The ventricles are four connected, fluid-filled cavities deep inside the brain: the two lateral ventricles, one in each cerebral hemisphere; the third ventricle, a thin slit on the midline between the left and right thalamus; and the fourth ventricle, between the pons and medulla in front and the cerebellum behind. [1][2]
All four are lit on the 3D brain. The cerebral aqueduct, the 1–2 mm channel that joins the third ventricle to the fourth through the midbrain, is not a separate shape in this model yet, so the gap between the two lit midline spaces is where it runs. [3][4]
The 3D brain needs JavaScript. The text below still says where the ventricles is and what it does.
Slice
They are all lit together; the side (sagittal) slice button cuts down the midline through them. Drag to turn, pinch or scroll to zoom, tap any part to name it; .
The two lateral ventricles are the largest. Each sits inside one cerebral hemisphere and is C-shaped, curving from the frontal lobe back over the thalamus and then down and forward into the temporal lobe. [5][1]
The third ventricle is a narrow slit on the midline in the diencephalon, between the left and right thalamus, with the hypothalamus forming its lower walls and floor. [6][2]
The fourth ventricle lies lower and further back, in the hindbrain: its floor is the back surface of the pons and upper medulla, and the cerebellum sits behind it. [2][7]
All of them develop from the hollow centre of the embryo’s neural tube, which is why they form one continuous system. [2][1]
What it does
They are filled with cerebrospinal fluid (CSF), made by the choroid plexus, a tissue found in all four ventricles; most of the fluid is produced in the two lateral ventricles. [8][9]
An adult has about 125–150 mL of CSF at any one time and makes roughly 500 mL a day (about 20 mL an hour), so the whole volume is replaced three to four times a day as it is reabsorbed into the blood. [9]
The fluid cushions the brain and lets it float: a brain with a mass of about 1,400–1,500 g has a net weight in CSF equivalent to only 25–50 g. [9]
The ventricles and the central canal of the spinal cord are lined with ependyma, a specialised layer of cells. [1]
Connections
Each lateral ventricle opens into the third ventricle through an interventricular foramen (foramen of Monro), a crescent-shaped hole near the midline with the fornix in front of it and the thalamus behind. [10][2]
The third ventricle drains backwards and down into the cerebral aqueduct (aqueduct of Sylvius), a tube about 15 mm long and 1–2 mm across that runs through the midbrain to the fourth ventricle. [3][2]
From the fourth ventricle the fluid leaves through three small openings, one median and two lateral apertures, into the subarachnoid space around the brain and spinal cord; at its lower tip the fourth ventricle also narrows into the central canal of the spinal cord. [7][2]
In the subarachnoid space, between the arachnoid mater and the pia mater, the fluid flows over the outside of the brain and spinal cord and is absorbed into the venous blood through the arachnoid granulations. [1][9]
Inside it
Choroid plexus sits in the body, atrium and inferior horn of each lateral ventricle and in the roofs of the third and fourth ventricles; there is none in the front and back horns of the lateral ventricles or in the cerebral aqueduct. [8][1]
Other names
Also called the ventricular system or the cerebral ventricles. The two lateral ventricles were at one time called the first and second ventricles, which is why the other two are the third and the fourth. [1][2]
The interventricular foramina are named after the Scottish physician Alexander Monro and the cerebral aqueduct after Franciscus Sylvius. [10][3]
Size
On this model the ventricles together fit in a box about 109 mm front to back, 88 mm top to bottom and 78 mm side to side (the box they fit in, not their volume). [4]
Centre of the third ventricle compared with the centre of the fourth ventricle: 25 mm further forward, 43 mm higher; 50 mm apart in a straight line. [4]
Centre of the left lateral ventricle compared with the centre of the third ventricle: 8 mm further back, 7 mm higher, 17 mm further from the midline; 20 mm apart in a straight line. [4]
Facts describe anatomy and normal function only. The 3D model is the adult male BodyParts3D brain (credits and licence); real brains vary in size and shape, so its measurements describe one model, not anyone’s brain.