The 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. It is responsible for flexing the lumbar spine, as when doing a 'sit-up'. It can play a role in
Respiration in the event the patient is short of breath.
If well-defined, the rectus abdominis is colloquially called a "six-pack." This is due to tendinous intersections within the muscle, usually at the level of the
Umbilicus (belly-button), the
Xiphisternum , and about halfway in-between.
The
Inferior Epigastric Artery and vein (or veins) run superiorly on the posterior surface of the rectus abdominis, enter the rectus fascia at the arcuate line, and help to supply the muscle with blood.
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 Rectus is enclosed in a sheath formed by the aponeuroses of the
Obliqui and
Transversus , which are arranged in the following manner. At the lateral margin of the Rectus, the aponeurosis of the
Obliquus Internus divides into two lamellæ, one of which passes in front of the Rectus, blending with the aponeurosis of the Obliquus externus, the other, behind it, blending with the aponeurosis of the Transversus, and these, joining again at the medial border of the Rectus, are inserted into the linea alba. This arrangement of the aponeurosis exists from the costal margin to midway between the umbilicus and symphysis pubis, where the posterior wall of the sheath ends in a thin curved margin, the linea semicircularis, the concavity of which is directed downward: below this level the aponeuroses of all three muscles pass in front of the Rectus. The Rectus, in the situation where its sheath is deficient below, is separated from the peritoneum by the transversalis fascia. Since the tendons of the Obliquus internus and Transversus only reach as high as the costal margin, it follows that above this level the sheath of the Rectus is deficient behind, the muscle resting directly on the cartilages of the ribs, and being covered merely by the tendon of the Obliquus externus.
