The roof of the inguinal canal is formed by the arching inferior fibres of internal oblique and transversus abdominis.
The internal oblique muscle fibres originate from the lateral two-thirds of the gutter formed by the infolding of the inguinal ligament. They arch inferomedially over the cord to become aponeurotic at the level of the lacunar ligament. The most lateral fibres pass anterior to rectus abdominis to insert into the pubic symphysis. Fibres originating more medially along the inguinal ligament insert laterally along the pectineal line as far as the free crescentic edge of the lacunar ligament.
The transversus abdominis muscular fibres originate from the lateral half of the inguinal ligament. They lie deep to the fibres of internal oblique and become aponeurotic at an earlier point in their arching course over the cord.
The aponeuroses of internal oblique and transversus abdominis are fused at their medial common insertion as the conjoint tendon.
The most inferior fibres of inferior oblique and transversus abdominis are supplied by the iliohypogastric and ilioinguinal nerves. If either are damaged e.g. transected at surgery, then there is no contraction of the roof of the canal in response to an elevation of intra-abdominal pressure. Consequently, there is then a predisposition to a direct inguinal hernia.
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