<P> The human rib cage is a component of the human respiratory system . It encloses the thoracic cavity, which contains the lungs . An inhalation is accomplished when the muscular diaphragm, at the floor of the thoracic cavity, contracts and flattens, while contraction of intercostal muscles lift the rib cage up and out . </P> <P> Expansion of the thoracic cavity is driven in three planes; the vertical, the anteroposterior and the transverse . The vertical plane is extended by the help of the diaphragm contracting and the abdominal muscles relaxing to accommodate the downward pressure that is supplied to the abdominal viscera by the diaphragm contracting . A greater extension can be achieved by the diaphragm itself moving down, rather than simply the domes flattening . The second plane is the anteroposterior and this is expanded by a movement known as the' pump handle .' The downward sloping nature of the upper ribs are as such because they enable this to occur . When the external intercostal muscles contract and lift the ribs, the upper ribs are able also to push the sternum up and out . This movement increases the anteroposterior diameter of the thoracic cavity, and hence aids breathing further . The third, transverse, plane is primarily expanded by the lower ribs (some say it is the 7th to 10th ribs in particular), with the diaphragm's central tendon acting as a fixed point . When the diaphragm contracts, the ribs are able to evert and produce what is known as the bucket handle movement, facilitated by gliding at the costovertebral joints . In this way, the transverse diameter is expanded and the lungs can fill . </P> <P> The circumference of the normal adult human rib cage expands by 3 to 5 cm during inhalation . </P> <P> Rib fractures are the most common injury to the rib cage . These most frequently affect the middle ribs . When several adjacent ribs incur two or more fractures each, this can result in a flail chest which is a life - threatening condition . </P>

Where are the 6th and 7th rib located