falx cerebri
Summary
The falx cerebri is a sickle-shaped fold of dura mater (meningeal layer) that descends in the longitudinal fissure between the two cerebral hemispheres. It anchors anteriorly to the crista galli and blends posteriorly with the tentorium cerebelli. It houses the superior and inferior sagittal sinuses and, with the tentorium, is key in subfalcine (cingulate) herniation.
Detail
ANATOMY: A midline, crescent/sickle-shaped dural reflection (inner meningeal layer of dura) that partially separates the right and left cerebral hemispheres in the longitudinal (interhemispheric) fissure. Anterior attachment: crista galli of the ethmoid bone. Posterior attachment: internal occipital protuberance and the upper surface of the tentorium cerebelli (where the straight sinus runs along the junction). Its superior convex border attaches to the skull along the sagittal sulcus, and its inferior free concave border lies just above the corpus callosum.
VENOUS SINUSES: Superior sagittal sinus runs along the attached (superior) margin; inferior sagittal sinus runs in the free (inferior) margin; the inferior sagittal sinus joins the great cerebral vein (of Galen) to form the straight sinus at the junction of the falx and tentorium. The superior sagittal sinus drains CSF via arachnoid granulations.
RELATED DURAL FOLDS: Tentorium cerebelli separates the occipital lobes from the cerebellum and forms the tentorial notch (incisura) for the midbrain. Falx cerebelli is a small fold between the cerebellar hemispheres. Diaphragma sellae roofs the sella turcica.
INNERVATION: Dura of the supratentorial region, including the falx, is supplied mainly by the trigeminal nerve (V1 via the tentorial/recurrent meningeal branch, plus V2 and V3 to the anterior and middle fossa). Pain is referred to the face/head (a basis for headache).
CLINICAL SIGNIFICANCE: - Subfalcine (cingulate) herniation: the most common herniation. A unilateral mass effect pushes the cingulate gyrus under the free edge of the falx. It can compress the anterior cerebral artery (ACA), causing contralateral lower-extremity weakness and sensory loss (ACA stroke pattern). - Falx meningioma: a parasagittal/falcine meningioma may compress the leg area of the motor cortex (paracentral lobule), causing contralateral leg spasticity or weakness and possible urinary incontinence. Imaging shows an extra-axial, dural-based, homogeneously enhancing mass with a "dural tail"; histology shows psammoma bodies and whorled cells. - Superior sagittal sinus thrombosis: can cause venous infarction/hemorrhage and raised ICP; associated with hypercoagulable states, pregnancy/postpartum, and OCP use. CT venography shows the "empty delta sign." - Subdural hematoma: bridging veins tear as they cross to the superior sagittal sinus (crescent-shaped, crosses suture lines but not dural reflections such as the falx and tentorium). Epidural hematomas (middle meningeal artery) do not cross suture lines but can cross the falx. - Falx calcification is common on imaging, usually incidental with age.
HIGH-YIELD HERNIATION SUMMARY: Subfalcine herniation (under the falx) → ACA compression. Uncal (transtentorial) herniation → CN III compression (blown pupil, "down and out" eye), PCA compression (contralateral homonymous hemianopia with macular sparing), Kernohan notch phenomenon (ipsilateral hemiparesis), and Duret hemorrhages (midbrain/pontine). Tonsillar herniation through the foramen magnum → medullary compression, cardiorespiratory failure.
Sources
- First Aid for the USMLE Step 1 (Neuroanatomy: meninges, herniation syndromes)
- Gray's Anatomy for Students
- Snell's Clinical Neuroanatomy
- Pathoma (CNS tumors, meningioma)
- Netter's Atlas of Human Anatomy
- Kaplan USMLE Step 1 Lecture Notes: Neuroscience
Reviewed by Medusa editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.