Internal oblique is lies beneath the external oblique for the majority of its course. Similar to the external oblique and transversus abdominis, the internal oblique muscle is also broad, thin and sheet-like, but it is thinner and less bulky. The fibers run at a right angle to those of the external oblique.
Origin of internal oblique:
Internal oblique arises from;
- Lumbar fascia
- Anterior two thirds of the iliac crest
- Lateral two thirds of the inguinal ligament
- Some fibers originate from the thoracolumbar fascia
Insertion of internal oblique:
Fibers of internal oblique muscle radiate as they pass upward and forward to be inserted into;
- Lower borders of the lower three ribs and their costal cartilages
- Xiphoid process
- Linea alba
- Symphysis pubis
There is a lower free border in the muscle that arches over the spermatic cord (in males) or round ligament of the uterus (in females). Behind, this free border descends to be attached to the pubic crest and the pectineal line. Near this insertion, the lowest tendinous fibers of internal oblique are joined by similar fibers from the transversus abdominis to form the conjoint tendon. Medial, the conjoint tendon is attached to linea alba but it has a lateral free border.
As the spermatic cord (or round ligament of the uterus) passes under the lower free border of internal oblique, it carries some muscle fibers with it. These muscle fibers from the cremaster muscle.
Blood supply of internal oblique:
Internal oblique receives its blood supply from the following arteries.
- Lower posterior intercostals and the subcostal arteries
- Superior and inferior epigastric arteries
- Superficial and deep circumflex iliac arteries
- Posterior lumbar arteries
Nerve supply to internal oblique:
Major share of nerve supply to internal oblique comes from lower five intercostals nerves and the subcostal nerve (ventral rami of lower six thoracic spinal nerves). A small contribution is made by the iliohypogastric and ilioinguinal nerves (ventral rami of first lumbar spinal nerve).
Actions of internal oblique:
Internal oblique performs following important functions.
- It enables lateral flexion of the trunk against resistance.
- It contributes to the maintenance of abdominal tone.
- It helps in defecation, micturition, parturition and vomiting by increasing the intra-abdominal pressure.
- It assists in forced expiration.