Now, when the diaphragm contracts, it descends and thus increases the depth of the chest cavity.  A quantity of air is now drawn into the lungs and causes them to expand, thus filling up the increased space.  As soon as the diaphragm relaxes, returning to its arched position and reducing the size of the chest cavity, the air is driven from the lungs, which then diminish in size.  After a short pause, the diaphragm again contracts, and the same round of operation is constantly repeated.

The walls of the chest being movable, by the contractions of the intercostals and other muscles, the ribs are raised and the breastbone pushed forward.  The chest cavity is thus enlarged from side to side and from behind forwards.  Thus, by the simultaneous descent of the diaphragm and the elevation of the ribs, the cavity of the chest is increased in three directions,—­downwards, side-ways, and from behind forwards.

It is thus evident that inspiration is due to a series of muscular contractions.  As soon as the contractions cease, the elastic lung tissue resumes its original position, just as an extended rubber band recovers itself.  As a result, the original size of the chest cavity is restored, and the inhaled air is driven from the lungs.  Expiration may then be regarded as the result of an elastic recoil, and not of active muscular contractions.

[Illustration:  Fig. 91.—­Diagrammatic Section of the Trunk. (Showing the expansion of the chest and the movement of the ribs by action of the lungs.) [The dotted lines indicate the position during inspiration.]]

211.  Varieties of Breathing.  This is the mechanism of quiet, normal respiration.  When the respiration is difficult, additional forces are brought into play.  Thus when the windpipe and bronchial tubes are obstructed, as in croup, asthma, or consumption, many additional muscles are made use of to help the lungs to expand.  The position which asthmatics often assume, with arms raised to grasp something for support, is from the need of the sufferer to get a fixed point from which the muscles of the arm and chest may act forcibly in raising the ribs, and thus securing more comfortable breathing.

The visible movements of breathing vary according to circumstances.  In infants the action of the diaphragm is marked, and the movements of the abdomen are especially obvious.  This is called abdominal breathing.  In women the action of the ribs as they rise and fall, is emphasized more than in men, and this we call costal breathing.  In young persons and in men, the respiration not usually being impeded by tight clothing, the breathing is normal, being deep and abdominal.

Disease has a marked effect upon the mode of breathing.  Thus, when children suffer from some serious chest disease, the increased movements of the abdominal walls seem distressing.  So in fracture of the ribs, the surgeon envelops the overlying part of the chest with long strips of firm adhesive plaster to restrain the motions of chest respiration, that they may not disturb the jagged ends of the broken bones.  Again, in painful diseases of the abdomen, the sufferer instinctively suspends the abdominal action and relies upon the chest breathing.  These deviations from the natural movements of respiration are useful to the physician in ascertaining the seat of disease.