Internal oblique

Where is the internal oblique?

It lies just under the external oblique, with the transversus abdominis under it. (StatPearls, Abdominal Wall; OpenStax A&P 2e, 11.4)

Slide your fingers into your front pockets and they point down and toward the midline, the line of the external oblique. The internal oblique’s fibers run the other way, up and toward the midline. (OpenStax A&P 2e, 11.4; StatPearls, Anterolateral Abdominal Wall and Anterolateral Abdominal Wall Muscles)

What does the internal oblique attach to?

Both StatPearls accounts have it arising from the iliac crest and the fascia of the lower back. Its fibers insert on the cartilages of the lower five ribs, and the lowest of them run across to the pubic tubercle. (StatPearls, Abdominal Wall and Anterolateral Abdominal Wall)

Above the arcuate line its aponeurosis splits around the rectus abdominis, to both walls of the rectus sheath. Below the line, all of it passes in front of the muscle, and every layer joins the linea alba. (StatPearls, Abdominal Wall)

Low down, its aponeurosis joins that of the transversus abdominis to form the conjoint tendon, and it gives off the cremasteric fibers that surround the spermatic cord. (StatPearls, Anterolateral Abdominal Wall)

What does the internal oblique do?

With the external oblique, it rotates the vertebral column and bends it to the side. (StatPearls, Abdominal Wall)

When the body needs more air, it acts as an accessory muscle of breathing, pulling the ribs down for active expiration. StatPearls names the anterolateral muscles, apart from the transversus abdominis, in this role. (StatPearls, Abdominal Wall)

The anterolateral muscles also stabilize the vertebral column, move the trunk and tense the abdominal wall. (StatPearls, Anterolateral Abdominal Wall)

Which nerves and arteries run beside the internal oblique?

The nerves of the abdominal wall, from the anterior rami of T7 to T12, run down and forward between it and the transversus abdominis. The iliohypogastric and ilioinguinal nerves also supply its lowest fibers. (StatPearls, Anterolateral Abdominal Wall and Abdominal Wall)

The lowest six intercostal arteries and the lumbar arteries run in the same plane, from the side toward the midline. (StatPearls, Anterolateral Abdominal Wall)

Where the sources differ

The two StatPearls accounts differ on the rest of its origin. The abdominal wall article adds the lateral inguinal ligament; the anterolateral abdominal wall article adds the iliopsoas fascia instead. (StatPearls, Abdominal Wall and Anterolateral Abdominal Wall)

OpenStax’s table, which groups the two obliques, gives their origin as ribs 5 to 12 and the ilium and their insertion as ribs 7 to 10, the linea alba and the ilium. StatPearls gives the internal oblique no rib origin and inserts it on the cartilages of the lower five ribs, the pubic tubercle and the linea alba. (OpenStax A&P 2e, 11.4; StatPearls, Anterolateral Abdominal Wall, Abdominal Wall and Anterolateral Abdominal Wall Muscles)