The falx cerebri is the sickle-shaped dural fold projecting vertically downward from the inner skull vault into the longitudinal fissure between the cerebral hemispheres. Its upper margin is attached to the inner calvarium from the crista galli anteriorly to the internal occipital protuberance posteriorly, where the superior sagittal sinus runs within its free upper border. Its lower free margin contains the inferior sagittal sinus. Posteriorly it merges with the tentorium cerebelli.
Limits excessive side-to-side displacement of the cerebral hemispheres during head acceleration; provides the structural housing for the superior and inferior sagittal sinuses; anchors the tentorium cerebelli at its posterior junction.
The falx cerebri is a frequent site of meningioma (falcine meningioma), which presents as a bilateral parasagittal or interhemispheric mass compressing the medial frontal lobes and causing lower limb weakness. Subfalcine herniation occurs when unilateral cerebral swelling or haematoma pushes the cingulate gyrus under the free lower edge of the falx, compressing the anterior cerebral artery and producing contralateral leg weakness. The superior sagittal sinus within the falx is the primary dural venous outflow and its thrombosis produces venous infarction with haemorrhage.
Mass effect from cerebral oedema or haematoma pushes the ipsilateral cingulate gyrus under the free lower edge of the falx cerebri, compressing the ACA against the falcine edge and producing contralateral leg weakness; CT demonstrates midline shift with the cingulate gyrus crossing below the falcine attachment line.
