Fluid space

Third ventricle

A thin, slit-like fluid chamber squeezed between the two halves of the thalamus.

The third ventricle is a narrow, fluid-filled slit right in the middle of the brain, sandwiched between the left and right thalamus. It collects fluid from both lateral ventricles and passes it down through the cerebral aqueduct. Its walls and floor are lined with important control centers, including the hypothalamus. Because it is so central and so narrow, a small blockage here can dam up fluid for the whole brain above it.

Where it is

In the very middle of the brain, between the two halves of the thalamus and above the hypothalamus. In most people, the two halves of the thalamus touch across it at a small bridge called the interthalamic adhesion.

What it does

Passing fluid on

It receives fluid from both lateral ventricles and sends it down into the cerebral aqueduct.

Making fluid

A strip of choroid plexus in its roof adds more cerebrospinal fluid.

Sensing the blood

Special spots in its walls have a leaky barrier on purpose, letting the brain sample the blood for things like its salt level. This helps control thirst.

A landmark

Small pockets in its outline reach into the stalks of the pituitary and pineal glands, so its shape marks where these glands are.

Think of it as a narrow hallway in the middle of a house, where rooms on both sides open onto it and a single staircase leads down.

Works with

ThalamusThe two halves of the thalamus form its side walls.
HypothalamusThe hypothalamus forms its floor and lower walls, and senses signals in the nearby blood.
Lateral ventriclesIt receives fluid from both lateral ventricles through small openings at its front.
Cerebral aqueductIts fluid drains out the back through the narrow cerebral aqueduct.
Pineal glandThe pineal gland sits just behind it, and a small pocket in its back wall reaches into the pineal stalk.
Pituitary glandA small pocket in its floor dips down into the stalk of the pituitary gland.

When it goes wrong

Colloid cyst

A rare, usually harmless cyst can grow at the front of the third ventricle and block fluid coming from the lateral ventricles. It can cause headaches and, rarely, a sudden dangerous rise in pressure, so a cyst that causes symptoms is treated, often by removing it.

A bypass for blockages

In a surgery called endoscopic third ventriculostomy, doctors make a tiny hole in its floor so fluid can flow around a blockage further down.

For learning only, not medical advice. If you have a health concern, talk to a doctor.

Surprising facts

In healthy young adults it is only a few millimeters wide, more like a slot than a room.
A minority of people have no bridge between the two halves of the thalamus across this chamber, and it is not clear that this makes any difference.

Where the name comes from

Named in order: the two lateral ventricles were historically counted as the first and second, so this one became the third.

Quick quiz

3 questions. Your answers are saved and come back in your daily review.

Which of these is a job of the third ventricle?
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Passing fluid on. It receives fluid from both lateral ventricles and sends it down into the cerebral aqueduct.
The third ventricle is part of which larger structure?
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Ventricular system. It belongs to the ventricular system.
Which part fits this description? "Like a narrow hallway in the middle of a house, where rooms on both sides open onto it and a single staircase leads down."
Show the answer
Third ventricle. It is the third ventricle. A thin, slit-like fluid chamber squeezed between the two halves of the thalamus.

Sources

Neuroanatomy, Ventricular System, StatPearls, NCBI Bookshelf
Third ventricle, Radiopaedia
Colloid Brain Cyst, StatPearls, NCBI Bookshelf

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