Rectus Abdominis Anatomy

• Rectus abdominis arises from two tendinous heads: the medial head from the pubic symphysis' anterior surface, and the lateral head from the pubic crest's lateral part and pubic tubercle.
• It inserts via four fleshy slips on the anterior thoracic wall, along a horizontal line that passes from the xiphoid process laterally, intersecting the 7th, 6th, and 5th costal cartilages.
• The muscle features three tendinous intersections: one at the xiphoid process tip, one at the umbilicus level, and one midway between the two; a fourth intersection may sometimes be present below the umbilicus.
• These tendinous intersections run horizontally in a zigzag pattern and attach to the rectus sheath's anterior wall, dividing the muscle into shorter segments.
• The rectus abdominis is enclosed within an aponeurotic sheath, which comes from the anterolateral abdominal wall's three paired flat muscles.
• Nerve supply is provided by the lower five intercostal and subcostal nerves (T7–T12).
• Primary action is flexion of the trunk on the pelvis.

Pyramidalis

• Pyramidalis is a rudimentary, small triangular muscle anterior to the lower rectus abdominis, within the rectus sheath.
• It originates from the front of the pubic body and the anterior pubic ligament.
• Insertion is into the linea alba, midway between the umbilicus and pubic symphysis.
• Innervated by the subcostal nerve (T12).
• It tenses the linea alba.
• The pyramidalis is absent in approximately 20% of individuals.

Functions of Anterior Abdominal Muscles

• They provide strong, expandable support for abdominal viscera against gravity and protect against injury.
• The muscles compress abdominal contents to increase intra-abdominal pressure, aiding expulsive and expiratory actions.
• Abdominal muscles facilitate trunk movement to maintain posture.
• Oblique and transverse muscles' tone offers firm, elastic support to abdominal viscera, preventing external injury.
• Oblique and transverse muscle contraction compresses the abdominal viscera, elevating intra-abdominal pressure.
• This increased pressure helps expel air during respiration, assists in forceful expiratory actions like coughing and sneezing, aids expulsive acts like defecation and parturition and is involved in heavyweight lifting which may sometimes cause hernia.
• Trunk flexion, especially in the lumbar region, is primarily performed by the rectus abdominis.
• Lateral trunk flexion results from unilateral oblique muscle contraction.
• Trunk rotation occurs through combined contraction of the external oblique on one side and the internal oblique on the opposite side.

Rectus Sheath

• An aponeurotic sheath enclosing the rectus abdominis and pyramidalis muscles (if present) on either side of the linea alba.
• Derived from the aponeuroses of the anterior abdominal wall's flat muscles.