Answer:
The ribs are moved by the intercostal muscles. The external intercostal contract to pull the ribs upwards and outwards. The internal intercostals contract to pull them downwards and inwards.
These two movements make the volume in the thorax bigger, so forcing the lungs to expand. The reduction in air pressure in the lungs results in air being pulled in through the nose and trachea. This movement of air into the lungs is known as ventilation.
The lungs are elastic and shrink back to their relaxed volume, increasing the air pressure inside them. This results in air being forced out again.
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