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Show 217 CLASS II. AVES. Birds are Oviparous Vertebrata, with double systems of cir~ cu1ation and respiration, organized for flight. Their lungs, undivided and attached to the ribs, are enveloped by a membrane pierced with large holes, which aUow the air to pass into several cavities of the chest, lower part of the abdomen, arm-pits, and even of the interior of the bones, so that not only is the surface of the pulmonary vessels bathed in the ambient fluid, but that also of an infinitude of vessels in other parts of the body. Thus, in certain respects, Birds respire by the branches of the aorta, as well as by those of the pulmonary artery, and the energy of their irritability is in proportion to their quantum of respiration. ( 1 ) The whole body is so constructed as to profit by this energy. Their anterior extremities, destined to sustain them in flight, could neither serve them for standing, nor for prehension; they are bipeds then, and pick up objects from the earth with their mouth; their body, consequently, is inclined before their legs, the thighs directed forwards, and the toes elongated, to form a sufficient base for it. The pelvis is very much extended in length, in order to furnish points of attachment to those muscles which support the trunk upon the thighs. There is even a suite of muscles reaching from the ~elvis to the toes, passing over the knee and heel, so that the Simple weight of the bird flexes the toes; it is thus that they . 0) Two common Swallows c~nsume as much pure air as a Guinea-Pig. LavoiSier, Mem. de Chimie, I, 119. Vot. I.-2 C |