The thoracic wall, sometimes called the chest wall, is the margin of the thoracic cavity, the space bounded by the spinal column and ribcage that contains the heart, lungs, trachea, and upper esophagus. This term is often used in reference to the muscles of this region alone, which include the intercostal muscles in the anterior ribcage, the pectoralis muscles to the outside of the ribcage, and occasionally the diaphragm muscle forming the lower boundary of the thoracic cavity. Thoracic wall may also, however, refer to the bones of the thoracic cage alone, or to the collective layers of skin, fat, muscle, connective tissue, and bone found within the chest wall.
In anatomy the torso is divided into three cavities: the thoracic cavity, abdominal cavity, and pelvic cavity. The pelvic cavity is the lowest of these and contained by the bones of the pelvis, and it houses the reproductive and urinary organs and the lower end of the digestive tract. Above this is the abdominal cavity, which is found between the pelvis and ribcage and contains the lower esophagus, stomach, liver, gallbladder, spleen, pancreas, intestines, and kidneys.
Enclosed by the ribcage is the thoracic cavity, which is bounded by the thoracic cage. This is made up of the spine behind, the ribs around and above, and the sternum or breastbone in front. Forming the lower boundary of the cavity is the diaphragm, a sheet of muscle stretching across the underside of the ribcage that is convex as the floor of the thoracic cavity and concave as the ceiling of the abdominal cavity.
Among and superficial to the bones of the thoracic cage, which protect the body’s most vital organs, are the thoracic wall muscles. The intercostals run between the individual ribs and are arranged in several layers to either side of the sternum. The internal intercostals are the deepest and angle downward and backward around the ribcage from front to back, while the external intercostals angle downward and forward. Other muscles found within the ribcage in the thoracic wall are the innermost intercostals, a smaller, deeper group of the internal intercostals that can be subdivided into the transversus thoracis and subcostalis muscles. Collectively, these muscles of the ribcage either pull the ribs apart, expanding the ribcage to allow inhalation of the lungs, or draw them together to collapse the ribcage during exhalation.
The diaphragm muscle below and the pectorals to the outside of the ribcage may also be counted among the muscles of the thoracic wall. Acting almost like a suction cup, the diaphragm pulls air into the lungs when it contracts by creating a vacuum in the thoracic cavity. The pectorals move the arms inward at the shoulder joint and depress the shoulders as a unit. They do not move the ribcage directly, so they tend to be excluded from the thoracic wall muscles, but their location in the chest to either side of the sternum sometimes allows them to be counted among the chest wall structures. Similarly, other structures in the chest wall to the outside of the ribcage may be included, such as the fascia or connective tissue between these muscles and the bones, the fatty tissues of the chest wall, and the skin.