Meninges and Cerebrospinal Fluid

Meninges and Cerebrospinal Fluid
 Both the brain and spinal cord are wrapped in three pro­tective membranes known as meninges (me-nin'jez). The outer meningeal layer, called the dura mater, is tough, white, fibrous connective tissue that lies next to the skull and vertebrae. In some areas, it splits into two layers, form­ing channels (called dural sinuses) that collect venous blood before it is returned to the circulatory system. Bleed­ing due to a head injury is called an epidural hematoma. When there is blood between the dura mater and bone, the injury is called a subdural hematoma. The middle meningeal layer is the arachnoid membrane, a weblike connective tissue covering with thin strands that attach to the pia mater, the inner and third meningeal layer. Be­neath the arachnoid membrane is the subarachnoid space, which is filled with cerebrospinal fluid. The pia mater is very thin and closely follows the contours of the brain and spinal cord.
Cerebrospinal Fluid 
Cerebrospinal fluid (CSF) is a clear tissue fluid that forms a protective cushion around and within the CNS. It also supplies the CNS with nutrients that have been filtered from the blood and collects wastes that are returned to the blood. CSF is a circulating fluid. CSF is produced by and filters out of specialized masses of capillaries called choroid plexuses into the lateral ventricles. Ependymal cells cover the choroid plexuses and aid in the production of CSF. CSF moves from the lateral ventricles into the third ventricle and then flows through the cerebral aqueduct into the fourth ventricle. From the fourth ventricle, it moves into the central canal of the spinal cord and then out into the subarachnoid spaces. After that, it returns to the veins of the brain.
Normally, the CSF within the brain maintains a con­stant pressure because it steadily drains into the circulatory system by way of the dural sinuses. However, if there is a blockage in an infant whose cranial sutures have not yet closed, the brain can enlarge due to fluid accumulation. This condition is called hydrocephalus, or "water on the brain." If fluid collects in an adult, the brain cannot en­large, and instead, it is pushed against the skull possibly causing injury.
The CNS is protected by the meninges and the cerebrospinal fluid.