B1.26.0 – The Ventricular System

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*As with all anatomy I suggest you search for each new term on google then click on the ‘Images’ tab at the top of the page.  Look at as many different pictures of each structure, from as many different angles as you can. Then look at it on the Visible Body. This will help you get a 3 dimensional image of the structure in your head.

The Ventricular System is a system of cavities and canals deep within the brain and spinal cord. They have a thin membranous lining called the Ependyma.  The whole Ventricular System is filled with Cerebrospinal Fluid. The Ventricular System consists of four ventricles connected by various communicating channels.

These are:

❍ Two lateral Ventricles (1st and 2nd Ventricles) located within the two cerebral hemispheres, each of which communicates via an inter-ventricular foramen to

❍ The third Ventricle located between the two Thalami of the brain.
The Third ventricle communicates inferiorly through the cerebral aqueduct (aqueduct of sylvius) to

❍ The fourth Ventricle located between the Cerebellum (posteriorly) and the Pons and Medulla (anteriorly).  The fourth Ventricle is continuous inferiorly with the central canal passing down the centre of the Spinal Cord.

In the roof of each of the four ventricles are located Choroid Plexi.
These are filter like structures through which Cerebrospinal Fluid is formed as a filtrate from arterial blood.  Arterial blood enters the Choroid Plexi from the cerebral arteries; then blood cells, proteins and other large particles are filtered out (remaining in the blood).  The pure colourless fluid that filters through this Choroid Plexi into the Ventricular System is Cerebrospinal Fluid.

HOW DOES CEREBROSPINAL FLUID GET OUT OF THE VENTRICULAR SYSTEM?

In the posterior and lateral walls of the fourth ventricle are three foramina –

The Foramen of Magendie (medial aperture), posteriorly, and
Two Foramina of Luschka (lateral apertures), bilaterally.

Cerebrospinal Fluid flows throughout the Ventricular System. It passes out through the Foramina of Magendie and Luschka into the sub-arachnoid space where it circulates throughout the Sub-arachnoid space around the Brain and Spinal Cord.

Cerebrospinal Fluid also seeps through the walls of the ventricles into the nerve tissue of the Brain and Spinal Cord.  From the sub-arachnoid space it seeps through the Pia Mater into the tissues of the Brain and Spinal Cord. Cerebrospinal Fluid also seeps out with the peripheral nerves of the spinal cord as they leave the Central Nervous System and travel out to the periphery.

REABSORBTION
Cerebrospinal Fluid is eventually returned to the blood via the Arachnoid Villi which protrude from the sub-arachnoid space through to the Superior Sagittal Sinus of the Brain. It re-joins the Venous blood which then drains from the Venous Sinuses via the Internal Jugular Vein to be returned to the heart.

So the direction of flow is,

• Arterial blood is pumped into the Choroid Plexi in the roofs of the Ventricles where it is filtered into cerebrospinal fluid.
• The lateral ventricles drain into the Third Ventricle via Inter-ventricular foramina.
• The third ventricle drains into the Aqueduct of Sylvius to the Forth Ventricle.
• It is in the Fourth Ventricle that the cerebrospinal fluid leaves the ventricles and enters the sub arachnoid space via the foramina of Luschka and Magendie. (It also travels down the central canal of the spinal cord.)
• It travels throughout the sub arachnoid space.
• Some of it seeps out with the peripheral nerves and is reabsorbed as an extracellular fluid.
• The bulk of it is reabsorbed by the arachnoid granulations of the arachnoid villi. These transform it into Venous blood as they deposit it into the Venous sinuses, particularly the superior sagittal sinus.

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