Internal abdominal oblique muscle

Origin and insertion

Internal abdominal oblique muscle has multiple sites of origin, which are distributed along  the anterolateral side of the trunk. According to their origin, the muscle fibers can be divided into the anterior, lateral and posterior fibers.

• Anterior fibers: traditionally are thought to arise from the lateral two-thirds of the superior surface of the inguinal ligament, forming a common attachment with the iliac fascia. However it is now known that they arise from a deeper structure known as the iliopectineal arch. The fibers pass inferomedially, arching over the inguinal canal and merge with the tendinous fibres of the transversus abdominis muscle to form the conjoint tendon. This tendon then inserts into the pubic crest and pecten pubis. In males, some of the anterior fibers extend into the spermatic cord and form the cremaster muscle.
• Lateral fibers: originate from the anterior two-thirds of iliac crest and then diverge superiorly and medially. The fibers then extend into an aponeurosis that contributes to the formation of the rectus sheath and inserts at the linea alba.
• Posterior fibers: originate from the posterior end of the iliac crest and the thoracolumbar fascia. The fibers then ascend superolaterally and insert into the inferior borders and tips of the lower three or four ribs and their cartilages. Here, these merge with the internal intercostals of the aforementioned ribs.

Relations

The internal abdominal oblique muscle lies on the lateral abdominal wall, comprising one of its three layers. It is found deep to the external abdominal oblique and superficial to the transverse abdominis.

The lateral fibres of the internal abdominal oblique muscle are continuous with the rectus sheath, the large aponeurosis of the anterior abdominal wall . The rectus sheath largely encloses the rectus abdominis and pyramidalis muscles, as well as many neurovascular structures of the anterior abdominal wall.

Below the costal margin, the upper three quarters of the aponeurosis of internal abdominal oblique muscle split into deep and superficial layers around the rectus abdominis muscle. The deep layer, together with the aponeurosis of transversus abdominis, form the posterior layer of the rectus sheath. The superficial layer, on the other hand, merges with the aponeurosis of external abdominal oblique to form the anterior layer of the rectus sheath.

Around the level of the lower quarter of the aponeurosis of internal abdominal oblique muscle, the aponeuroses of the external abdominal oblique, internal abdominal oblique and transversus abdominis converge and lie anterior to the rectus abdominis muscle. The point at which they converge is known as the arcuate line, and is located roughly 2.5cm below the umbilicus.

The abdominal muscles and their relations and attachments can be a bit confusing. To get a clear picture and wrap your head around the most important concepts, check out the following page:

Innervation

Internal abdominal oblique muscle is mainly supplied by the lower six thoracic spinal nerves (T7-T12), namely the terminal branches of the lower five intercostal nerves and the subcostal nerve. Additionally, the iliohypogastric and ilioinguinal nerves (L1) provide minor contributions to the nervous supply of this muscle.

Blood supply

Internal abdominal oblique muscle receives its arterial blood supply from the following arteries:

• Lower posterior intercostal and subcostal arteries
• Superior and inferior epigastric arteries
• Superficial and deep circumflex iliac arteries 
• Posterior lumbar arteries

Function

Internal abdominal oblique muscle has several functions that are dependent upon which parts of the muscle contracts. Upon bilateral contraction, the internal abdominal oblique flexes the trunk. Simultaneously, it causes compression of the intra-abdominal viscera thereby increasing the intra-abdominal pressure. This action is utilized for functions such as forced expiration, micturition, defecation etc. If the rib cage is fixed, the bilateral contraction of the internal abdominal oblique lifts the anterior part of the pelvis and alters the degree of pelvic tilt. Upon unilateral contraction, internal abdominal oblique causes ipsilateral flexion of the trunk, as well as ipsilateral rotation of the trunk.

Along with other muscles of the abdominal wall, internal abdominal oblique muscle is important for maintaining normal abdominal wall tension. Therefore the tonic contraction of these muscles has a protecting as well as a supporting role. Additionally, weakness of the internal abdominal oblique or other abdominal muscles increases the risk for abdominal hernias.

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