Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Musculoskeletal System
Muscular System
Muscles and Fascia of the Trunk — Abdominal Muscles

The abdomen is the part of the trunk located between the Thorax above and the pelvis below. Its cavity is the Abdominal cavity, the walls of which are formed above by the Diaphragm, below by the pelvic bones and Muscles as well as the pelvic floor (Perineum). The posterior wall of the abdominal cavity is formed by THE Vertebral Column and the paired quadratus lumborum Muscle. The anterior and lateral walls of the abdominal cavity are likewise formed by the paired external and internal oblique, transversus abdominis, and rectus abdominis muscles.

The external oblique muscle of the abdomen is broad and thin, originating by digitations from the eight lower Ribs. The muscle then runs downward and forward, continuing into a broad tendon (aponeurosis) that attaches to the iliac crest and the Pubic Symphysis. In the region between the anterior part of the ilium and the pubic tubercle, the aponeurosis of the external oblique folds inward and thickens, forming the inguinal ligament. Along the midline of the anterior abdominal wall, the aponeurosis of the external oblique joins the corresponding tendon of the opposite side, forming the linea alba (white line of the abdomen). This line extends from the xiphoid process above to the pubic symphysis below.

Approximately at the midpoint of the linea alba lies the umbilical ring (navel), which is closed by Connective Tissue. In embryos and fetuses, Blood Vessels (umbilical vessels) pass through the umbilical ring. The umbilical ring can also be a site for The formation of umbilical hernias.

The internal oblique muscle of the abdomen lies beneath the external oblique. It originates from the thoracolumbar fascia, the iliac crest, and the inguinal ligament, and runs upward and forward. This muscle inserts into the cartilages of the lower ribs, and its broad aponeurosis contributes to the Formation of the linea alba. The transversus abdominis muscle lies in the third layer, beneath the two preceding oblique muscles. It originates from the inner surface of the six lower ribs, the thoracolumbar fascia, the iliac crest, and the inguinal ligament. The broad aponeurosis of this muscle blends into the linea alba. The rectus abdominis muscle is situated laterally to the linea alba. It originates from the xiphoid process of the Sternum and the cartilages of ribs V–VII. The muscle bundles run vertically downward and attach to the pubic crest and the pubic symphysis. Both the right and left rectus abdominis muscles are enclosed within their own strong tendinous sheath (rectus sheath), formed by the aponeuroses of the external and internal oblique and transversus abdominis muscles. The rectus abdominis muscles pull the rib cage downward and flex the trunk. The oblique muscles also bend the trunk forward, assist in its rotation to the right and left, and aid in Respiration due to their attachments to the ribs.

The quadratus lumborum muscle is located laterally to the lumbar spine. It originates from the 12th rib and the transverse processes of lumbar vertebrae I–IV. The muscle inserts into the iliac crest and the transverse processes of the lower lumbar vertebrae. Upon contraction, this muscle bends the spine to its side.

Upon contraction, the Abdominal muscles increase intra-abdominal pressure, which is essential for maintaining Internal Organs in their natural positions. Intra-abdominal pressure facilitates bowel evacuation (defecation), urination, and, in women, the expulsion of the fetus from the Uterus during childbirth. Due to these Functions, the abdominal muscles form what is known as the abdominal press.

Externally, beneath the subcutaneous tissue, the abdominal muscles are covered by the superficial fascia, which is a continuation of the superficial Fascia of the chest. On the abdominal cavity side, the abdominal walls are lined by the transversalis fascia (endoabdominal fascia).

Due to their complex Structure, weak spots exist in the abdominal walls where hernias may develop. These include the linea alba, the umbilical ring, and the Inguinal Canal. The slit-like inguinal canal is located above the inguinal ligament, between the aponeurosis of the external oblique muscle anteriorly and the transversalis fascia posteriorly. The upper wall of the inguinal canal is formed by the lower margins of the internal oblique and transversus abdominis muscles. The superficial (external) ring of the inguinal canal lies above the medial part of the inguinal ligament. Through the inguinal canal pass the Spermatic Cord in men and the round ligament of the uterus in women.