Meninges, Ventricles and Cerebrospinal Fluid

Three concentric membranes surrounding the brain and spinal cord, from outer to inner: dura mater, arachnoid mater, pia mater.

Dura Mater

• The tough, outermost, fibrous layer.

Periosteal layer:

• The outer layer, functioning as the internal periosteum of the skull bones, firmly adherent to the inner cranial surface (particularly at suture lines), a key point, since this layer is absent around the spinal cord (only the meningeal layer continues into the vertebral canal), explaining why the cranial and spinal epidural spaces differ anatomically (the cranial "epidural space" is only a true potential space when pathologically created, e.g., by an extradural haematoma stripping the periosteal dura from bone; the spinal epidural space is a normal, fat-filled anatomical space).

Meningeal layer:

• The inner layer, continuous with the spinal dura mater, closely applied to the arachnoid mater below it (the subdural space is a potential, not a normal anatomical, space between dura and arachnoid).

Dural reflections:

• Where the meningeal dural layer folds inward, forming septa that partially subdivide the cranial cavity and provide structural support/limit brain displacement.

Falx cerebri:

• A sickle-shaped, vertical dural fold in the midline longitudinal fissure, separating the two cerebral hemispheres, attached anteriorly to the crista galli and posteriorly continuous with the tentorium cerebelli; contains the superior and inferior sagittal sinuses along its attached and free borders respectively.

Tentorium cerebelli:

• A horizontal, tent-shaped dural fold separating the cerebral hemispheres (above) from the cerebellum (below), attached to the petrous temporal bones and the occipital bone, with a central opening (the tentorial notch/incisura, through which the midbrain passes), this notch is the critical anatomical site of transtentorial (uncal) herniation (see Applied Anatomy); contains the transverse sinuses along its attached border.

Falx cerebelli:

• A small, vertical dural fold in the posterior fossa, partially separating the two cerebellar hemispheres.

Diaphragma sellae:

• A small, horizontal dural fold forming a roof over the pituitary fossa (sella turcica), with a small central opening for the pituitary stalk (infundibulum) to pass through.

Arachnoid Mater

• A thin, web-like, avascular membrane closely applied to the inner dural surface, separated from the pia mater by the subarachnoid space (containing CSF and the major cerebral vessels), connected to the pia by fine trabeculae spanning this space.

Pia Mater

• A thin, highly vascular membrane closely investing the brain surface itself, following every sulcus and gyrus contour (unlike the arachnoid, which bridges over sulci), continuous with the perivascular sheaths of vessels entering the brain substance.

Endothelium-lined venous channels running between the two dural layers, draining the brain's venous blood (and, via emissary veins, communicating with extracranial veins).

Superior sagittal sinus:

• Runs along the attached (superior) border of the falx cerebri, from the frontal region to the confluence of sinuses posteriorly; receives cerebral veins and, importantly, is the site of arachnoid granulations (CSF reabsorption).

Inferior sagittal sinus:

• Runs along the free (inferior) border of the falx cerebri, draining into the straight sinus.

Straight sinus:

• Formed by the union of the inferior sagittal sinus and the great cerebral vein (of Galen), running along the junction of the falx cerebri and tentorium cerebelli, draining into the confluence of sinuses (typically preferentially into the left transverse sinus, though this is variable).

Transverse sinuses (paired):

• Run along the attached border of the tentorium cerebelli, from the confluence of sinuses to the sigmoid sinuses; typically asymmetric in size (the right often larger, receiving preferential drainage from the superior sagittal sinus, while the left often receives preferential drainage from the straight sinus).

Sigmoid sinuses (paired):

• Continue from the transverse sinuses, curving down to exit the skull via the jugular foramen, becoming the internal jugular vein.

Cavernous sinus (paired):

• Located on either side of the sella turcica/sphenoid body, receiving the superior/inferior ophthalmic veins and the sphenoparietal sinus; traversed by the internal carotid artery and CN VI, with CN III, IV, V1, V2 running within its lateral wall.

Superior petrosal sinus:

• Runs along the petrous temporal bone's superior border, draining the cavernous sinus into the transverse sinus.

Inferior petrosal sinus:

• Runs along the petrous temporal bone's inferior border, draining the cavernous sinus into the internal jugular vein directly (via the jugular foramen).

Major connections and drainage (summary pathway):

• Superior sagittal + straight sinuses → confluence of sinuses → transverse sinuses → sigmoid sinuses → internal jugular veins, with the cavernous sinus draining separately via the superior/inferior petrosal sinuses into the transverse sinus and internal jugular vein respectively, this network has no valves, meaning flow direction can be influenced by pressure gradients, relevant to the various infection/thrombosis spread patterns discussed throughout this Atlas.

Lateral ventricles (paired):

• C-shaped cavities within each cerebral hemisphere, each with a body, and anterior (frontal), posterior (occipital), and inferior (temporal) horns, containing choroid plexus (except in the frontal/occipital horns), the largest CSF-containing spaces.

Interventricular foramina (of Monro):

• Paired small openings connecting each lateral ventricle to the third ventricle, located just anterior to the thalamus.

