BONES OF THE THORAX

The thorax resembles a truncated cone, each pair of ribs having a greater diameter than that above, so the rib cage is 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 so the intercostal spaces widen 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 the gladiolus; mobility is slight. The xiphoid is lance shaped or bifid and usually calcifies in later life. When angulated forward it may be mistaken for an abdominal mass.

Each rib is a flattened arch. Each typical rib has two connections with the vertebral column. Each rib articulates with two adjacent vertebrae and their intervertebral disk at a gliding synovial joint, and with the transverse process of the upper vertebra by a second synovial joint. 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 the gladiolus 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 of the rib above; many variations are found. The 11th and 12th ribs are vertebral or floating ribs.

THORACIC WALL MUSCLES

The intercostal spaces contain the internal and external intercostal muscles that attach to adjacent rib margins; contraction draws the bones together. When the first rib is fixed by contraction of the scaleni, contraction of the intercostals, the levatores costarum, and the serratus posterior superior pull the ribs upward. When the last rib is fixed by contraction of the quadratus lumborum, the subcostales and the transversus thoracis draw the ribs downward.