Transversus thoracis m.

The transversus thoracis muscle is a thin plane of muscular and tendinous fibers situated upon the internal surface of the front wall of the chest. It is in the same layer as the subcostal muscles and the innermost intercostal muscles.

It arises on either side from the lower third of the posterior surface of the body of the sternum, from the posterior surface of the xiphoid process, and from the sternal ends of the costal cartilages of the lower three or four true ribs.

Its fibers diverge upward and lateralward, to be inserted by slips into the lower borders and inner surfaces of the costal cartilages of ribs 2-6 (sometimes also rib 7). The lowest fibers of this muscle are horizontal in their direction, and are continuous with those of the transversus abdominis. The intermediate fibers are oblique, while the highest are almost vertical.

The muscle is supplied by the intercostal nerves, which are the anterior rami of the thoracic spinal nerves.

It is almost completely without function, but it separates the thoracic cage from the parietal pleura. When contracted, it can depress the ribs and assist in exertional expiration by decreasing the transverse diameter of the thoracic cage.