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The linea semilunaris is an anatomical feature of the rectus abdominis, the large superficial muscle of the abdomen. A paired feature found to either side of the muscle, this nearly vertical line indicates the lateral or outside border of the rectus abdominis where it meets the external and internal oblique muscles on either side. It arises from the cartilage joining the ninth rib to the sternum or breastbone and continues all the way to the pubic tubercle, a feature of the pubis bone in the pelvis. The linea semilunaris gets its name from the fact that the line curves slightly around each segment of the rectus abdominis, those divisions of the muscle that give it the nickname of the “six-pack” muscle.
Extending from the front side of the lower ribcage to the lower pelvis, the rectus abdominis spans the entire abdomen from top to bottom, but not from side to side. This muscle covers little more than half the width of the torso, with the obliques covering the sides of the abdomen. It gets its distinguishing shape by several lines bordering or crossing the muscle belly.
The linea alba runs down the middle of the muscle, dividing it into two longitudinal sections. The linea semilunaris runs down either side of the muscle, separating it from the obliques. Three tendinous inscriptions, or lengths of tendon-like fibers, cross the muscle laterally, dividing it into four sections on either side from top to bottom. When exposed, the muscle in reality resembles an “eight-pack.”
As the outside border of the rectus abdominis, the linea semilunaris is distinguished from the reddish fibers of the muscle belly by its white color. This is due to its composition not of muscle fibers but rather of aponeurosis fibers. An aponeurosis consists of the same fibrous, collagen-based tissue that makes up ligaments and tendons, but rather than being shaped like a band or rope, it is more like a broad, flat sheet of tissue. Each layer of muscle in the abdomen is enclosed and therefore separated from the other muscles by a sheath of this tissue, and it is the intersection of the aponeuroses of the rectus abdominis, internal obliques, and external obliques that form the linea semilunaris.
The external and internal obliques form a superficial and deep layer of muscle, respectively, that is found to either side of the rectus abdominis. Where the superficial surface of the internal oblique muscle meets the deep surface of the external oblique muscle is a blending of the aponeuroses encasing both muscles. The medial or inside border of these aponeuroses also blends with the lateral or outside border of the aponeurosis of the rectus abdominis, resulting in a line of fibrous tissue with muscle fibers to either side of it — this is the linea semilunaris.