Rib cage

The rib cage or thoracic cage is an endoskeletal enclosure in the thorax of most vertebrates that comprises the ribs, vertebral column and sternum, which protect the vital organs of the thoracic cavity, such as the heart, lungs and great vessels and support the shoulder girdle to form the core part of the axial skeleton.

A typical human thoracic cage consists of 12 pairs of ribs and the adjoining costal cartilages, the sternum (along with the manubrium and xiphoid process), and the 12 thoracic vertebrae articulating with the ribs. The thoracic cage also provides attachments for extrinsic skeletal muscles of the neck, upper limbs, upper abdomen and back, and together with the overlying skin and associated fascia and muscles, makes up the thoracic wall.

In tetrapods, the rib cage intrinsically holds the muscles of respiration (diaphragm, intercostal muscles, etc.) that are crucial for active inhalation and forced exhalation, and therefore has a major ventilatory function in the respiratory system.

There are thirty-three vertebrae in the human vertebral column. The rib cage is associated with TH1−TH12. Ribs are described based on their location and connection with the sternum. All ribs are attached posteriorly to the thoracic vertebrae and are numbered accordingly one to twelve. Ribs that articulate directly with the sternum are called true ribs, whereas those that do not articulate directly are termed false ribs. The false ribs include the floating ribs (eleven and twelve) that are not attached to the sternum at all.

The terms true ribs and false ribs describe rib pairs that are directly or indirectly attached to the sternum respectively. The first seven rib pairs known as the fixed or vertebrosternal ribs are the true ribs (Latin: costae verae) as they connect directly to the sternum via their own individual costal cartilages. The next five pairs (eighth to twelfth) are the false ribs (Latin: costae spuriae) or vertebrochondral ribs, which do not connect directly to the sternum. The first three pairs of vertebrochondral ribs (eighth to tenth) connect indirectly to the sternum via the costal cartilages of the ribs above them, and the overall elasticity of their articulations allows the bucket handle movements of the rib cage essential for respiratory activity.

The phrase floating rib (Latin: costae fluctuantes) or vertebral rib refers to the two lowermost (the eleventh and twelfth) rib pairs; so-called because they are attached only to the vertebrae and not to the sternum or any of the costal cartilages. These ribs are relatively small and delicate, and include a cartilaginous tip.

The spaces between the ribs are known as intercostal spaces; they contain the instrinsic intercostal muscles and the neurovascular bundles containing intercostal nerves, arteries and veins. The superficial surface of the rib cage is covered by the thoracolumbar fascia, which provides external attachments for the neck, back, pectoral and abdominal muscles.

Each rib consists of a head, neck, and a shaft. All ribs are attached posteriorly to the thoracic vertebrae. They are numbered to match the vertebrae they attach to – one to twelve, from top (T1) to bottom. The head of the rib is the end part closest to the vertebra with which it articulates. It is marked by a kidney-shaped articular surface which is divided by a horizontal crest into two articulating regions. The upper region articulates with the inferior costal facet on the vertebra above, and the larger region articulates with the superior costal facet on the vertebra with the same number. The transverse process of a thoracic vertebra also articulates at the transverse costal facet with the tubercle of the rib of the same number. The crest gives attachment to the intra-articular ligament.