to the CSF and this fluid passes via the cerebral aqueduct to the 4 th ventricle. The choroid plexus of the 4 th ventricle also contributes to the total CSF volume. The CSF then passes into the subarachnoid space at the lateral foramina and the medial foramen. The CSF enters the cerebellomedullary cistern, a large fluid space that lies dorsal to the medulla and caudal to the cerebellum. Having thus entered the subarachnoid space around the medulla oblongata, the CSF circulates throughout the subarachnoid space to cover the entire surface of the brain and spinal cord. It circulates dorsally and then ventrally around the spinal cord. A lumbar cistern is an enlarged subarachnoid space located caudal to the termination of the spinal cord in the vicinity of the cauda equina. It then passes to the base of the brain and finally over the dorsal surface of the cerebral hemispheres. The flow of CSF is thought to be due to the pulsation of the blood in the choroid plexuses. With each pulsation, the CSF pressure rises and surges toward the lateral apertures. The cilia on the ependymal cells may contribute to the flow.
Absorption of CSF
There are no lymphatics in the tissues of the brain and spinal cord for drainage of the CSF. However, it has been shown that the speed of removal is rapid. A dye injected into the CSF appears in the venous blood within 30 seconds. In most animals the major mode of CSF absorption is by the action of the arachnoid villi, which are arachnoidal invaginations into the venous sinuses of the dura mater. Thus, CSF is formed and absorbed throughout the ventricles and subarachnoid space. It is in constant motion and progresses generally toward the surface of the cerebral hemispheres and along the spinal cord. With the rate of absorption being independent of intracranial pressure, absorption is the primary homeostatic mechanism for maintenance of the intracranial pressure.
Functions of the CSF
CSF surrounds and permeates the entire central nervous system. Consequently, the brain and spinal cord are suspended and buoyed by CSF which physically protects them by acting as a cushioning agent and shock absorber . The CSF helps modulate pressure changes that occur within the cranial cavity. In conjunction with the cerebral blood flow, it helps regulate the intracranial pressure. Since the cranium is a bony, non-distenable structure, an increase in blood volume can only occur if another substance within the cranial cavity is proportionally reduced in volume. Reduction in CSF permits a reciprocal increase of blood volume and vice-versa.
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