at the front by the ribs and breast

at the front by the ribs and breastbone (or sternum), and at the bottom by the dome-shaped muscle known as diaphragm, which separates the lungs from the lower cavities of the abdomen. The structure surrounding the thoracic cavity is referred to as the thoracic cage. The act of respiration takes place through the action of the thoracic cage, which enables the lungs to act as a kind of bellows, allowing air to flow inwards and outwards.