Transversus thoracis muscle
Transversus thoracis (triangularis sternae, sternocostalis) is a muscle found on the inner surface of the anterior chest wall. It belongs to the intrinsic muscles of the chest wall, along with the intercostals, subcostal, levatores costarum and serratus posterior muscles. Transversus thoracis is organized into several slips that radiate from the body and xiphoid process of sternum towards the costal cartilages of ribs 2-6.
Just like all the intrinsic chest muscles (except for levatores costarum), transversus thoracis is innervated by the adjacent intercostal nerves. Like all these muscles, transversus thoracis helps to move the ribs during forced breathing and support the thoracic cage during the breathing process.
This article will discuss the anatomy and function of the transversus thoracis muscle.
|Origin||Inferoposterior surface of body of sternum and xiphoid process; sternal ends of costal cartilages of ribs 4-7|
|Insertion||Internal surface of costal cartilages of ribs 2-6|
|Action||Depresses ribs during forced expiration; Supports intercostal spaces and thoracic cage|
|Blood supply||Sternal branches of internal thoracic artery|
Origin and insertion
The transversus thoracis muscle originates from three points; inferior third of the posterior surface of the body of sternum, posterior surface of the xiphoid process and sternal ends of the costal cartilages of ribs 4-7. Its fibers diverge and course superolaterally, forming 4-5 slips on each side of the sternum. The muscle slips serially insert into the inner surfaces of the costal cartilages of ribs 2-6, respectively.
The inferiormost fibers of transversus thoracis lie in the horizontal plane and are continuous with the transversus abdominis muscle. The intermediate fibers run obliquely in a superolateral direction, whilst the superiormost fibers lie nearly vertically.
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Transversus thoracis lies deep to the central part of the thoracic cage, forming the anterior wall of the anterior mediastinum. It separates the pleura from the intercostal nerves. The upper border of transversus abdominis is in direct contact with the inferior border of transversus thoracis. The superior epigastric artery and vein pass anterior to both muscles.
Transversus thoracis is innervated by the second to fifth thoracic intercostal nerves. These nerves are the anterior rami of spinal nerves T2-T6.
Transversus thoracis is an accessory respiratory muscle that is active during forced expiration. It pulls ribs 2-6 towards the sternum during forced expiration, which results in depression of those ribs. This action consequently decreases the anteroposterior diameter of the thoracic cavity.
Besides this, transversus thoracis stiffens the thoracic wall during inspiration, thus preventing paradoxical movements of the chest wall.
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.
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