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20 THE BREATHING APPARATUS.
shape. The ribs increase in length, in obliquity, and in the size of their bows from above downwards as far as the seventh or eighth; the lengths of the cartilages between the ends of the ribs and the sternum also greatly increase from the first down to the seventh (Fig. 12); these cartilages being composed of a yielding elastic substance considerably increase the freedom of movement of the chest wall. The cartilages of the eighth, ninth, and tenth ribs are joined together, the cartilages of the eleventh and twelfth ribs are quite free. The seven upper ribs are called true ribs, the lower five false ribs, and the last two floating ribs. Whenever the ribs move they are raised upwards, and owing to their peculiarity in shape and arrangement, whenever they are raised the chest wall is expanded from side to side, especially at the level of the seventh rib, and from before backwards by the thrusting forward of the sternum.
26. The Breathing Muscles.—Between each pair of ribs are two muscles with fibres running obliquely downwards, called the external and internal intercostals. The fibres of the outside muscle run downwards and inwards (Fig. 12, upper part), those of the inside muscle run downwards and outwards (Fig. 12, lower part): these raise the ribs when they contract and so increase the size of the chest from side to side and from before backwards. The lungs because of the air-pressure follow the movement of the chest wall, and air goes into them whenever the chest cavity is enlarged.
27. The Floor of the Chest.—A muscle called the diaphragm forms the floor of the chest, and separates the chest from the abdomen (Fig. 13). It consists of muscular fibres outside, where it is attached to the cartilages of the