The internal oblique muscle is one of the muscles that form the anterior abdominal wall. Inferiorly, it contributes towards the formation of the inguinal ligament.

origin: originates along the whole length of the lumbar fascia, from the anterior two-thirds of the intermediate line of the iliac crest and from the lateral two-thirds of the grooved upper surface of the inguinal ligament
insertion:
- inserts onto the inferior border of the costal cartilages of the lower 3 ribs (10^th-12^th) in continuity with the internal intercostal muscles.
- becomes aponeurotic at the level of the 9^th costal cartilage, which fuses at the midline at the linea alba.
innervation: lower intercostal nerves (T7-T12), ilioinguinal and iliohypogastric nerves
action: compression of abdominal content, forced expiration, flexes and bends the trunk

Gross anatomy

The internal oblique muscle lies deep to the external oblique muscle and superficial to the transversus abdominis muscle. It represents a continuation of the layer of the internal intercostal muscles of the thorax ¹. The muscle fibers pass in a superomedial direction. Laterally, it is muscular and medially it forms an aponeurosis that merges into the linea alba at the midline ².

The internal oblique muscle has a free lower border, in comparison to the external oblique muscle that has a free posterior border which forms the anterior boundary of the lumbar triangle of Petit. The free lower border of the internal oblique muscle is located anterior to the deep inguinal ring. It arches over the spermatic cord – laterally there are muscle fibers anterior to the cord, and medially there are tendinous fibers behind the cord ¹. Inferiorly, it attaches onto the pectineal line with the transversus aponeurosis, which forms the conjoint tendon.

Below the level of the costal margin and above the arcuate line, the internal oblique aponeurosis splits anteriorly and posteriorly around the rectus muscle to form the anterior and posterior rectus sheath. Below the arcuate line all three aponeuroses (external and internal obliques and transversus) converge anteriorly. Above the arcuate line, there is a posterior sheath, and below this line, there is none. The arcuate line is formed 2.5 cm below the level of the umbilicus¹(or, using surface landmarks, midway between the umbilicus and pubic crest).

Function

Along with other abdominal wall muscles, the internal oblique muscle flexes and bends the trunk, assists forced expiration by depressing the lower ribs, and helps to maintain intra-abdominal pressure in defecation, micturition and childbirth.