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THE BREATHING APPAEATUS.
20. Breathing.—The voluntary muscles engaged in breathing, that require special attention, are those that move the chest walls in inspiration and those that control the air in expiration. The function of respiration consists of an inspiration—a taking-in of air—followed by an expiration—an emission of air. The air enters the lungs whenever any diameter of the chest is increased in size, and air is emitted when the chest returns to its original size. This function may be performed in many ways.
21. Breathing of Repose.- -In ordinary quiet breathing the diameter of the chest from the top to the bottom is increased in size by the descent of the contracting diaphragm, and the requisite quantity of air for physiological needs is taken in, to be expelled again by the ascent of the relaxing diaphragm: this purely involuntary action is repeated about 17 to 20 times a minute by the healthy adult.
22. Breathing of Action and Voice.—In the fuller breathing required for action the chest is increased in size from side to side and from before backwards by the movement pf the ribs upwards and outwards and by a movement of the breast-bone forwards (this is also accompanied by a modified downward descent of the