Internal Oblique

The internal oblique is one of the three flat muscles of the lateral abdominal wall, situated deep to the external oblique and superficial to the transversus abdominis. It plays a crucial role in trunk rotation and lateral flexion, as well as in increasing intra-abdominal pressure. Its fibers run in a superomedial direction, roughly perpendicular to the external oblique. Anatomically, the internal oblique contributes to the formation of the rectus sheath and the inguinal canal, making it significant both functionally and clinically.

Structure

The internal oblique is a thin, broad muscle with a fan-like shape. It occupies the middle layer between the external oblique and the transversus abdominis in the lateral abdominal wall. Its fibers are muscular laterally and aponeurotic medially.

Origin

• Thoracolumbar fascia (posteriorly)
• Iliac crest (anterior two-thirds of the intermediate line)
• Lateral half of the inguinal ligament

Insertion

• Inferior borders of ribs 10 to 12
• Linea alba via aponeurosis
• Pubic crest and pecten pubis via the conjoint tendon

Fiber Direction

The fibers of the internal oblique run superomedially — upward and toward the midline — which is opposite to the direction of the external oblique. This crisscrossing arrangement enhances the structural strength of the abdominal wall.

Layer Relationships

The internal oblique lies:

• Deep to: External oblique
• Superficial to: Transversus abdominis

The neurovascular plane of the anterior abdominal wall lies between the internal oblique and transversus abdominis, where the segmental nerves and vessels run.

Function

The internal oblique works synergistically with the other abdominal muscles to perform various essential tasks. These include:

• Trunk rotation: When acting unilaterally, it rotates the trunk to the same side.
• Lateral flexion: Bends the torso sideways.
• Abdominal compression: Increases intra-abdominal pressure, aiding in forced expiration, urination, defecation, and parturition.
• Trunk stabilization: Provides core stability and support to the vertebral column.

Contribution to Rectus Sheath

The internal oblique plays a critical role in the formation of the rectus sheath, especially above the arcuate line:

• Above the arcuate line: Its aponeurosis splits into anterior and posterior layers, which contribute to both the anterior and posterior parts of the sheath surrounding the rectus abdominis.
• Below the arcuate line: The aponeurosis does not split but passes entirely anterior to the rectus abdominis, contributing only to the anterior sheath.

Contribution to Inguinal Canal

The internal oblique is also involved in the structure of the inguinal canal:

• Its lower fibers arch over the canal and join with fibers of the transversus abdominis to form the conjoint tendon, which supports the posterior wall of the canal.
• In males, it contributes muscle fibers to the cremaster muscle, which elevates the testis in response to temperature or stimulation.

Nerve Supply

The internal oblique is innervated by segmental nerves that travel in the plane between it and the transversus abdominis:

• Thoracoabdominal nerves (T7–T11)
• Subcostal nerve (T12)
• Iliohypogastric and ilioinguinal nerves (L1)

These nerves provide both motor and sensory innervation, and injury to them during abdominal surgeries may weaken the abdominal wall.

Blood Supply

Arterial blood is supplied through multiple sources:

• Lower posterior intercostal arteries
• Subcostal artery
• Superior epigastric artery
• Inferior epigastric artery
• Deep circumflex iliac artery

Venous drainage follows the arterial supply, draining into the internal thoracic, external iliac, and lumbar veins.

Relations

The internal oblique is closely related to:

• Superficially: External oblique
• Deeply: Transversus abdominis
• Medially: Contributes to the lateral part of the rectus sheath
• Inferomedially: Forms part of the conjoint tendon near the pubic region

Clinical Significance

• Inguinal hernias: Weakness or failure of the conjoint tendon may predispose to direct inguinal hernias.
• Cremasteric reflex: This reflex tests the function of the genitofemoral and ilioinguinal nerves and relies on the integrity of internal oblique-derived cremaster fibers.
• Surgical approach: The internal oblique must be separated or incised during open hernia repair or C-section approaches, requiring anatomical precision to avoid nerve damage.
• Core muscle injuries: Athletes with repetitive trunk rotation may suffer internal oblique strain, especially near its rib attachments.

Embryological Origin

The internal oblique, like the other abdominal muscles, originates from the paraxial mesoderm. It develops as part of the myotome-derived musculature that migrates to the lateral abdominal wall during embryogenesis. Its layered relationship with the transversus abdominis and external oblique reflects this shared lineage and coordinated developmental pattern.