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.
The abdominal internal oblique muscle, also internal oblique muscle or interior oblique, is an abdominal muscle in the abdominal wall that lies below the external oblique muscle and just above the transverse abdominal muscle.
Anatomy of Internal Oblique
Its fibers run perpendicular to the external oblique muscle, beginning in the thoracolumbar fascia of the lower back, the anterior 2/3 of the iliac crest (upper part of hip bone) and the lateral half of the inguinal ligament. The muscle fibers run from these points superomedial (up and towards the midline) to the muscle’s insertions on the inferior borders of the 10th through 12th ribs and the linea alba. In males, the cremaster muscle is also attached to the internal 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
Insertion: Linea alba, Xiphoid process and the inferior ribs.
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
- Intercostal nerves T8, T9, T10, T11, subcostal nerve(T12), iliohypogastric nerve, ilioinguinal nerve
- The major share of nerve supply to internal oblique comes from the 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 the first lumbar spinal nerve).
- The internal oblique is supplied by the lower intercostal nerves, as well as the iliohypogastric nerve and the ilioinguinal nerve.
Function
- Compresses abdomen and rotates vertebral column.
- The internal oblique performs two major functions. Firstly as an accessory muscle of respiration, it acts as an antagonist (opponent) to the diaphragm, helping to reduce the volume of the chest cavity during exhalation. When the diaphragm contracts, it pulls the lower wall of the chest cavity down, increasing the volume of the lungs which then fill with air.
- Conversely, when the internal obliques contract they compress the organs of the abdomen, pushing them up into the diaphragm which intrudes back into the chest cavity reducing the volume of the air-filled lungs, producing an exhalation.
- Secondly, its contraction causes ipsilateral rotation and side-bending. It acts with the external oblique muscle of the opposite side to achieve this torsional movement of the trunk. For example, the right internal oblique and the left external oblique contract as the torso flexes and rotates to bring the left shoulder towards the right hip. For this reason, the internal obliques are referred to as same-side rotators.
