. The human body and health : an elementary text-book of essential anatomy, applied physiology and practical hygiene for schools . bon dioxide passes from the blood tothe air cell. The air in the air cells must be constantly changed tokeep up the supply of oxygen and to get rid of the carbondioxide. This is done by the action of many muscles,producing an inward and then an outward flow of airthrough the tracheaand tubes of thelungs. This pro-cess is called breath-ing and is necessaryfor respiration. How Breathingis Maintained. —Breathing consistsof inspiration^ orinhaling air intothe lungs, an
. The human body and health : an elementary text-book of essential anatomy, applied physiology and practical hygiene for schools . bon dioxide passes from the blood tothe air cell. The air in the air cells must be constantly changed tokeep up the supply of oxygen and to get rid of the carbondioxide. This is done by the action of many muscles,producing an inward and then an outward flow of airthrough the tracheaand tubes of thelungs. This pro-cess is called breath-ing and is necessaryfor respiration. How Breathingis Maintained. —Breathing consistsof inspiration^ orinhaling air intothe lungs, and ex-piration^ or exhal-ing air out of thelungs. The air oratmosphere presses upon every object it touches in the same way as waterpresses on anything immersed in it. Air flows towardthe place where the pressure is least. The pressure of airin a vessel or cavity may be decreased by increasing thesize of the vessel or cavity and not permitting any air toenter. When the lungs are removed from the body of a mouseand placed in a bottle, Avith a glass tube extending fromthe trachea out through a hole in a tight rubber cork, and. diaphragm] Fig. 95. ^ The position of the diaphragm andrihs when the chest cavity is enlarged to admitair into the hmgs is shown at the right. Thefigure on the left shows how the chest cavity isdecreased to j)ush out the air from the lungs. 140 THE RESPIRATORY SYSTEM the air then pumped out of the bottle, the lungs will ex-pand. The air runs down the tube and presses out everyair sac, thus hlling the space that tlie air had life air goes into the lungs when the pressure onthe outside of the lungs is decreased by enlarging thecavity of the chest (Fig. 95). This decrease of pressureis produced by the contraction of certain muscles, andwhen these muscles are relaxed, the pressure of the ribson the lungs causes the breathing out of the air. How Breathing is Regulated. — Breathing occurs usuallyat the rate of about fifteen times per minute. Du
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