All parts of the brain › Interventricular foramina

Where are the interventricular foramina?

The interventricular foramina (foramina of Monro) are two short channels near the midline, deep in the brain, that connect the left and right lateral ventricles to the third ventricle. Each has the fornix in front of it and the thalamus behind it. [1]

The 3D brain needs JavaScript. The text below still says where the interventricular foramina are and what they do.

Slice

Opens cut by a front (coronal) slice through the interventricular foramina. Drag to turn, pinch or scroll to zoom, tap any part to name it; .

The 3D atlas draws the left and right foramina as one small shape across the midline, so they are lit and measured together as one part. [2]

Part of the ventricles — see the whole of it lit.

Next to it, each lit on its own page: fornix, thalamus, third ventricle, lateral ventricles.

Where it is

They lie on the underside of the lateral ventricles near the midline, and open into the third ventricle where its roof meets its front surface. [1]
In front of each foramen is the fornix, and behind it is the thalamus. [1]
The columns of the fornix arch down in front of them and behind the anterior commissure. [3]
The lamina terminalis, the thin front wall of the third ventricle, starts at them and runs down to the base of the optic stalk. [4]

What it does

They let the cerebrospinal fluid made in the lateral ventricles reach the third ventricle and then the rest of the ventricular system. [1]
Most cerebrospinal fluid is made in the two lateral ventricles; from there it passes through the interventricular foramina into the third ventricle, then through the cerebral aqueduct to the fourth ventricle. [5]
Their walls contain choroid plexus, the tissue that makes the fluid, continuous with the plexus of the lateral ventricles above and the third ventricle below. [1]

Connections

End branches of the medial posterior choroidal arteries, and the superior thalamostriate, superior choroid and septal veins, also pass through them. [1]
In development the lateral ventricles grow as outpouchings of the third ventricle and stay joined to it throughout; the foramina take shape slowly, moving forward and outward as the fornix grows. [1]

Inside it

Not tissue but openings: each is a hole (Latin foramen, plural foramina) through which cerebrospinal fluid flows. [1]

Other names

Also called the foramina of Monro, after the Scottish physician Alexander Monro, who described one in 1764 and again in his 1783 Observations on the Structure and Functions of the Nervous System. The eponym is disputed: authors back to Galen had noted that the ventricles connect. [1]

Size

Each is normally crescent-shaped, and becomes rounder and larger with the size of the lateral ventricles. [1]
Monro described one as “an oval hole, large enough to admit a goose quill”. [1]
On this model the one shape the atlas draws for both interventricular foramina measures about 5 mm front to back, 11 mm top to bottom and 13 mm side to side (the box it fits in, not its volume). [2]
Centre of the interventricular foramina compared with the centre of the left fornix: 14 mm further forward, 9 mm higher, 12 mm closer to the midline; 20 mm apart in a straight line. [2]
Centre of the interventricular foramina compared with the centre of the left thalamus: 18 mm further forward, 8 mm higher, 12 mm closer to the midline; 23 mm apart in a straight line. [2]

Sources

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.