The rectus abdominis muscle is a paired muscle running vertically on each side of the anterior wall of the human abdomen (and in some animals). There are two parallel muscles, separated by a band of connective tissue called the linea alba (white line). It extends from the pubic symphysis inferiorly to the xiphisternum and lower costal cartilages superiorly.

The Rectus abdominis is a long flat muscle, which extends along the whole length of the front of the abdomen, and is separated from its fellow of the opposite side by the linea alba.

It is much broader, but thinner, above than below, and arises by two tendons; the lateral or larger is attached to the crest of the pubis, the medial interlaces with its fellow of the opposite side, and is connected with the ligaments covering the front of the symphysis pubis.

The muscle is inserted by three portions of unequal size into the cartilages of the fifth, sixth, and seventh ribs.

The upper portion, attached principally to the cartilage of the fifth rib, usually has some fibers of insertion into the anterior extremity of the rib itself.

Some fibers are occasionally connected with the costoxiphoid ligaments, and the side of the xiphoid process.

The Rectus is crossed by fibrous bands, three in number, which are named the tendinous inscriptions; one is usually situated opposite the umbilicus, one at the extremity of the xiphoid process, and the third about midway between the xiphoid process and the umbilicus.

These inscriptions pass transversely or obliquely across the muscle in a zigzag course; they rarely extend completely through its substance and may pass only halfway across it; they are intimately adherent in front to the sheath of the muscle.

Sometimes one or two additional inscriptions, generally incomplete, are present below the umbilicus.

The linea alba is a fibrous structure that runs down the midline of the abdomen in humans and other vertebrates. The name means white line and the linea alba is indeed white, being composed mostly of shiny white collagen connective tissue.

It is formed by the fusion of the aponeuroses of the abdominal muscles, and it separates the left and right rectus abdominis muscles. In muscular individuals its presence can be seen on the skin, forming the depression between the left and right halves of a “six pack.”

Because it consists of only connective tissue, and doesn't contain important nerves or blood vessels, a median incision through the linea alba is a common surgical approach.