Membranes which cover the brain

The term meninges comes from the Greek for "membrane" and refers to the three membranes that surround the brain and spinal cord. The membrane layers discussed in detail below from the outside in are the: dura mater, arachnoid mater, and pia mater. Their positioning around the brain can be seen in the image to the right. The brain is soft and mushy, and without structural support it would not be able to maintain its normal shape.

In fact, a brain taken out of the head and not properly suspended e. While the bone of the skull and spine provide most of the safeguarding and structural support for the central nervous system CNS , alone it isn't quite enough to fully protect the CNS. The meninges help to anchor the CNS in place to keep, for example, the brain from moving around within the skull.

They also contain cerebrospinal fluid CSF , which acts as a cushion for the brain and provides a solution in which the brain is suspended, allowing it to preserve its shape. The outermost layer of the meninges is the dura mater, which literally means "hard mother. The dura provides the brain and spinal cord with an extra protective layer, helps to keep the CNS from being jostled around by fastening it to the skull or vertebral column, and supplies a complex system of veinous drainage through which blood can leave the brain.

The arachnoid gets its name because it has the consistency and appearance of a spider web. It is much less substantial than the dura, and stretches like a cobweb between the dura and pia mater. By connecting the pia to the dura, the arachnoid helps to keep the brain in place in the skull. Between the arachnoid and the pia there is also an area known as the subarachnoid space , which is filled with CSF. The cranium protects the brain from injury and along with the bones that protect the face are called the skull.

Between the skull and brain is the meninges, which consist of three layers of tissue that cover and protect the brain and spinal cord. These membranes, however, can become infected when someone gets meningitis, and because the meninges are in direct contact with the brain, it can be dangerous. From the outermost layer inward they are the dura mater , arachnoid and pia mater. The dura mater or pachymeninx is made up of two layers of whitish membrane.

The outer layer endosteal is called the periosteum. An inner layer meningeal , lines the inside of the entire skull and creates little folds or compartments in which parts of the brain are protected and secured. Because the two layers are indistinguishable except in a few areas, it's simpler to consider the dura as one layer, which serves as both endocranium and meninx.

Instead of being considered as separating two layers, the venous sinuses are usually described as being situated within a single dura. The two special folds of the dura in the brain are called the falx and the tentorium. The falx separates the right and left half of the brain and the tentorium separates the upper and lower parts of the brain. The dura, like the scalp, is supplied by both cranial chiefly the trigeminal and cervical nerves.

The brain itself is normally insensitive, and headaches are commonly either of vascular intracranial or extracranial or dural origin.

The leptomeninges include the arachnoid and pia mater. These layers bound the subarachnoid space, which is limited externally by a water-tight layer of connective tissue, the arachnoid, and internally by a thinner layer, the pia mater. The pia mater adheres intimately to the surface of the brain and spinal cord. There is a trabecular structure of connections between the arachnoid and the pia that bridges the subarachnoid space which is otherwise full of circulating CSF.

The arachnoid is a thin and delicate membrane that surrounds the brain loosely and is separable from the dura by a potential space into which subdural hemorrhage may occur. There is a space between the dura and the arachnoid membranes that is called the subdural space. The arachnoid is made up of delicate, elastic tissue and blood vessels of varying sizes.

The arachnoid dips into the longitudinal interhemispheric fissure but not into the sulci. Near the dural venous sinuses, the arachnoid has microscopic projections, called arachnoid villi, which are believed to be concerned with the absorption of CSF. Enlargements of the villi, known as arachnoid granulations, enter some of the sinuses and their associated lateral lacunae and are visible to the naked eye.

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