The External Oblique Muscle is the outermost and largest of the three-layered lateral abdominal muscles. Located on the lateral and anterior abdominal wall, it stands out for its oblique (slanted) fiber structure and assists with trunk movement, increasing intra-abdominal pressure, and breathing.

Anatomy

Morphology and Connection Points

The External Oblique Muscle is a broad, thin, quadrilateral muscle. It is found as muscle fibers on the lateral abdominal wall and as an aponeurotic structure on the anterior wall. Although superficial, it is generally invisible externally due to subcutaneous fat. The muscle originates as eight distinct fleshy processes on the lateral surfaces and inferior borders of the 5th to 12th ribs. The upper five processes grow from above downward and intertwine with the fibers of the serratus anterior muscle. The lower three processes become smaller as they descend and merge with the latissimus dorsi muscle. The fibers direct downward and forward. The lowest fibers of the muscle run vertically, inserting on the outer lip of the iliac crest. The middle and upper fibers become aponeurotic as they progress. This aponeurosis forms the anterior sheath of the rectus abdominis and crosses (decussates) with the fibers of the contralateral external oblique muscle on the linea alba. The aponeurosis of the external oblique muscle also contributes to the structure of the inguinal ligament and inguinal canal. The internal oblique muscle lies just beneath this muscle, and together they form the multilayered structure of the abdominal wall.

Embryological Origin and Classification

The External Oblique Muscle forms the most superficial layer of the anterolateral abdominal wall muscles, embryologically derived from the ventrolateral muscle mass. This structure, differentiated from the paraxial mesoderm, forms a functional triad with the internal oblique and transversus abdominis muscles. During the postnatal period, it plays an active role in various functions such as respiration, posture, and abdominal pressure control.

Nerve Innervation and Vascular Supply

The Muscle is innervated by the ventral branches of the lower six thoracoabdominal nerves (T7–T12) and the subcostal nerve (T12). These segmental nerves provide motor functions to the muscle and also provide neural support to the abdominal skin via sensory branches. Vascular supply is provided by the lower intercostal arteries above the muscle and by branches of the deep circumflex iliac artery and iliolumbar artery below. These vessels branch along the muscle's fiber orientation, ensuring effective perfusion.

Function and Clinical Relationships

The External Oblique Muscle is a versatile muscle. Its primary function is to increase intra-abdominal pressure; during this function, it pulls the rib cage down and compresses the abdominal organs, assisting in Valsalva maneuvers, vomiting, and coughing. It also actively contributes to respiration, particularly during forced exhalation. Its motor functions include ipsilateral lateral flexion (bending to the same side) and contralateral rotation (turning to the opposite side). For example, the right external oblique muscle works in conjunction with the left internal oblique muscle to rotate the trunk to the left. The robust structure of this muscle also contributes to postural control and trunk stabilization. Clinically, the external oblique muscle is particularly important due to its proximity to anatomical structures associated with abdominal wall hernias (e.g., inguinal hernias). In surgical procedures, particularly laparotomy or open hernia repair, the aponeurosis of this muscle must be carefully preserved or appropriately planned. Additionally, weakness or functional impairment of this muscle may impair abdominal stability, causing back pain or posture disorders.