The internal oblique, external oblique, rectus abdominis, and transversus abdominis are four pairs of flat muscles that hold in and protect the soft organs in the abdominal cavity, These muscles are layered on top of one another and their fibers run in various direction. This is an arrangement that provides mechanical strength for physical reasons similar to those responsible for the strength of the material plywood. So although anteriorly, the abdominal viscera has no hard bony protection, it is usually adequately supported.
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The main muscles that support and protect abdominal viscera are the pairs of internal oblique, external oblique, rectus abdominis, and transversus abdominis. These muscles form layers on top of one another in an arrangement that maximizes the strength of the anterior and lateral abdominal walls. The external obliques are the most superficial pair; their fibers run downward (latero-medially). The the internal obliques are beneath (dorsal) to the external obliques; the fibers of the former run both up ward and downward. The transversus abdominis lies underneath both oblques and its fibers run horizontally. This variation in fiber directions contributes significantly to the strength of the abdominal wall. Further reinforcement is added by the fourth component of the anterior abdomonal wall, namely the rectus abdominis. This muscle runs medially from the pubis symphysis to the rib cage. It inserts into the aponeurosis that joins the three pairs of lateral muscles and which extends from pubis symphysis to stennum. The strong rectus sheath formed by the aponeurosis and the rectus abdominus is the main factor that keeps abdomen from distending anteriorly. The linea alba is the strong connective tissue line that marks the center of the rectus sheath.