The human rib cage, or thoracic cage, is a semi-rigid enclosure of bone and cartilage that forms the thorax, the region of the body between the neck and the abdomen. This structure encloses the thoracic cavity and the organs within it. The rib cage’s architecture provides a combination of strength and flexibility, forming a protective barrier while allowing for the movements necessary for breathing.
Bony Framework of the Thoracic Cage
The skeleton of the thoracic cage is formed by the sternum, the ribs, and the thoracic vertebrae. The sternum, or breastbone, is a flat bone in the anterior midline of the chest composed of three parts. The uppermost section is the manubrium, which connects to the clavicles and the first two pairs of ribs.
The central and longest part is the body of the sternum, which articulates with the costal cartilages of ribs two through seven. The most inferior and smallest part is the xiphoid process, a cartilaginous structure that gradually ossifies with age.
Posteriorly, the foundation of the rib cage is formed by the 12 thoracic vertebrae, designated T1 through T12. These vertebrae are part of the spinal column and have specific facets where they articulate with the ribs. Each of the 12 pairs of ribs curves from its posterior attachment at a vertebra toward the front of the body. The progressive increase in vertebral body size from T1 to T12 supports the increasing load down the spine.
Classification of Ribs
The 12 pairs of ribs are categorized into three groups based on their anterior attachment to the sternum. The first seven pairs are known as “true ribs” or vertebrosternal ribs. They are named this way because their costal cartilage—a type of hyaline cartilage—connects directly from the rib bone to the sternum.
The next three pairs, ribs 8, 9, and 10, are referred to as “false ribs” or vertebrochondral ribs. Their costal cartilages do not attach directly to the sternum. Instead, the cartilage of each of these ribs connects to the cartilage of the rib immediately above it, creating an indirect linkage.
The final two pairs, ribs 11 and 12, are termed “floating ribs” or vertebral ribs. These ribs are the shortest and are distinct because they do not connect to the sternum at all. Their posterior attachment is to the thoracic vertebrae, but their anterior ends terminate within the musculature of the abdominal wall.
Role in Respiration and Organ Protection
The primary functions of the rib cage are to protect the organs within the thoracic cavity and to facilitate breathing. The structure forms a protective shield around the heart and lungs. The cage also covers the upper portions of abdominal organs, including the liver and spleen, safeguarding them from external trauma.
The rib cage’s role in respiration is dynamic and involves coordinated movement with associated muscles. During inhalation, the diaphragm contracts and moves downward, while intercostal muscles contract, elevating and expanding the rib cage. This motion increases the volume of the thoracic cavity, which reduces internal pressure and causes air to flow into the lungs. During exhalation, these muscles relax, causing the rib cage to depress and contract, decreasing thoracic volume and forcing air out.
Joints and Cartilage
The thoracic cage’s integrity and flexibility are maintained by a network of joints and cartilage. Costal cartilage is the hyaline cartilage that connects the ribs to the sternum. This tissue provides the structure with a degree of elasticity, which is necessary for the expansion and contraction of the chest wall during breathing.
Several types of joints, or articulations, facilitate the movements of the rib cage, allowing for the slight gliding and rotational movements required for respiration.
- Costovertebral joints connect the heads of the ribs to the thoracic vertebrae.
- Costotransverse joints link the tubercle of the rib to the transverse process of the corresponding vertebra.
- Costochondral joints are where the ribs meet their respective costal cartilages and permit no movement.
- Sternocostal joints are the articulations between the costal cartilages of the true ribs (1-7) and the sternum.
- Interchondral joints are found between the costal cartilages of the false ribs (8-10), enabling them to move in concert.