Rectus abdominis muscle
The rectus abdominius muscle is one of four muscles of the anterior abdominal wall. It acts as a flexor of the spine and an accessory muscle of respiration. In those with low body fat, is clearly visible beneath the skin. In this article we will discuss the gross and functional anatomy of the rectus abdominis muscle. We will also discuss the clinical relevance of the structure, and provide a summary of key points at the end of the article. We will finally conclude with some review questions to test the reader’s understanding of the article content.
The rectus abdominis muscle is paired muscle that runs vertically, either side of the linea alba, on the anterior surface of the abdominal wall. The linea alba is a band of connective tissue that divides the two halves of the muscle vertically. The linea semilunaris is the tendinous intersection that separates the lateral edge of the muscle from the external oblique and internal oblique muscles that lie on the lateral surface of the anterior abdominal wall. It usually extends from tip of the ninth costal cartilage to the pubic tubercle.
Origins & Insertions
The rectus abdominis muscle is attached superiorly to the xiphoid process and costal margins of 5th, 6th and 7th ribs (principally the fibers of the 5th rib). Inferiorly it is attached by two tendons; the larger one is attached to the pubic crest, from the pubic tubercle to the pectineal line, while the small, medial tendon is attached to the pubic symphysis.
There are three tendinous intersections in the rectus abdominis muscle. One of these horizontal intersections is present at the level of umbilicus, another at the level of xiphoid process and third mid way between them. These fibrous bands divide the muscle into segments, resulting in a grid iron ‘six pack’ shape in those with low body fat. The intersections are believed to be representations of myosepta which delineate the muscle forming myotomes.
The Rectus Sheath
The rectus abdominis muscle itself lies within the rectus sheath, which is formed by the merging of the aponeurosis of transversus abdominis, external and internal oblique abdominal muscles. External oblique is the most superficial muscle of the anterior abdominal wall. The transversus abdominis is the deepest of the three muscles and its fibers run in a horizontal direction. The internal oblique aponeurosis divides into two. The anterior part of the internal oblique aponeurosis passes in front of the rectus abdominis muscle with the external oblique aponeurosis. The posterior division of the internal oblique aponeurosis passes behind the rectus abdominis muscle with the transversus abdominis aponeurosis.
However, below the arcuate line (which lies one third of the way from the umbilicus to the pubic crest), all three of the muscular aponeuroses pass anterior to the rectus abdominis, and the posterior surface of the muscle is covered only by transversalis fascia, and parietal peritoneum.
The muscle itself is a flexor of the spine, and also acts as an accessory muscle of respiration. It compressed the abdomen, which raises the diaphragm further due to superior displacement of the abdominal contents, allowing for more air to be released during exhalation.
The blood supply to the rectus abdominius muscle arises from a number of vessels. The inferior epigastric artery and vein arise from the external iliac artery and vein respectively. They run along the posterior surface of the muscle, and enter the rectus sheath at the level of the arcuate line.
The superior epigastric is another vessel that supplies the rectus abdominius muscle, and is a branch of the internal thoracic (internal mammary) artery. The internal thoracic is a branch of the subclavian artery near its origin. In addition small terminal branches of the lower three posterior intercostal arteries, subcostal and deep circumflex artery also provide some contribution.
The rectus abdominius muscle is innervated by the thoracoabdominal nerves, which enter the rectus sheath by piercing its anterior surface. They pass between the transversus abdominis and internal oblique muscle layers, and pierce the sheath of the rectus abdominis muscle. The nerves are simply the anterior divisions of the 7th to 11th lower intercostal nerves, that continue to supply the abdominal wall after the intercostal spaces they supplied end medially.
The transverse rectus abdominius muscle flap is a free flap used for breast reconstruction surgery. A segment of the muscle is incised and raised along with the perforating inferior epigastric artery and anastomosed with branches of the internal mammary/internal thoracic artery. Nowadays, the deep inferior epigastric artery perforator flap (skin, fascia and fat only i.e. no muscle) has replaced the TRAM flap as it requires less recovery time.
A hernia is an abnormal protrusion of an organ beyond its usual cavity. The anterior abdominal wall and inguinal region are common sites for hernias. A hernia can be irreducible (cannot be returned to the abdominal cavity by pressure), obstructed (the intestine becomes blocked, resulting in frank constipation and abdominal pain), incarcerated (stuck in its cavity) or strangulated (loss of blood supply due to compression of the vessels).
Spigelian Hernia - This occurs when the fascia or musculature of the linea semilunaris is weak, allowing bowel to herniate through. The linea semilunaris separates the lateral margin of the rectus abdominius from the lateral abdominal muscles.
Umbilical Hernia - This is when the posterior surface of the umbilicus is weak or becomes weakened, and it causes an outpouching of bowel. It occurs in children (most commonly African Caribbean children), where it usually disappears itself. It also occurs in conditions where the intra-abdominal pressure is raised e.g. ascites from liver disease, or pregnancy.
Paraumbilical hernia - This occurs when the fascia or musculature around the umbilicus is weak, allowing bowel to herniate through.
Epigastric hernia - This occurs when the fascia or musculature of the epigastric region is weak, allowing bowel to herniate through the linea alba.
Incisional hernia - This occurs when the fascia or musculature after a surgical incision is weak, allowing bowel to herniate through.
Direct Inguinal hernias - It is an acquired hernia. It involves the bowel herniating through a region known as Hasselbach’s triangle, which is bordered by the lateral margin of the rectus abdominis muscle medially, the inferior epigastric artery and vein laterally, and the inguinal ligament inferiorly. Hernia exits through the superficial inguinal ring.
Indirect inguinal hernias - This occurs if there is a patent processus vaginalis, or a congenital weakened area around the region of the processus. This allows bowel to herniate through. The bowel herniates medially to the inferior epigastric artery and vein, and hence enters the deep inguinal ring. Over time, or acutely, this may traverse the length of the inguinal canal, and pass through the superficial inguinal ring (at the superolateral aspect of the scrotum), and may enter the scrotum. This is termed an inguinoscrotal hernia.
This is a rare defect of the anterior abdominal wall, when the muscular layers fail to close, resulting in the abdominal content being held outside the abdominal cavity in a sac. It is associated with chromosomal abnormalities such as Edward’s syndrome (trisomy 18) and Patau syndrome (trisomy 13).
If the anterior abdominal wall fails to close during foetal development, the contents of the abdominal cavity may herniate out. This is similar to Omphalocele, but it usually does not involve the umbilical cord, and is to the right of the umbilical site. Some parts of organs may be suspended in amniotic fluid. The disorder is less associated with other defects than omphalocele. Surgical treatment is difficult as the abdominal cavity shrinks and the abdominal organs swell as they develop.