Cranial Cavity and Dural Folds
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Direct answer
Four folds of dura mater partition the cranial cavity: the falx cerebri between the cerebral hemispheres, the tentorium cerebelli forming the roof of the posterior cranial fossa, the small falx cerebelli between the cerebellar hemispheres, and the diaphragma sellae covering the pituitary fossa. Together they divide the cavity into supratentorial and infratentorial compartments and stiffen the brain against displacement. Their edges enclose the dural venous sinuses, and their free margins — above all the tentorial hiatus through which the midbrain passes — determine where brain tissue herniates when intracranial pressure rises.
What you must remember
- Falx cerebri: a sickle-shaped vertical fold in the longitudinal fissure; attached to the crista galli in front and the internal occipital protuberance behind; the superior sagittal sinus runs in its attached upper border, the inferior sagittal sinus in its free lower margin.
- Tentorium cerebelli: tent-like, horizontal-ish sheet separating occipital lobes from cerebellum; its attached border runs from the posterior clinoid processes along the petrous temporal ridge (grooving the superior petrosal sinus) to the internal occipital protuberance.
- Free margin of the tentorium: a U-shaped border attached separately to the anterior clinoid processes, forming the tentorial hiatus that transmits the midbrain; the free and attached borders cross at the apex of the petrous bone, leaving anterior and posterior clinoid attachments.
- Falx cerebelli: small midline fold below the tentorium, containing the occipital sinus; the diaphragma sellae roofs the sella turcica with an aperture for the pituitary stalk.
- Straight sinus: formed at the junction of falx cerebri and tentorium cerebelli by union of the inferior sagittal sinus and the great cerebral vein (of Galen).
- Subfalcine herniation: the cingulate gyrus slips under the falx, compressing the anterior cerebral artery — contralateral leg weakness.
- Uncal herniation: medial temporal lobe forced through the tentorial hiatus stretches the third nerve (ipsilateral dilated, fixed pupil) and presses the opposite peduncle against the tentorial edge (Kernohan's notch — same-sided, false-localising hemiplegia).
- Tonsillar herniation (coning): cerebellar tonsils descend through the foramen magnum and compress the medulla — the reason lumbar puncture is dangerous in raised intracranial pressure.
Working through a falling conscious level
A man with a left extradural haematoma drowses, then develops a dilated right pupil and a right hemiplegia — the dural folds explain each step. The clot first pushes the cingulate gyrus beneath the falx, then drives the uncus of the temporal lobe through the tentorial hiatus. The oculomotor nerve, stretched over the tentorial edge as it runs to the cavernous sinus, loses its parasympathetic fibres first: the pupil dilates on the side of the herniation before eye movements fail. If the opposite cerebral peduncle is then jammed against the sharp free tentorial edge, corticospinal fibres are crushed — weakness on the same side as the lesion, a false localising sign examiners love. Continued pressure cones the cerebellar tonsils through the foramen magnum, and compression of the vital centres in the floor of the fourth ventricle kills the patient; that is why a falling conscious level after head injury outranks every other observation.
Where students slip
The commonest written error is placing the superior sagittal sinus in the free edge of the falx — the free lower edge carries the inferior sagittal sinus, while the superior sagittal occupies the attached upper border. The second slip is the free margin of the tentorium ending at the anterior clinoid processes, not the posterior. Asked why an uncal herniation dilates a pupil, weaker candidates say "third nerve compression" without a side; the complete answer names the nerve stretched over the tentorial edge, the ipsilateral pupil, and adds Kernohan's notch for the misleading same-sided hemiplegia.
Frequently asked questions
Which dural fold contains the straight sinus?
The straight sinus lies at the junction of the falx cerebri and the tentorium cerebelli, formed by the union of the inferior sagittal sinus with the great cerebral vein. It runs backwards to the confluence of the sinuses.
Which structures pass through the tentorial hiatus?
The midbrain, accompanied by the third and fourth nerves and the posterior cerebral and superior cerebellar arteries at the margins. The oculomotor nerve is the structure compressed against the free edge.
Why does an uncal herniation cause a dilated pupil on the same side as the lesion?
The parasympathetic pupilloconstrictor fibres in the oculomotor nerve are stretched against the free tentorial edge on the side of herniation. Losing them leaves the dilator pupillae unopposed, so the pupil dilates and becomes fixed ipsilaterally.
What is Kernohan's notch?
It is the groove crushed into the contralateral cerebral peduncle as it is forced against the sharp free edge of the tentorium during uncal herniation. The result is a hemiplegia on the same side as the causating lesion — a false localising sign.
What is the role of the diaphragma sellae?
It is a circular dural fold forming the roof of the pituitary fossa, pierced by a small central aperture through which the infundibulum (pituitary stalk) passes. It separates the pituitary from the optic chiasma above.