meninges

meninges is the plural of meninx — Greek for membrane. The term encompasses a group of three membranes that provide mechanical protection and support to the delicate tissue of the central nervous system — the brain and the spinal cord. Moving inwards from the skull towards the brain, or from the vertebral canal towards the spinal cord, the three meninges are: dura mater or pachymeninx (Greek pachy, meaning thick), arachnoid mater, and pia mater. Arachnoid and pia are also called leptomeninges (thin membranes).

The dura consists of an outer layer, rich in blood vessels and nerves, and an inner layer, firmly attached to the arachnoid. At some sites within the skull, these two layers are separated, forming channels for blood draining from the brain into the veins — the venous sinuses. The sagittal sinus, for example, curves from front to back over the midline of the brain — ‘in the line of an arrow’ (Latin sagitta, an arrow). The dura also extends into membranes that subdivide the cranial cavity into compartments: the central, vertical falx cerebri (Latin falx — a sickle — which describes its shape), which separates the cerebral hemispheres, and the tentorium cerebelli, a ‘tent’ stretched over the cerebellum, forming the roof of the posterior fossa of the skull, which contains the cerebellum and brain stem. The free front margin of the tentorium fits closely round the back of the brain stem; the brain stem can be damaged, for example if a tumour growing above or below the ‘tent’ encroaches on this narrow space. The dura also forms a diaphragm above the pituitary gland, through which passes the pituitary stalk, joining the gland to the hypothalamus.

The blood supply to the dura is provided by an artery (middle meningeal) that is vulnerable to laceration by fracture of the skull; this can cause an epidural haemorrhage between the skull and the dura.

The dura of the brain is in continuation with the dura of the spinal cord, which is separated from the periosteum (the covering of the vertebral bones) by a narrow epidural space. Epidural anaesthesia to eliminate sensation from lower regions of the body, especially in childbirth, involves injection of drugs into this space. The dura tapers at the lower end of the spinal cord, forming a sheath around its thin remnant (the filum terminale). At the gaps between the vertebral bones, on each side, the dura forms a sleeve around the nerve roots that carry sensory and motor information to and from the spinal cord down its whole length.

The arachnoid surrounds the brain (bridging the sulci — the furrows on the surface of the cerebral cortex), and also the spinal cord and the cranial and spinal nerves. The space between arachnoid and pia is called the subarachnoid space, and contains the cerebrospinal fluid (CSF), which drains out of the cerebral ventricles. The space is narrow over the convexity of the brain and wider below and around the brain stem, where large spaces are formed, called cisternae by obvious reference to water cisterns.

The subarachnoid space surrounds the spinal cord and extends beyond its lower end, forming a ‘cistern’ that extends from the upper lumbar to the sacral part of the bony canal. This provides a site from which CSF can be sampled through a hollow needle for diagnostic purposes — the so-called lumbar puncture.

The subarachnoid space contains arteries and veins, which can be the site of abnormalities such as cerebral aneurysms — ‘blow-outs’ of artery walls, or malformations of the blood vessels. Rupture of these is the cause of subarachnoid haemorrhage.

The subarachnoid space is continuous with the cavities of the cerebral ventricles. The rate of formation of new CSF within the cerebral ventricles is matched by a continuous flow through the subarachnoid space, back into the bloodstream. The route for this is provided by protrusions of the leptomeninges into the sagittal venous sinus. These protrusions are the arachnoid villi. They act as passive, pressure-dependent valves that discharge the CSF from the subarachnoid space into the sinus.

The innermost meningeal layer, the pia mater, is closely applied to the surface of the brain tissue and carries many small arteries and veins. The pia and arachnoid follow the branches of the surface blood vessels where they penetrate the brain tissue, so that a microscopic CSF-containing space surrounds them as far as the capillaries.

Meningitis — inflammation of the meninges — can be a dramatically severe and dangerous illness if due to infection by meningococcus bacteria — but is often relatively innocuous when (now most commonly) caused by one of many possible virus infections.

Francesco Scaravilli

See also blood–brain barrier; cerebrospinal fluid; cerebral ventricles.