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Major Systems and Physiology

The Thoracic Wall

The skeletal and muscular shell of the thorax encloses the heart and lungs, 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 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 12 thoracic vertebrae, 12 pairs of ribs, and the sternum.


The thorax resembles a truncated cone, each pair of ribs having a greater diameter than that above, making the rib cage much smaller at the top than at the base. The ribs are separated by intercostal spaces numbered from the rib above. The first rib slopes slightly downward from back to front. Each succeeding rib has a greater slope, the intercostal spaces widening from top to bottom.


The sternum (Fig. 8-1) consists of the manubrium, body, (gladiolus), and xiphoid cartilage. There is a fibrocartilage (rarely synovial) joint between the manubrium and body; mobility at this joint is slight. While it is cartilaginous at birth, the xiphoid begins calcifying in childhood and this continues throughout life. The xiphoid is commonly monofid, lance shaped and caudally oriented. Variations are very common and include bifid and trifid divisions, xiphoidal foramina as well as ventral and dorsal projections. When angulated forward, the xiphoid can be mistaken for an abdominal mass.

FIG. 8-1

The Bony Thorax. The left clavicle is removed exposing the underlying first rib. The xiphoid and rib cartilages 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. All sternal rib ends continue as costal cartilages. The first to seventh ribs are usually termed true ribs or vertebrosternal because their costal cartilages join directly to 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 body with two synovial joints. The eighth to twelfth ribs are false ribs without anterior attachment to the sternum. The eighth, ninth, and tenth ribs are vertebrochondral, each costal cartilage usually joining the cartilage of the rib above. The 11th and 12th ribs are vertebral or floating ribs without anterior attachment. Important variations include supranumerary ribs such as the more common variation cervical rib articulated to ...

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