The thoracic cage is a bony and cartilaginous structure that encases and protects vital organs of the chest, including the heart and lungs. It forms a conical enclosure that is narrower at the superior inlet and broader at the inferior outlet.
The sternum is a dagger-shaped bone situated in the middle of the anterior chest wall. It consists of three parts: the manubrium, the body, and the xiphoid process.
The junction between the manubrium and the body forms a prominent anterior ridge known as the sternal angle (angle of Louis). Midwives and doctors rely on this angle as a vital surface landmark because it aligns with the second costal cartilage, marking the point where the trachea bifurcates and the second rib attaches. At the very bottom, the small xiphoid process serves as an anatomical marker for the upper limit of the liver and the lower boundary of the thoracic cavity.
Ribs are classified into three distinct groups based on how they connect anteriorly to the sternum.
Every typical rib features a head that articulates with the vertebral bodies, a neck, a tubercle that articulates with the transverse process of a vertebra, and a long shaft that bends sharply forward at the costal angle.
The posterior wall of the thoracic cage is formed by the twelve thoracic vertebrae. These bones possess specific features that distinguish them from cervical and lumbar vertebrae, most notably the presence of costal facets on their bodies and transverse processes. These smooth surfaces form joints with the heads and tubercles of the ribs.
The joints formed between the rib heads and the vertebral bodies are called costovertebral joints, while the joints between the rib tubercles and the transverse processes are called costotransverse joints. These gliding joints permit the subtle elevating and depressing movements of the ribs that drive normal breathing cycles.
The spaces situated between adjacent ribs are known as intercostal spaces, and there are eleven of them on each side of the body. Each space contains three layers of intercostal muscles that help move the chest wall during respiration.
Running along the inferior margin of each upper rib inside the costal groove is the main neurovascular bundle. This bundle contains the intercostal vein, artery, and nerve, arranged in that exact order from superior to inferior (VAN: Vein, Artery, Nerve). Midwives, doctors, and nurses must remember this anatomical arrangement when performing procedures like thoracentesis or chest tube insertion, always inserting needles along the upper border of the lower rib to avoid cutting these vital vessels.
The thoracic cage is not a rigid box; it changes shape continuously to facilitate breathing. During inspiration, the contraction of the diaphragm and intercostal muscles increases the volume of the thoracic cavity, drawing air into the lungs.
Two distinct movements characterize rib motion:
During expiration, the muscles relax, and the elastic recoil of the lungs and chest wall decreases the thoracic volume, pushing air back out.
Pathologies affecting the thoracic cage directly impair respiratory function. A common traumatic injury is a rib fracture, which causes sharp localized pain worsened by deep breathing or coughing. If multiple adjacent ribs are fractured in multiple places, a segment of the chest wall becomes free-floating, leading to a dangerous condition called flail chest. In flail chest, the isolated segment moves paradoxically inward during inspiration and outward during expiration, severely compromising ventilation.
Clinicians inspect the chest for symmetry, abnormal deformities like pectus excavatum (sunken chest), and signs of respiratory distress such as the use of accessory muscles in the neck and shoulders.