The thorax comprises the major greater portion of the chest, and contains the rib cage (costal cartilages). There are six principal muscles of the thorax. These are the intercostales externi, intercostales interni, subcostales, transversus thoracis, levatores costarum, serratus posterior superior, serratus posterior inferior, and diaphragm. The abdomen lies below the thorax, overlying the stomach. The muscles of the abdomen are the obliquus externus, obliquus internus, transversus, rectus, and pyramidalis. The muscles work to assist respiration, digestion, and to protect internal organs.
The intercostal muscles are thin and plate-like segments made of muscle and fiber. They are found in the intercostal spaces; the spaces between the ribs. Their names are derived from their relation to one another. The intercostales externi are nearer to the surface than the intercostales interni. There are 11 of the external muscles on each side. Each one is attached from the lower part of a rib at one end and the upper part of the next lower rib, cross-meshing the ribs together. There are also 11 of the internal muscles on each side of the ribcage. They are also connected to adjacent ribs, but at the back of the ribs, and are meshed in the opposite pattern to the external muscles. The effect is to provide great structural support.
The subcostals connect a rib to the second or third rib below it. Each muscles runs in the same direction as the intercostales interni.
The transversus thoracis is also a thin plane of muscle and fiber. Found on the front wall of the chest, the muscle runs from the bottom of the thorax upward and laterally outward, and inserts into the cartilage of the second to the sixth ribs. The lowest of these inserted muscles is almost horizontal, while the topmost muscles is almost vertical.
There are 12 levatores costarum muscles on each side of the thorax. These muscles are small and tendon-like. They connect the ribs to the central vertebral column.
The serratus posterior superior is another thin muscle. Its shape differs from the other muscles, because it is shaped as a quadrilateral. It is located at the upper and back part of the thorax. From there, it is oriented downward (inferiorly) and outward, almost laterally, and inserts into the upper part of the second through fifth ribs. Thus, they provide support to these ribs from above. The serratus posterior inferior rises upward from the lower region of the thorax and inserts into the fourth pair of ribs.
The abdomen is also well muscled. The muscles of the abdomen are divided into two groups: the anterolateral muscles and the posterior muscles. As their names imply, the muscles in the first group are found at the front and side of the abdomen, while those of the second group are found in the lower back.
The anterolateral muscle group consists of: obliquus externus, obliquus internus, transversus, rectus, and pyramidalis. The obliquus externus muscles are the horizontal muscles, arranged in four pairs from the lower abdomen upward, which are prominent in bodybuilders. The obliquus internus muscles are smaller and more vertically oriented. Both muscle types help to anchor the ribs to the abdomen.
Underneath the obliquus muscles is the transversus. It is oriented more horizontally, fanning out to inserted in the pubic bone.
The rectus abdominus is a long flat muscle. It runs the entire length of the front of the abdomen, front the pubic area at its lower reaches to the fifth through seventh ribs at it upper reaches.
The pyramidalis is a small triangular-shaped muscle that is located at the lower part of the abdomen. It runs from the pubic area on either side of the abdomen and arches upward to near the navel.
The posterior muscle group is made of psoas major, psoas minor, iliacus, and the quadraturs lumborum. They all function to stabilize the lower portion of the rib cage from the back.
The muscles of the thorax and abdomen provide structural support for the rib cage and function in the rotational twisting, bending and flexing motions that these regions of the body exert. Injury to these muscle groups can severely compromise range of motion.
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