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The transversus thoracis is a thin, flat layer of muscles and tendons connecting the lower back of the sternum, or central breastbone, to the 2nd through 6th ribs inside the rib cage — each side is mirrored on the opposite side of the body. This muscle is part of the intercostal muscles, all of which are located between the ribs and attach to them. The primary function of these muscles is to assist with breathing activities by moving the ribs when the lungs contract and expand. The contraction and expansion movements form an essential part of the breathing process by promoting air movement within the lungs.
The transversus thoracis begins as part of the fibers of the transversus abdominis muscle that end above the diaphragm in the rib cage area. These fibers start out in the horizontal direction, following the way the fibers are arranged in the transversus abdominis, then begin to slope upward and outward as they attach to various ribs, becoming nearly vertical by the time they attach to the 2nd rib. These rib attachment areas can vary significantly among different individuals, and they can also be formed quite differently in one person on opposite sides of the chest.
The transversus thoracis is also called the transverse thoracic muscle. This flat muscle extends along the front chest wall between the rib cage and parietal pleura, the membrane covering the inside chest wall and upper part of the diaphragm. It is a skeletal muscle, in that it has the characteristics of these types of muscles: a banded appearance due to the admixture of light and dark rows of expansion and contraction cells as well as an attachment to the sternal bone. However, as with other skeletal muscles involved in the respiratory function, it has the capability of working without conscious direction.
The chest and abdomen contain other torso muscles, such as the abdominals, which also support breathing functions, but the intercostal muscles are the primary chest muscles involved in respiration. They have three layers: the external intercostals, the internal intercostals, and the innermost intercostals — with the transversus thoracis belonging to the innermost intercostal layer. These muscles help to maintain space separations between the ribs, along with establishing rhythmic rib cage movements during breathing. To do this, the external intercostals contract to lift and expand the ribs for inhaling, while the transversus thoracis and other intercostals contract to lower the ribs for exhaling. The transversus thoracis can also force the lungs to sharply exhale by strongly contracting.