The chest, or thorax, lies between the neck and the abdomen. Inside the thorax lie the heart, lungs, and major blood vessels. The ribcage surrounding them is formed by the backbone, ribs, costal cartilages, and sternum (breastbone).
The ribcage is strong enough to protect the vital organs, but flexible enough to expand and contract for breathing. Attached to the ribcage are the muscles of the chest. Together with the diaphragm, many of these muscles help with breathing.
They move superiorly, inferiorly, anteriorly and posteriorly to facilitate breathing (their flexibility in their movement increases/decreases the size of the thoracic cavity; assisting the lungs in respiration. Control of these movements is via the diaphragm, external intercostals, and the intercartilaginous portion of the internal intercostals). They play a role in erythropoiesis during development (at birth, the erythropoiesis sites change, it recedes in long bones and persists in flat bones, like ribs).
Picture Credit : Google