. Kirkes' handbook of physiology . esented only by minute and scattered cartilag-inous flakes. And when the bronchi by successive branches are reducedto about -fa of an inch, mm., in diameter, they lose their cartilaginous ele-ment altogether and their walls are formed only of a tough, fibrous, elasticmembrane with circular muscular fibers. They are still lined, however,by a thin mucous membrane with ciliated epithelium, the length of the cellsbearing the cilia having become so far diminished that the cells are almostcubical. In the smaller bronchi the circular muscular fibers are relative
. Kirkes' handbook of physiology . esented only by minute and scattered cartilag-inous flakes. And when the bronchi by successive branches are reducedto about -fa of an inch, mm., in diameter, they lose their cartilaginous ele-ment altogether and their walls are formed only of a tough, fibrous, elasticmembrane with circular muscular fibers. They are still lined, however,by a thin mucous membrane with ciliated epithelium, the length of the cellsbearing the cilia having become so far diminished that the cells are almostcubical. In the smaller bronchi the circular muscular fibers are relativelymore abundant than in the larger bronchi and form a distinct circular coat. The Lungs and Plurae. The lungs occupy the greater portionof the thorax. They are of a spongy elastic texture, and on section appearto the naked eye as if they were in great part solid organs, except wherebranches of the open bronchi or air-tubes may have been cut across and showon the surface of the section. In fact, however, the lungs are hollow organs. Fig. 222.—Transverse Section of the Chest. composed of a mass of air cavities all of which communicate finally withthe common air-tube, the trachea. Each lung is enveloped by a serous membrane, the pleura, which ad-heres closely to its surface and provides it with its smooth and slipperycovering. This same membrane lines the inner surface of the chest continuity of this membrane, which forms a closed sac as in the caseof other serous membranes, will be best understood by reference to figure appearance of a space, however, between the pleura which covers thelung, visceral layer, and that which lines the inner surface of the chest, parietallayer, is inserted in the drawing only for the sake of distinctness. Theselayers are, in health, everywhere in contact, one with the other; and betweenthem is only just as much fluid as will insure frictionless movement in theirexpansion and contraction. THE FINER STRUCTURE OE THE LUNG 249 When con
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