The transversus thoracis muscle (), also known as triangularis sterni, lies internal to the thoracic cage, anteriorly. It is usually a thin plane of muscular and tendinous fibers, however on athletic individuals it can be a thick 'slab of meat', situated upon the inner surface of the front wall of the chest. It is in the same layer as the

subcostal muscles The subcostales (singular: subcostalis) (Infracostales) consist of muscular and aponeurotic fasciculi, which are usually well-developed only in the lower part of the thorax; each originates from the inner surface of one rib In vertebrate an ...

and the innermost intercostal muscles.

Structure

It arises on either side from the lower third of the posterior surface of the body of the

sternum The sternum (: sternums or sterna) or breastbone is a long flat bone located in the central part of the chest. It connects to the ribs via cartilage and forms the front of the rib cage, thus helping to protect the heart, lungs, and major bl ...

, from the posterior surface of the

xiphoid process The xiphoid process (), also referred to as the ensiform process, xiphisternum, or metasternum, constitutes a small cartilaginous process (extension) located in the inferior segment of the sternum, typically ossified in adult humans. Both the Gr ...

, 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 the

second The second (symbol: s) is a unit of time derived from the division of the day first into 24 hours, then to 60 minutes, and finally to 60 seconds each (24 × 60 × 60 = 86400). The current and formal definition in the International System of U ...

, third, fourth, fifth, and sixth ribs. 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. This muscle varies in its attachments, not only in different subjects, but on opposite sides of the same subject. The muscle is supplied by the anterior rami of the

thoracic spinal nerves A spinal nerve is a mixed nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body. In the human body there are 31 pairs of spinal nerves, one on each side of the vertebral column. These are grouped into th ...

(intercostal nerves).

Function

It is almost completely without function, but it separates the thoracic cage from the

parietal pleura The pleurae (: pleura) are the two flattened closed sacs filled with pleural fluid, each ensheathing each lung and lining their surrounding tissues, locally appearing as two opposing layers of serous membrane separating the lungs from the medi ...

. It depresses the ribs. Contraction of this muscle aids in exertional expiration by decreasing the transverse diameter of the thoracic cage.

Additional images

File:Gray503.png, Transverse section of thorax, showing relations of pulmonary artery. File:Gray819.png, Diagram of the course and branches of a typical intercostal nerve.

References

External links

* - "Thoracic Wall: Removal of Intercostal Muscles" * Muscles of the torso {{muscle-stub