genera it presents dilatations and contractions, and in others it is variously curved, two slender muscles, which run along its sides towards the sternum, serving to contract it. In many of the song-birds several pairs of small muscles are attached to the lower larynx where the tube bifurcates, by which they are enabled to control this organ, which is the producer of their note. [Illustration: Fig. 46.] [Illustration: Fig. 47.]