The brain is generally described as having three coverings-the dura, arachnoid, and pia mater. Surgically speaking, the pia and arachnoid are one over the greater part of their extent, and may be treated together.
The dura mater lines the entire cavity of the cranium, and is continuous with the dura of the spine through the foramen magnum. It sends prolongations along the nerves- e.g., the optic, to fuse with the sclerotic ; the seventh and eighth nerves, to the internal ear. It is a tough fibrous membrane, which acts both as internal periosteum of the skull and as a covering for the brain. It is said to be the chief source of blood-supply to the skull, but in operations on the brain large osteoplastic flaps may be raised from the dura without untoward effect. Like all periostea, of course, the dura mater has no bone-forming function, the function of a periosteum being to supply blood to the bone, and to act as a limiting membrane to the osteoblasts.
In cases where the skull has been damaged, the dura, from its tough, unyielding character, forms, along with the scalp, an excellent protection for the underlying brain. The dura is rather loosely adherent over the vault, a fact well seen in raising osteoplastic flaps from it, but is more firmly attached over the base, and particularly over the petrous, sella turcica, and cribriform plate, fractures of these parts being generally associated with tearing of the dura. It is more adherent to the vault during infancy and old age, and frequently becomes firmly attached in pathological conditions. By means of its processes, the falx cerebri and tentorium cerebelli, it supports the brain, and limits the transmission of impulses received by injury from one part to another. It forms the great blood sinuses of the skull by splitting. The dura does not possess a true lymphatic system, and while it contains numerous lymphatic spaces, it does not readily yield a passage to purulent processes. Extradural abscess, even when large, and causing considerable tension, frequently remains external to the dura, and does not give rise to leptomeningitis. Extradural abscess is sometimes caused by extension through the bone of septic mischief from scalp wounds. The pus may not involve either pia or brain, but partial destruction of the overlying bone may occur, toxins escape, and oedema of the overlying soft parts be caused, probably some considerable time after the primary wound has healed. This condition is known as 'Pott's puffy tumour.'
While extradural hemorrhage is generally localized, it is often caused by rupture, from fracture, of the middle meningeal artery, and may then be of sufficient extent to cause death. The vessel lies in a groove, or even a tunnel, in the bone, and the anterior branch is the one most often affected. The effused blood separates the dura from the cranium, and causes pressure on the brain, while the vessel remains within its osseous canal. The artery has been torn by blows without fracture of the skull. In all such cases it is important to remember that unconsciousness often comes on gradually, the patient being able to speak intelligently and walk about for perhaps an hour or so after the accident. The dura is separated from the pia arachnoid by a slight space-the subdural space -which contains cerebro-spinal fluid.
The pia arachnoid may be considered surgically as a single flocculent membrane. The arachnoid portion does not dip down into the sulci and is smooth on its durai aspect, while it is intimately connected with the pia underneath by a fine filamentous structure whose interstices are filled with cerebrospinal fluid, forming the subarachnoid space. In most places this space is insignificant, but particularly at the base of the brain it expands greatly, forming the water-bed of the brain. This bed is of great importance surgically, as it prevents the brain, in severe injuries, from being impacted against the base of the skull. It extends forwards as far as the optic nerves, the largest spaces being the cisterna pontis, under the pons and medulla, and cisterna magna, between the roof of the fourth ventricle and under surface of the cerebellum. The frontal lobes, resting on the anterior fossa, do not possess this support, and thus are not infrequently lacerated by injury.
The pia mater is a delicate membrane which supports a rich vascular network for the supply of the brain, the whole being floated by the cerebro-spinal fluid. The pia may thus be regarded as the blood-supplying covering of the brain, just as the dura supplies the skull. The pia not only dips down into the sulci, and turns in at the transverse fissure to form the velum interposition and choroid plexuses, but also sends prolongations along the cerebral vessels into the brain (perivascular sheaths), so that it is easy to understand how a degree of encephalitis must almost certainly accompany leptomeningitis.
The pia, like the dura, is continuous with that of the cord, and similarly sends prolongations along the cranial and spinal nerves.
The continuity of cerebro-spinal fluid between the outside and inside of the brain and the cord is of importance. When the brain expands, as it does at each cardiac impulse, fluid is forced from the cranial cavity to the spinal portion, and returns on diastole, the pressure being thus kept constant. In congested conditions of the ganglia also fluid would be displaced through the foramen of Magendie and other communications, and equilibrium thus restored.
Subdural hemorrhage, unlike the extradural variety, is often very extensive, extending both above and below the arachnoid, and sometimes even to the cord. It may arise from rupture of the vessels of the pia arachnoid, or, if the dura be torn, may extend from either the middle meningeal artery or from one of the sinuses, such as the superior longitudinal, cavernous or, lateral. Even after blood has clotted in the subdural space, it may slip from its position, which perhaps was over the vertex, and cause death by pressure on the medulla.
Meningitis, or inflammation of the cerebral membranes, may affect the dura mater (pachymeningitis) or the pia arachnoid (leptomeningitis). Pachymeningitis is generally due to pyogenic organisms, and, if not accompanied by leptomeningitis, is generally localized. Leptomeningitis, when due to pyogenic organisms, is generally diffuse, often extending along the membranes of the cord, and accompanied by a degree of encephalitis. Pyogenic infection of the cerebral membranes may arise from many causes, notably middle-ear disease. Tubercle is generally conveyed by the blood-stream, and involves the pia mater, particularly at the base of the brain and along the Sylvian fissure.
A congenital protrusion of the membranes, or meningocele, occurs most frequently posteriorly, through the occiput, and less frequently at the root of the nose. When accompanied by brain matter it is called a meningo-encephalocele or en-cephalocele, and when the protruded portion is distended by ventricular fluid it is called hydrencephalocele.