15.12: Muscles of the Abdomen
The abdominal wall encircles the abdominal cavity, providing flexible protection and shielding the internal organs from harm. It is bordered at the top by the xiphoid process and costal margins, at the back by the vertebral column, and at the bottom by the pelvic bones and inguinal ligament. The abdominal wall is divided into two regions — the anterolateral and posterior regions.
Anterolateral Region
The anterolateral region comprises five paired muscles classified into the lateral and anterior groups. The lateral group consists of three flat muscles — the external oblique, the internal oblique, and the transversus abdominis. The internal oblique muscles run perpendicular to the superficial external oblique muscles, while the transversus abdominis muscle mainly runs transversely around the abdominal wall. The aponeuroses of these muscles interlace and form the linea alba, a fibrous band of connective tissue running vertically from the xiphoid process of the sternum to the pubic symphysis.
The anterior muscles, specifically the rectus abdominis, and pyramidalis, are positioned bilaterally to the linea alba. The long, flat rectus abdominis muscle extends from the sternum and ribs to the pubic bone. Just beneath the lower portion of the rectus abdominis lies the pyramidalis, a small and often variable muscle that not everyone has. When present, it's located in front of the rectus abdominis and attaches to the pubic bone and linea alba.
Posterior Region
The posterior abdominal wall is located between the last thoracic and lumbar vertebrae and is made up of paired muscles, including the quadratus lumborum, psoas major, psoas minor, and iliacus. The quadratus lumborum is located deep in the lower back on either side of the lumbar spine. It is a key stabilizer muscle that extends from the iliac crest to the lower ribs and lumbar vertebrae, aiding in maintaining upright posture. The psoas major is a fusiform muscle that runs from the sides of the lumbar vertebrae to the lesser trochanter process of the femur. It works closely with the iliacus, a fan-shaped muscle that merges with the psoas major to form the iliopsoas muscle. Finally, the psoas minor, present in only about half of the population, runs parallel to the psoas major but is a much smaller and thinner muscle. When present, it assists in flexing the pelvis and spine.
