. The structure and life of birds . m, underfavourable conditions, a flight of fifty miles in anhour is no great exertion. In all birds we findthe same striking contrast between the excellenceand the small size of the lungs. Though they arespongy, they have but little elasticity. When a manexpands his chest, the lungs are distended and the airrushes in to fill the vacuum caused. A birds lungsvary little in size. They are prolonged into spaciousair-sacks, the most characteristic part of the breath-ing apparatus, which renders elasticity of lungsunnecessary. These air-sacks are extensions of the
. The structure and life of birds . m, underfavourable conditions, a flight of fifty miles in anhour is no great exertion. In all birds we findthe same striking contrast between the excellenceand the small size of the lungs. Though they arespongy, they have but little elasticity. When a manexpands his chest, the lungs are distended and the airrushes in to fill the vacuum caused. A birds lungsvary little in size. They are prolonged into spaciousair-sacks, the most characteristic part of the breath-ing apparatus, which renders elasticity of lungsunnecessary. These air-sacks are extensions of themembrane which forms the walls of the two bronchi we have already described asleading to the lungs. They run through the lungsdividing as they go, and end in these great ex-pansions by the help of which a bird is able to get 8o THE STRUCTURE AND LIFE OF BIRDS chap. more work, in proportion to the size of his body, outof his exceedingly small and light lungs than a mancan out of his far heavier apparatus. Any one who ICS. Fig. 23—Diagram after Heider. Air-sacks excepting the cervical. The lungs areshaded dark. Ahs, abdominal sack ; as, anterior thoracic ; B, entrance of bronchialmembrane; H, humerus; ics, interclavicular sack, surrounding trachea; and 1, 2,3, 4, its extensions ; 2 opens between the pectoral muscles ; rs, posterior thoracicsack ; t, trachea. See Fig. 25. wishes to see the air-sacks—and to see them is muchbetter than only to read of them—should take somebird of moderate size, such as a pigeon, cut throughthe windpipe somewhere in the neck, insert a blowing- vi FORM AND FUNCTION Si tube and tic the windpipe round it with a piece offine string or cotton, then inflate them. The wholebreast and abdomen will be seen to rise and windpipe should then be tied up and the air-sacks left in a state of inflation. Next the centralpart of the sternum must be got out of the way. Cutit longitudinally on either side of the keel from thehinder almost to
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