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THE BREATHING APPARATUS. 17
diaphragm). In breathing for voice the outward movement of the ribs is still more marked in inspiration. The expiration of repose and action is effected by the elastic recoil of the displaced organs and of the expanded chest walls and lungs; the expiration of voice has to be carefully controlled by voluntary muscles.
23. The Lungs.—The easiest way to get a good conception of the structure and function of the lungs is to purchase a sheep's pluck. Wash it thoroughly in water, and cut away the heart and all the fat and membrane and vessels attached to it so as to expose the windpipe and its branches into each lung. Examine the windpipe and note the arrangement of the rings of cartilage ending above in the enlarged cricoid cartilage with the other laryngeal cartilages attached to it (Fig. 11), also examine the vocal cords inside the larynx. The windpipe divides into two branches, one for each lung, and these branches divide and subdivide like the branches of a tree or shrub until every part of the lung is reached by a branch (Fig. 11). If a tree were hollow throughout and placed upside down it would resemble somewhat the arrangement of air-tubes and cells in the lung. The hollow trunk corresponds to the windpipe, the hollow branches dividing and subdividing into smaller branches represent the bronchial tubes, the hollow twigs represent the smallest or capillary bronchial tubes, and the hollow leaves represent the air-cells (of which there are said to be 600,000,000 in the lungs).
24. The Lungs are hollow and elastic.—If the
sheep's lungs are uninjured and fresh they can be distended by fixing a glass tube into the upper part of the windpipe, and by blowing air into it by means of a bellows or by the mouth. This proves that the lungs are hollow, v t. 2