Transversus Thoracis

What is the Transversus Thoracis

The transversus thoracis muscle, also known as the transverse thoracic muscle or triangularis sternae, is a thin band-like intrinsic muscle found on the inner surface of the front of the thoracic wall. It belongs to the group of intrinsic muscles of the thoracic wall with the intercostals, subcostals, serratus posterior, and levatores costarum.

Transversus thoracis are considered accessory respiratory muscles as they help with forced breathing.

Anatomy

Location and Attachments

Origin	Posterior surface of xiphoid process; the lower back surface of the body of sternum; costal cartilages of 4th to 7th ribs
Insertion	Inner surface of the costal cartilage of 2nd to 6th ribs

Origin

The muscle originates and multiple thin muscular slips radiating from 3 points of origin:

• The inner or posterior surface of the xiphoid process
• The lower part of the posterior surface of the sternal body
• The costal cartilages and the sternal end of the 4th to 7th ribs — the lowest four true ribs

The flat ligamentous muscle slips course superolaterally, radiating toward the higher ribs.

Insertion

One by one, the slips are inserted into the inner aspect of the costal cartilages of the first six ribs, typically spanning from the 2nd to the 6th rib. The point of insertion can extend from the cartilage to the costochondral joints and the costal ends of the respective ribs.

The muscular slips that attach at the 6th ribs course almost horizontally. Those attaching into the higher ribs course at a 30-45° angle, becoming almost vertical at the highest insertion points.

Anatomical variations have been observed in these attachment points, even between the attachments on the left and right sides of the same individual.

Relations With Surrounding Muscles and Structures

The transversus thoracis muscle shares the same structural characteristics as the transversus abdominis muscle, while its lowermost muscular fibers also maintain continuity with it.

It lies deep to the thoracic cage on the anterior side of the body — the 2nd to 7th ribs, to be more exact. It is a vital position that allows the radiating strap-like slips of the muscle to contribute to the wall of the anterior mediastinum, particularly in the area directly behind the lower half of the sternum. 

The transversus thoracis muscle also lies deep to the internal intercostal muscles, the upper six vessels of the anterior intercostal artery, and the intercostal nerves. The parietal pleura and endothoracic fascia lie superficially. So, the transversus thoracis muscle also acts as a barrier between the pleura and the intercostal nerves.

Function

Action

Depressing the ribs

The functions of this muscle were unclear to experts for a long time. Later studies have shown that the muscle plays an important role in the movements of the ribs during respiration.

As mentioned, the muscular bands course outward and upward as they attach to ribs almost vertically. It allows them a considerable mechanical advantage over the ribs to depress them, especially during forced exhalation, and stabilize the rib cage. In fact, with its unique shape and position, the transversus thoracis stabilizes the anterior side of the thoracic wall, keeping it from collapsing during inspiration.

Innervation

Nerve

Intercostal nerves

It is innervated by the anterior rami of the thoracic spinal nerve 2 to 6 (T2-T6), also known as the 2nd to 6th thoracic intercostal nerves.

Blood Supply

Artery

Intercostal arteries

The blood supply to this muscle comes from the anterior intercostal arteries that branch from the internal thoracic artery. The sternal branches of the internal thoracic artery serve as the primary source of vasculature.