Meninges

of CSF flow. It can be used to visually demonstrate the direction of CSF flow and for distinguishing arachnoid cysts from open CSF spaces. It is also possible to calculate CSF flow velocity in any location and to quantify volumetric flow.

MENINGES

Three layers of meninges cover the brain and spinal cord (cf. spinal meninges). These are, from without inwards - the dura mater, the arachnoid mater and the pia mater.

Dura mater

This is a tough membrane described as having two layers. The outer layer is, in fact, the periosteum of the inner aspect of the skull and is continuous through all the foramina and sutures of the skull w i th the periosteum on the outside of the skull. The inner layer is the dura mater proper. This is, however, densely adherent to the outer layer in all places except where the layers separate around the dural venous sinuses and where the inner layer projects inwardly as the falx cerebri and cerebelli, and as the tentorium cerebelli and the diaphragma sellae.

The falx cerebri is a sickle-shaped dural septum in the median sagittal plane attached to the crista galli in the midline of the floor of the anterior cranial fossa and along the midline of the inner aspect of the vault of the skull to the margins of the superior sagittal sinus. Posteriorly it is wider; here it is attached to the upper surface of the tentorium cerebelli, enclosing the straight sinus. It projects into the interhemispheric fissure and its inferior free edge lies close to the corpus callosum. The inferior sagittal sinus is within this free edge.

The diaphragma sellae is a horizontal fold of dura that almost completely covers the pituitary fossa, w i th a small opening for the pituitary stalk.

The tentorium cerebelli is a horizontal septum of dura mater that separates the occipital lobes from the superior surface of the cerebellum. Its outer attached border arises from each posterior clinoid process and is attached along the upper border of the petrous bones, enclosing the superior petrosal sinus on each side. The tentorium is attached to the occipital bone along the margins of the transverse sinuses to the internal occipital protuberance. The anterior free edge of the tentorium, called the tentorial notch, is attached to the anterior clinoid processes and surrounds the midbrain. There is no venous sinus in this free edge. The tentorium is sloped upwards from its attached edge to its free edge and has a midline ridge where the falx cerebri is attached to its superior surface. Close to the posterior clinoid processes an outpouching in the attached edge of the tentorium forms a recess around the trigeminal nerve and ganglion, the trigeminal cave.

The uncus of the temporal lobe and the posterior cerebral artery lie close to the free edge of the tentorium and may be compressed against it if there is increased pressure in the supratentorial part of the brain.

The falx cerebelli is a low elevation of dura that projects a small distance into the cerebellar interhemispheric fissure. It is attached from the internal occipital protuberance along the internal occipital crest to the posterior margin of the foramen magnum. The small occipital venous sinus lies in its attached edge.

Arachnoid mater

This layer of meninges is a delicate membrane which is impermeable to CSF. It lines the dura mater, separated from it only by a thin layer of lymph in the subdural (potential) space. It is separated from the pia mater by the subarachnoid space, which contains the CSF. The arachnoid mater projects into the interhemispheric fissure and into the root of the sylvian fissure, but otherwise does not dip into the sulci. It surrounds the cranial and spinal nerves in a loose sheath as far as their exit from the skull and vertebral canal.

Arachnoid mater herniates through holes in the dura into the venous sinuses and venous lakes as arachnoid v i l li (cf. CSF absorption), and only in these v i l li is the arachnoid pervious to CSF.

Pia mater

This meningeal layer is closely adherent to the brain surface and dips into all the sulci. It continues along all the cranial and spinal nerves and fuses w i th their epineurium. It is also invaginated into the surface of the brain by the entering cerebral arteries. It invaginates w i th the choroid vessels into the ventricles, and the layer of pia mater and ependyma together thus formed over these vessels is called the tela choroidea of the ventricles.

Arterial supply of the meninges

The arterial supply is found mostly between the periosteal and inner layers of the dura mater. Most of the supratentorial dura is supplied by the middle meningeal artery, a branch of the maxillary artery that enters the cranium through the foramen spinosum. It passes laterally in the floor of the middle cranial fossa, then superiorly and anteriorly along the greater wing of sphenoid, where it divides into anterior and posterior branches.

There is additional arterial supply to the meninges i n: • The anterior cranial fossa from meningeal branches of the ophthalmic and anterior and posterior ethmoid arteries; and • Over the cavernous sinus by meningeal branches of the carotid artery and by the accessory meningeal artery from the maxillary artery, which passes through the foramen ovale.