The rectus sheath is located in the front abdominal region and contains the rectus abdominis and pyramidalis muscles. It is composed of many layers of shiny white tendons. These broad, flat tendons are called aponeuroses, and include only a few blood vessels and nerves. Blood is supplied to the sheath through the epigastric arteries.
The rectus abdominis muscles are parallel bars of muscle separated by a thin line of connective tissue called the linea alba. The entire rectus abdominis muscle is located within the rectus sheath. Well-defined rectus abdominis muscles are commonly referred to as six-pack abs.
Another muscle located within the rectus sheath is a small triangular muscle called the pyramidalis. The lower portion of the muscle is attached to the pubic symphysis and the crest of the pubic bone. It is secured to the front of the pubis bone by the anterior pubic ligament. The upper portion of the pyramidalis decreases in size until the superior end inserts into the linea alba, located halfway between the belly button and pubic bone. Contracting the pyramidalis causes the linea alba to tense.
The aponeuroses of the internal and external abdominal oblique muscles, as well as the transverse abdominis muscle, form the laminae, or layers, of the rectus sheath. Laminae are often divided into anterior (head) and posterior (tail) sections. The anterior sheath begins at a point called the arcuate line, located just below the umbilicus, or belly button.
Blood flows to the sheath through two different epigastric arteries. The inferior epigastric artery is a branch off the external iliac artery, and is found between the posterior rectus sheath and the rectus abdominis muscle. As the muscle contracts during movement, this artery slides smoothly with the muscle. An offshoot of the external thoracic artery becomes the superior epigastric artery. Located between the posterior rectus sheath and the rectus abdominis muscle, the artery enters the sheath just below the cartilage of the seventh ribs.
An uncommon injury to the sheath is called a rectus sheath hematoma (RSH). The abdominal pain that accompanies the condition is often mistaken for a symptom of another illness. RSH is classified according the location and severity of the hematoma, which is a collection of blood outside of a blood vessel. Significant swelling can occur as a result of the formation of a hematoma, and some patients may require a blood transfusion as part of the treatment process. Most people diagnosed with RSH report a healing period of up to four months.