= Solution
\b[Ordinary inspiration is active enlargement of the <thorax>; ordinary expiration is chiefly passive recoil after the inspiratory <muscles> relax.] The movements increase and decrease the vertical, anteroposterior and transverse dimensions together. Their relative prominence varies with posture and the individual; there is no exclusive division into abdominal and costal kinds of <breathing>.
The principal inspiratory <muscle> is the <respiratory diaphragm>. Its peripheral fibres shorten towards the central tendon, lowering and flattening its domes. This increases the vertical dimension of the <thorax> and displaces the abdominal contents downwards and forwards, making the relaxed abdominal wall project. Its costal attachments also act on the lower <ribs>: with abdominal support opposing unlimited descent, shortening can lift and expand the lower costal margin. Thus the <respiratory diaphragm> does more than pull down a piston. Its motor supply is through the <phrenic nerves>, principally from cervical segments C3–C5.
The <external intercostal muscles> elevate the <ribs> and help keep the intercostal spaces from being drawn inwards as <pleural pressure> falls. The interchondral, or parasternal, portions of the <internal intercostal muscles> also contribute to inspiration. It would therefore be inaccurate to label every part of the <internal intercostal muscles> expiratory. Coordinated activity braces the <rib cage> so that the shortening of one layer moves the <ribs> rather than simply deforming the spaces between them.
Rotation about the posterior articulations raises the anterior ends of the upper <ribs> and the <sternum>, increasing the anteroposterior diameter: the usual pump-handle description. The more laterally directed portions of the lower <ribs> rise and move outwards, increasing the transverse diameter: the bucket-handle description. These are descriptions of components of an oblique movement, not two independent sets of hinges. The <scalene muscles> can stabilize and elevate the uppermost <ribs>, with recruitment varying in quiet <breathing>. The <sternocleidomastoid muscle>, when the head is fixed, is an accessory elevator of the <sternum> in greater inspiratory effort; it is not required for an ordinary quiet breath.
The <lungs> follow the expanding chest wall because their surfaces remain coupled to it through the thin fluid-filled <pleural cavity>. More negative <pleural pressure> increases the distending <transpulmonary pressure>; enlargement of the <alveoli> briefly makes <alveolar pressure> lower than <atmospheric pressure>, so air enters. At the end of inspiration, airflow stops when these last two <pressures> equalize. During quiet expiration, relaxation of the <respiratory diaphragm> and inspiratory <intercostal muscles> allows the stretched <lungs> and chest-wall system to recoil towards their resting balance. The domes rise, the <ribs> and <sternum> descend, and <alveolar pressure> becomes slightly positive relative to <atmospheric pressure> while air leaves. Active <abdominal muscles> and the interosseous portions of the <internal intercostal muscles> become important in forced expiration, rather than supplying the main force in quiet expiration.<Anatomy and Physiology 2e by J. Gordon Betts and others/22.3 The Process of Breathing>[Mechanisms of Breathing and Pulmonary Ventilation; Figures 22.16–22.17]{ref}
A simple mechanical demonstration uses two elastic bags in an airtight transparent vessel, connected through its stopper to the air, with a movable rubber sheet forming the base. Pulling the sheet down expands the vessel and inflates the bags; letting it return allows deflation. A side-tube <manometer> can show the associated <pressure> change. This illustrates expansion by lowering the floor, but its rigid vessel cannot demonstrate the movements of the <ribs>, the action of the <intercostal muscles>, or the real geometry of the <pleural cavity>. Observing lower-chest widening and abdominal displacement during quiet <breathing> supplies those missing observations without confusing the model with the anatomy.
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