Transversus thoracis muscle
Anatomy and supply
The transversus thoracis muscle is a thin muscle on the inner surface of the anterior thoracic wall. Its origin lies on the dorsal surface of the xiphoid process and the body of the sternum. From there five insertion tendons run craniolaterally to the cartilage of the 2nd to 6th ribs giving it a serrated appearance. The exact number of tendons and the places of insertion vary greatly, making the transversus thoracis one of the most variable muscles in the human body. Caudally, its fibers course almost parallel to those of the transversus abdominis, which is where it got its name from. In rare cases, these two muscles can even be found merged together. The transversus thoracis is supplied by intercostal nerves arising from the thoracic nerves (Th2-Th6).
The contraction of the transversus thoracis pulls the rib cartilages caudally, thus supporting expiration. However, it is not considered to be among the primary respiratory muscles such as the intercostal muscles or the diaphragm. Firstly, it is barely active in the supine position. Secondly, many people have a poorly developed transversus thoracis and show no breathing difficulties. For those reasons, it is only regarded as an accessory muscle of expiration.
The transversus thoracis plays an important role in heart surgery as it lies in immediate relation to the internal thoracic artery, clinically known as the internal mammary artery (IMA). Due to its exit from the subclavian artery and the close proximity to the heart, this artery suits particularly well as a transplant vessel for a coronary artery bypass. Moreover, it only needs to be anastomosed on one side. When harvesting the artery, surgeons prefer to start between the first rib and the highest insertion tendon of the transversus thoracis, as this is usually the easiest place to detect and dissect the vessel. More caudally, the internal thoracic artery is increasingly covered by fibers of the transversus thoracis muscle.