Major Systems and Physiology
The skeletal and muscular shell of the thorax encloses the thoracic visceral organs, powers breathing and is the mechanical platform for arm and neck motion. It is bounded anteriorly by the sternum and ribs, laterally and posteriorly by the ribs, and supported posteriorly by the spine. The inferior boundary is the diaphragm and rib margins. Superiorly, it is bounded by the clavicles and soft tissues of the neck. The thoracic wall includes the bodies of the 12 thoracic vertebrae, the 12 pairs of ribs, and the sternum.
The thorax resembles a truncated cone, each pair of ribs having a greater diameter than that above, so the sternovertebral dimension is much smaller at the top than at the base. The ribs are separated by intercostal spaces, each space taking its numbers from the rib above. The first rib slopes slightly downward from vertebra to sternum; each succeeding rib has a greater slope increasing the width of the intercostal spaces progressively from top to bottom.
The sternum (Fig. 8–1) consists of the manubrium, the gladiolus, and the xiphoid cartilage. There is a fibrocartilage (rarely synovial) joint between the manubrium and gladiolus; mobility is slight. The xiphoid cartilage is either lance-shaped or bifid and may be mistaken for an abdominal mass when angulated forward; it usually calcifies in later life.
The Bony Thorax.The left clavicle is removed exposing the underlying first rib. The cartilages of the xiphoid and ribs are stippled. Note the surface landmarks: the suprasternal notch, the angle of Louis, and the infrasternal notch. The two lower rib margins form the intercostal angle.
Each rib is a flattened arch. Each typical rib has two connections with the vertebral column: the head is bound to two adjacent vertebrae and their intervertebral disk at a gliding synovial joint; a second synovial joint on the articular tubercle of the rib's neck articulates with the transverse process of the upper vertebra. The sternal rib ends continue as costal cartilages. The first to seventh ribs are true ribs since their costal cartilages join the sternum. The costal cartilage of the first rib connects to the manubrium at a fibrous joint. The other six true ribs attach to the sternum by synovial joints. The second rib attaches to both the manubrium and gladiolus at their fibrocartilage with two synovial joints. The first, tenth, eleventh, and twelfth ribs are atypical, each articulating with a single vertebra. The eighth to twelfth ribs are false ribs. The eight, ninth, and tenth ribs are vertebrochondral, each costal cartilage usually joining the cartilage ...