Rectus abdominis is a long strap-like muscle. It extends through the entire length of the anterior abdominal wall. It is broader above and is thinner below. The two rectus muscles are separated from each other by the linea alba. Each rectus muscle is enclosed between the aponeurosis of the external oblique, internal oblique and transversus abdominis muscles, which form the rectus sheath.
Rectus abdominis is different from most muscles of the body. Its fibers are interrupted by three fibrous bands of tendinous intersections, which divide the muscle into different sections. The tendinous intersections have a regular position. The first lies opposite the free end of xiphoid process. The second lies at the level of umbilicus and the third about midway between the first and second. These tendinous intersections usually fuse with the rectus sheath lying anterior to the muscle.
The medial border of the rectus abdominis muscle is closely related to linea alba through its entire length. The lateral border, however, may be visible on the surface of anterior abdominal wall as a curved groove, the linea semilunaris. It extends from the tip of the ninth costal cartilage to the pubic tubercle.
Origin of rectus abdominis:
Rectus abdominis arises by two heads. The first head, the medial one, arises from the front of symphysis pubis while the second head, the lateral and larger, from the pubic crest.
Insertion of rectus abdominis:
The muscle is inserted into the fifth, sixth and seventh costal cartilages and the xiphoid process.
Blood supply of rectus abdominis:
Rectus abdominis is supplied by the superior and inferior epigastric arteries. The inferior epigastric artery has a bigger share than the superior because it tends to be larger in size. Small terminal branches from the lower three posterior intercostal arteries, the subcostal artery, the posterior lumbar arteries and the deep circumflex artery may provide some contribution.
Nerve supply to rectus abdominis:
This muscle is innervated by the terminal branches of the anterior rami of lower six thoracic nerves via the lower intercostal nerves and the subcostal nerve.
Actions of rectus abdominis:
Rectus abdominis, along with other muscles of anterior abdominal wall, performs a number of important functions.
- It helps in compression of the abdominal contents to facilitate the processes of micturition, parturition and defecation.
- It flexes the vertebral column.
- It acts as an accessory muscle of expiration.
- It contributes to the maintenance of the tone of abdominal wall.