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...