Muscles of the Anterior Trunk
Some of the trunk muscles have been given nicknames by gym rats. For instance, the "pecs" are the pectoralis major muscles at each breast. Deep to pectoralis major are the paired pectoralis minor muscles. Between the ribs are three layers of intercostal muscles: external intercostals (most superficial), inner intercostals (intermediate), and innermost intercostals (deepest intercostal muscles).Although the internal intercostal muscles are deep to the external intercostal muscles, paying attention to the direction of muscle fibers for each muscle can help you identify them correctly. From an anterior view, internal intercostal muscle fibers run in an inferolateral direction from superior to inferior, whereas external intercostal muscle fibers run in an inferomedial direction from superior to inferior.
Extending from the back and wrapping around the sides of the rib cage are the paired serratus anterior muscles. This muscle’s anterior edges are serrated like the teeth of a saw because this muscle’s origins are on ribs 1 through 8 and each serration is the attachment point to another rib.
Rectus abdominis, the medial, paired muscles that can create an abdominal "six-pack," is enclosed by the rectus sheath, also known as the rectus fascia. The rectus sheath is formed by aponeuroses (white, fibrous, flat, tendon-like tissue) from the external oblique, internal oblique, and transverse abdominis muscles. The rectus sheath fuses along a medial line called the linea alba, literally meaning the "white line." Linea alba is a fibrous tissue attached to the pubic symphysis and the xiphoid process of the sternum and is a point of insertion for abdominal muscle.