The linea alba is the vertical line that divides the rectus abdominis or “six-pack” muscle into left and right halves. Actually composed of eight sections of muscle that are delineated by several lines of connective tissue — three horizontal lines and the linea alba — the rectus abdominis extends from the sternum to the pelvis. Like the rest of the body’s connective tissue, the linea alba is made up of collagen and elastin fibers rather than muscle fibers and is white in color. In addition to dividing the rectus abdominis bilaterally, on its deep surface is an attachment point for the other abdominal muscles: the external obliques, internal obliques, and the transverse abdominis.
Originating on the pubis bone in the central anterior pelvis, the rectus abdominis muscle is a superficial muscle, meaning that it lies close to the skin and is visible in the abdomen in the absence of excess stored body fat. It runs up the center of the abdomen all the way to the rib cage. At that point, it attaches to the lower sternum, or breastbone, and to the cartilage of the ribs on either side of the sternum.
Continuous along the length of the muscle is the linea alba, a slender white line nestled between the bulging segments of muscle to either side of it. This line is classified as connective tissue both for its structure and its function. It is made up of the same fibers as tendons, which hold muscle to bone, and ligaments, which hold bone to bone. It functions to unite the muscles of the abdominal wall.
Above the linea alba are only stored body fat and skin, so on lean people, it is visible through the skin as a longitudinal hollow along the midline of the anterior torso. Beneath this line, however, are the aponeuroses of several other abdominal muscles. An aponeurosis is simply a layer of connective tissue arranged in a sheet rather than a band that typically separates layers of adjacent muscles.
The rectus abdominis is encompassed by one such layer of tissue, as are the obliques and transverse abdominis beneath it. These other three muscles of the abdominal wall, all of which are also bilateral, lie to either side of the linea alba toward the sides of the waist rather than the center of the abdomen. When viewed in cross section, the aponeuroses of each muscle can be seen approaching and connecting to the linea alba from the underside.
In order of the most superficial layer to the deepest, the aponeurosis of the external oblique muscle lies immediately beneath the rectus abdominis and attaches to either side of the linea alba. The internal oblique muscle is directly beneath that and attaches posterior to the external oblique. The transverse abdominis and its aponeurosis approach from beneath these two.