. Essentials de biologie a présenté des problèmes. La biologie. La RESPIRATION et l'excrétion 383 BroncKial moTiary de, md-a-rlery se divise en deux bronches. Les bronches dans les poumons se divisent en un grand nombre de petits tubes, les bronches, qui divisent un peu comme les petites branches d'un arbre. Cette branche- ment augmente la surface de la des tubes d'air dans les poumons. Les bronches, en effet tous les passages d'air, sont hned avec cellules ciliées. Les cils de ces cellules sont constamment en mouvement, en battant avec une course rapide vers l'extrémité du tube, c'est, vers la bouche. Par conséquent

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. Essentials of biology presented in problems. Biology. RESPIRATION AND EXCRETION 383 BroncKial From, md-moTiary a-rlery divides into two bronchi. The bronchi within the lungs break up into a great number of smaller tubes, the bronchial tubes, which divide somewhat like the small branches of a tree. This branch- ing increases the surface of the air tubes within the lungs. The bronchial tubes, indeed all the air passages, are hned with ciliated cells. The cilia of these cells are constantly in motion, beating with a quick stroke toward the outer end of the tube, that is, toward the mouth. Hence, if any foreign material should get into the windpipe or bronchial tubes, it will be expelled by the action of the cilia. It is by means of cilia that phlegm is raised from the throat. Such action is of great im- portance, as it prevents the filling of the air passages with foreign matter. The bronchi end in very minute air sacs called alveoli, — little pouches having elastic walls, — into which air is taken when we inspire or take a deep breath. In the walls of the alveoli are numerous capil- laries, the ends of arteries which pass from the heart into the lung. I It is through the very thin walls of the alveoli that an inter- change of gases takes place which results in the blood giving up part of its load of carbon dioxide, and taking up oxygen in its place. ^ The Pleura. — The lungs are covered with a thin elastic membrane, the pleura. This forms a bag in which the lungs arc hung. Between the walls of the bag and the lungs is a space filled with lymph. By this means the lungs are prevented from rubbing against the walls of the chest. '' Breathing. — In every full breath there are two distinct movements, inspiration (taking air in) and expiration (forcing air out). In man an inspiration is produced by the contraction of the muscles between the ribs together with the contraction of the diaphragm, the muscular wall just below the heart and lungs; this results in pulling

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