External oblique is a broad, thin, sheet-like muscle. It is the largest and most superficial of the muscles of anterior abdominal wall.
Origin of External Oblique:
External oblique arises from the outer surfaces of the lower eight ribs (Rib 4 to 12).
Insertion of External Oblique:
After its origin, the muscle fibers fan out to be inserted into the following structures;
- Xiphoid process
- Linea alba
- Pubic crest
- Pubic tubercle
- Anterior half of iliac crest
Most of the fibers of external oblique are inserted by means of a broad aponeurosis. There is a triangular shaped defect in the aponeurosis above and medial to pubic tubercle. This defect is known as superficial inguinal ring.
The lower border of the aponeurosis between the anterior superior iliac spine and pubic tubercle is folded backward on itself, forming the inguinal ligament (Ligament of Poupart). The medial end of this ligament is known as lacunar ligament.
It must be appreciated that most posterior fibers of the muscle, passing down to the iliac crest form a posterior free border.
Blood supply of external oblique:
External oblique has a profuse blood supply. The following arteries are involved;
- Lower posterior intercostal and subcostal arteries
- Superior and Inferior epigastric arteries
- Superficial and deep circumflex iliac arteries
- Posterior lumbar arteries
Nerve supply to External oblique:
External oblique receives its nerve supply from lower five intercostal nerves and the subcostal nerve. Iliohypogastric and Ilioinguinal nerves also innervate this muscle.
Actions of External Oblique:
External oblique performs following important functions.
- It plays a major role in maintenance of abdominal tone.
- It helps in completion of micturition, parturition and defecation and vomiting by compressing the abdominal contents.
- It assists in flexing and rotating the trunk.
- It assists in forced expiration.