Meninges

The meninges are the three connective-tissue membranes which envelop the brain and spinal cord in human beings and the rest of vertebrate animals. Their main function is to protect the brain and other portions of the central nervous system from external concussion (blow) and infection, be it bacterial or viral. The meninges develops from a primitive meninx (singular) in the embryo. Later, as the fetus develops, this primeval membrane differentiates into three distinct layers: the dura mater (adjoining the periosteum of the skull), the arachnoid, and primary pia mater (adjoining the brain and forming the folds of the brain’s vascular plexuses).

In the brain region, dura mater is the outer membrane, which is the toughest meningeal layer. Externally, it forms the periosteum of the inner surface of the skull. Internally, it sends fibrous longitudinal and transverse processes (projections) into the cranial cavity. The main processes are the falx cerebri, which is wedged between the two cerebral hemispheres, and the tentorium cerebelli, which separates the cerebellum from the lower surface of the occipital lobes of the cerebrum. Sinuses form, when the dura mater splits, in certain places in the cranium; venous blood flows into them from the brain. In the spinal cord region, the dura mater consists of two layers, outer (periosteum) and inner. These are divided by the epidural space, which is filled with adipose tissue and venous plexuses.

The arachnoid is the middle membrane of the meninges. It is tenuously attached to the externally adjacent dura mater (specifically the dural border cell layer), and no natural space occurs at the dura-arachnoid interface. Thus, in a spinal puncture, dura mater and arachnoid are penetrated simultaneously as if a single layer. However, a separation of the arachnoid membrane from the dura mater may occur as a result of a trauma or pathologic processes, thus creating what is commonly called a subdural hematoma. The arachnoid mater is named for the delicate, spiderweblike filaments that extend from its deep surface, through the cerebrospinal fluid of the subarachnoid space, to the pia mater.

The pia mater is the primitive and innermost layer of the three meninges. It is vascular and adheres to the surface of the spinal cord and brain, extending into the brain’s fissures, sulci, and depressions. This membrane also contains the blood vessels that feed the external surface of the cerebral cortex. The subarachnoid space, between the arachnoid and the pia mater, is filled with cerebrospinal fluid. The pia mater follows exactly the outer markings of the cerebrum and also the ependymal lining circumference of the choroid membranes and plexus. Although it also invests the cerebellum, it does not envelop it so intimately as it does the cerebrum, not dipping down into all the smaller sulci. The pia mater and the arachnoid are collectively called leptomeninges, as distinguished from dura mater or pachymeninx.

A schematic picture showing all three layers of the human meninges: the dura mater, arachnoid, and pia mater.