The transversus thoracis muscle (), also known as triangularis sterni, lies internal to the

thoracic cage The rib cage, as an enclosure that comprises the ribs, vertebral column and sternum in the thorax of most vertebrates, protects vital organs such as the heart, lungs and great vessels. The sternum, together known as the thoracic cage, is a sem ...

, 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 Subcostal may refer to: * Subcostal nerve * Subcostal arteries The subcostal arteries, so named because they lie below the last ribs, constitute the lowest pair of branches derived from the thoracic aorta, and are in series with the intercostal a ...

and the

innermost intercostal The innermost intercostal muscle is a layer of intercostal muscles. It may also be called the intima of the internal intercostal muscles. It is the deepest muscular layer of the thorax, with muscle fibres running vertically (in parallel with the i ...

muscles.

Structure

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

sternum The sternum 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 blood vessels from injury. Sh ...

, from the posterior surface of the

xiphoid process The xiphoid process , or xiphisternum or metasternum, is a small cartilaginous process (extension) of the inferior (lower) part of the sternum, which is usually ossified in the adult human. It may also be referred to as the ensiform process. Both ...

, and from the sternal ends of the

costal cartilages The costal cartilages are bars of hyaline cartilage that serve to prolong the ribs forward and contribute to the elasticity of the walls of the thorax. Costal cartilage is only found at the anterior ends of the ribs, providing medial extension. ...

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 the unit of time in the International System of Units (SI), historically defined as of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds ...

, 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 transverse abdominal muscle (TVA), also known as the transverse abdominis, transversalis muscle and transversus abdominis muscle, is a muscle layer of the anterior and lateral (front and side) abdominal wall which is deep to (layered below) t ...

; the

intermediate fibers Intermediate fibers, also known as fast oxidative-glycolytic fibers, are fast twitch muscle fibers which have been converted via endurance training. These fibers are slightly larger in diameter, have more mitochondria A mitochondrion (; ) i ...

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 pulmonary pleurae (''sing.'' pleura) are the two opposing layers of serous membrane overlying the lungs and the inside of the surrounding chest walls. The inner pleura, called the visceral pleura, covers the surface of each lung and dips be ...

. 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