Third ventricle:

• A narrow, slit-like midline cavity between the two thalami (often bridged by the interthalamic adhesion/massa intermedia, a variable structure), bounded by the hypothalamus inferiorly.

Cerebral aqueduct (of Sylvius):

• A narrow channel through the midbrain, connecting the third and fourth ventricles, the narrowest point of the entire ventricular system, and consequently the most common site of obstruction causing obstructive (non-communicating) hydrocephalus (see Applied Anatomy).

Fourth ventricle:

• A diamond-shaped cavity between the pons/medulla (anteriorly) and the cerebellum (posteriorly), continuous inferiorly with the spinal cord's central canal.

Median aperture (of Magendie):

• A single midline opening in the fourth ventricle's roof, allowing CSF to exit into the cisterna magna (subarachnoid space).

Lateral apertures (of Luschka, paired):

• Two openings at the lateral recesses of the fourth ventricle, allowing CSF to exit into the subarachnoid space around the cerebellopontine angle.

Formation:

• Produced predominantly by the choroid plexus within each ventricle (lateral, third, and fourth ventricles), via active secretion and ultrafiltration of blood plasma; a small additional contribution arises from the brain's extracellular fluid/ependymal surfaces generally.

Choroid plexus:

• Specialised, highly vascular tissue projecting into the ventricles, formed by invaginated pia mater and ependymal cells (together forming the tela choroidea at these specific sites), present within the lateral ventricles (body and inferior horns, absent from the frontal/occipital horns), the roof of the third ventricle, and the roof of the fourth ventricle.

Circulation:

• Lateral ventricles → (via interventricular foramina) → third ventricle → (via cerebral aqueduct) → fourth ventricle → (via median and lateral apertures) → subarachnoid space, circulating around the brain and spinal cord within this space, bathing the CNS before reabsorption.

Reabsorption:

• Predominantly via arachnoid granulations (also termed arachnoid villi), small protrusions of arachnoid mater through the dura into the superior sagittal sinus (and, to a lesser extent, other dural sinuses), allowing one-way, pressure-gradient-dependent bulk flow of CSF into the venous blood.

Arachnoid granulations:

• The primary site of CSF-to-blood reabsorption, functioning essentially as pressure-sensitive one-way valves, flow occurs only when CSF pressure exceeds venous sinus pressure, preventing reflux of blood into the subarachnoid space under normal circumstances.

Hydrocephalus:

• Excess accumulation of CSF within the ventricular system, from either obstructive (non-communicating) causes (a physical blockage somewhere along the ventricular pathway, most classically at the narrow cerebral aqueduct, or at the fourth ventricle's outlet apertures) or communicating causes (impaired reabsorption at the arachnoid granulations, e.g., following subarachnoid haemorrhage causing granulation blockage by blood products, or meningitis-related inflammatory scarring), the distinction determines which specific part of the ventricular pathway is dilated on imaging, and guides treatment (e.g., endoscopic third ventriculostomy for aqueductal obstruction, vs ventriculoperitoneal shunting for communicating causes).

Raised intracranial pressure:

• Can result from excess CSF (hydrocephalus), a mass lesion (tumour, haematoma), or diffuse cerebral oedema, the skull's rigid, fixed-volume nature (the Monro-Kellie doctrine: brain + CSF + blood volume within a fixed cranial volume) means any pathological volume increase must be compensated by displacement of one of the other two components (typically CSF and venous blood initially), and once this compensatory reserve is exhausted, pressure rises steeply and disproportionately, risking herniation.

Lumbar puncture anatomy:

• Performed below the spinal cord's termination (conus medullaris, typically L1-L2 in adults), classically at the L3-L4 or L4-L5 interspace (guided by the intercristal/supracristal line connecting the iliac crests, corresponding approximately to L4), to safely access the lumbar cistern (the CSF-filled subarachnoid space surrounding the cauda equina below the conus) without risking direct spinal cord injury.

Subdural haemorrhage:

• Bleeding into the potential subdural space (between dura and arachnoid), classically from rupture of the bridging veins that cross this space to drain the cerebral cortex into the superior sagittal sinus, these veins are particularly vulnerable to shearing forces from brain movement relative to the fixed dura (e.g., in acceleration-deceleration trauma, or in cerebral atrophy where the brain has more room to move, explaining the higher incidence in elderly patients and infants).
• Classically produces a crescent-shaped haematoma on imaging (following the brain's contour, not limited by suture lines, since the subdural space is continuous across the calvaria).

Epidural (extradural) haemorrhage:

• Bleeding into the potential space between the skull and the periosteal dura, classically from rupture of the middle meningeal artery following a pterion fracture.
• Classically produces a lentiform (biconvex) haematoma on imaging, limited by suture lines (since the periosteal dura's firm attachment there restricts the haematoma's lateral spread), a key distinguishing imaging feature from subdural haemorrhage.

Subarachnoid haemorrhage:

• Bleeding into the subarachnoid space itself, classically from rupture of a berry (saccular) aneurysm at the circle of Willis, presenting with sudden, severe "thunderclap" headache, blood within the CSF-filled subarachnoid space can subsequently impair CSF reabsorption at the arachnoid granulations (via blood product-induced fibrosis/blockage), causing delayed communicating hydrocephalus as a recognised complication.