Ventricular system

The ventricular system is a set of structures in the brain continuous with the central canal of the spinal cord. The system comprises four ventricles:
 * right and left lateral ventricles
 * third ventricle
 * fourth ventricle.

Each ventricle contains a choroid plexus that produces cerebrospinal fluid (CSF) used to bathe and cushion the brain and spinal cord within their bony confines.

Ventricles
There are four cerebral ventricles: the paired lateral ventricles and the third and fourth ventricle. The two lateral ventricles are relatively large and C-shaped, roughly wrapping around the dorsal aspects of the basal ganglia. It is in the lateral ventricles of the embryo that the successive generation of neurons gives rise to the 6-layered structure of the neocortex, constructed from the inside out during development. Each lateral ventricle extends into the frontal, temporal and occipital lobes via the frontal (anterior), temporal (inferior), and occipital (posterior) horns, respectively.

The lateral ventricles both communicate via the foramina of Monro with the third ventricle, found centrally within the midbrain. The third ventricle communicates via the cerebral aqueduct with the fourth ventricle, found within the hindbrain. The three foramina to the subarachnoid space are found here, permitting cerebrospinal fluid produced in the ventricles to surround the brainstem, cerebellum, and cerebral cortex. The fourth ventricle is also continuous with the central canal, allowing CSF to bathe the inside surface of the spinal cord as well.

Flow of cerebrospinal fluid
Cerebrospinal fluid is produced by modified ependymal cells of the choroid plexus within the lateral, third, and fourth ventricles. CSF flows from the lateral ventricles via the foramina of Monro into the third ventricle, and then the fourth ventricle via the cerebral aqueduct in the brainstem. From there it can pass into the central canal of the spinal cord or into the cisterns of the subarachnoid space via three small foramina: the central foramen of Magendie and the two lateral foramina of Luschka.

The fluid then flows around the superior sagittal sinus to be reabsorbed via the arachnoid villi into the venous system. CSF within the spinal cord can flow all the way down to the lumbar cistern at the end of the cord around the cauda equina where lumbar punctures are performed.

The aqueduct between the third and fourth ventricles is very small, as are the foramina, which means that they can be blocked, causing high pressure in the lateral ventricles. This is the cause of hydrocephalus--otherwise known as water on the brain--and is an extremely serious condition due to both the damage caused by the pressure as well as nature of whatever caused the block (possibly a tumour or inflammatory swelling).

Protection of the brain
The brain and spinal cord are covered by a series of tough membranes called meninges, which protect these organs from rubbing against the bones of the skull and spine. The cerebrospinal fluid within the skull and spine is found between the pia mater and the arachnoid meninges and provides further cushioning.

Role in disease
Diseases of the ventricular system include abnormal enlargement (hydrocephalus) and inflammation of the CSF spaces (meningitis, ventriculitis) caused by infection or introduction of blood following trauma or hemorrhage. Interestingly, scientific study of CAT scans of the ventricles in the late 1970s revolutionized the study of mental illness. Researchers found that patients with schizophrenia had enlarged ventricles compared to healthy subjects. This became the first "evidence" that mental illness was biological in origin and led to a reinvigoration of the study of such conditions via modern scientific techniques.

Embryology
The structures of the ventricular system are embryologically derived from the centre of the neural tube.