Answer
The four pairs of muscles that form most of the anterior abdominal wall are the external oblique, the internal oblique , the transversus abdominis and rhe rectus abdominis muscles. These muscles are arranged in layers on top of one another. The external oblique are the most superficial, and the internal obliques are layered under them . The transversus abdominis, which have horizontal fibers , are layered under the obliques. The fibers of these abdominal muscles run in different directions. In the case of the external obliques the fibers run downward and latero-medially; the fibers of the internal oblique also run latero-medially, but trend first upward and then later downward. The transversus abdominis lies beneath the obliques and its fibers run horizontally. These muscles are joined in a medial aponeurosis . A the pair of rectus abdomins muscles form the antero-medial abdominal wall and are joined medially by the connective tissue linea alba. The whole rectus muscle is joined with the general abdominal aponeurosis to form the rectus sheath.
The layering of muscle fibers and their variation in fiber directions, from muscle to muscle, enhances the strength of the abdominal wall over that which would be provided by a single muscle layer with fibers running in one direction. This rationale is taken from strength of materials evidence obtained by using plywood in construction. Plywood is made of several layers of wood glued together. The layers of wood are arranged so that the grains of each layer trend in different directions.. Experience has shown that plywood made in this way has greater mechanical strength and tensile resistance than a single block of wood with all fibers running in the same direction.
Variation in fibers direction gives greater resistance to multi-dimensional shear forces and minimizes the development of shear-cracks or failures.
Work Step by Step
The musculature of the anterior and lateral abdominal walls consists of several layers of muscles. These include the external obliques, the internal obliques, the tranversus abdominis, and the rectus abdominis. There are no hard bony structures in the anterior abdominal wall to protect the soft viscera. nevertheless,the musculature of the wall provide adequate support and protection because of the physical arrangement of the muscles. The external oblique is superficial to the internal oblique, and the transversus abdominis is layered beneath the obliques. The different layers provides greater strength and resistance to stress than would a thick single layer of muscle. In addition to the layering, there is variation in the direction of trending of muscle fibers. Thus the fibers of the external obliques run medially and downward, while those of the internal oblige run medially and upward and then downward, As their names indicate the fibers of the transversus abdominis run horizontally while those of the rectus abdominis run vertically. This variation in the directions of fibers gives greater mechanical strength to the abdominal muscle wall and enhances its resistance to multidimensional stresses and shears.
