The ribs are twelve pairs of curved bones forming the protective framework of the chest, known as the thoracic cage. This structure shields vital organs. Humans typically possess 24 ribs, twelve on each side, all connecting posteriorly to the 12 thoracic vertebrae of the spine. Rib classification depends on their varied connections to the sternum, or breastbone, at the front of the body.
The Three Primary Classifications
Ribs are categorized into three main types based on their anterior attachment to the sternum.
The first seven pairs (Pairs 1–7) are known as True Ribs, or vertebrosternal ribs. These ribs connect directly to the sternum via their own individual strips of costal cartilage.
Following the True Ribs are the False Ribs, also referred to as vertebrochondral ribs. These three pairs (Pairs 8–10) do not have an independent connection to the sternum. Instead, their costal cartilage joins the cartilage of the rib immediately above them, creating an indirect attachment to the sternum through the seventh rib’s cartilage.
The final two pairs (Pairs 11–12) are the Floating Ribs, or vertebral ribs. These are the shortest and are unique because they do not attach to the sternum or any other costal cartilage. They are only anchored to the thoracic vertebrae in the back and terminate within the abdominal wall musculature.
Detailed Anatomy of a Rib
Each rib features several distinct anatomical components. The posterior end is the head, which is wedge-shaped and typically contains two articular facets. These facets form joints with the corresponding thoracic vertebra and the vertebra directly above it.
The head narrows into the neck, a short, flat segment connecting the head to the body, or shaft, of the rib. The shaft is the long, curved portion of the bone. Where the neck meets the shaft, a roughened area called the tubercle contains a facet that articulates with the transverse process of the vertebra.
The shaft is marked by the costal groove along its inferior border, which protects the intercostal nerves and blood vessels. At the anterior end of the shaft is the flexible costal cartilage, a segment of hyaline cartilage. This cartilage provides an elastic connection, allowing the rib cage to move and expand.
Key Roles of the Rib Cage
The rib cage forms a semi-rigid yet flexible structure that performs several biological functions. Its primary role is protecting the thoracic organs, including the heart, lungs, and major blood vessels.
The structure’s mobility is central to respiration, or breathing. The joints and flexible costal cartilage allow the rib cage to expand and contract with the movements of the diaphragm and intercostal muscles. When inhaling, these muscles contract, lifting the ribs and expanding the chest cavity volume.
This volume increase creates a pressure difference, allowing air to rush into the lungs. When exhaling, the muscles relax, and the ribs return to their resting position, decreasing the chest cavity volume and passively pushing air out. The rib cage also provides attachment points for the muscles of the neck, back, and upper abdomen, contributing to trunk stability and movement.