The internal oblique muscle arises from the upper surface of the inguinal ligament, the iliac crest, and the thoracolumbar fascia and is at a right angle to the external oblique muscle. The fibers run in inferolateral to superomedial direction. The aponeurosis of the internal oblique split near the rectus muscle and cover the rectus muscles. The anterior layer combines with the aponeurosis of the external oblique muscle to form the anterior rectus sheath and the posterior layer combines with the aponeurosis of the transversus abdominis to form the posterior rectus sheath. The lowermost fibers of the internal oblique muscle run inferomedially and accompany the spermatic cord into the scrotum as the cremasteric muscle.
Transversus abdominis is the innermost muscle layer of the abdominal wall. This muscle arises from the inguinal ligament, the iliac crest, the thoracolumbar fascia, and the lower costal cartilages. It runs transversely through the midline and forms aponeurosis which passes posterior to the rectus sheath above the semicircular line and anteriorly below it. The inferior fibres of the muscle run inferomedially and form the aponeurotic arch which is a major surgical landmark.
The transversalis fascia lies beneath the transversus abdominis muscle and forms a complete fascial envelope around the abdominal cavity. This fascia binds together all the muscles and aponeurosis and increases the strength and structural integrity of the abdominal wall.
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