Overview

The third ventricle is a vertical, slit-like midline cavity a few millimetres wide. Its boundaries are:
• lateral walls: the thalamus above and the hypothalamus below, separated by the hypothalamic sulcus. The interthalamic adhesion often bridges the cavity.
• anterior wall: the lamina terminalis (the closed rostral end of the neural tube) and the anterior commissure, with the interventricular foramina opening at its anterosuperior corners.
• roof: the tela choroidea carrying two thin strips of choroid plexus, with the fornix and internal cerebral veins above.
• floor: from front to back, the optic chiasm, the infundibulum and tuber cinereum, the mammillary bodies and the posterior perforated substance.
• posterior wall: the pineal gland and the posterior and habenular commissures, with the opening of the cerebral aqueduct below.

The cavity extends into four recesses: the optic and infundibular recesses in front and below, and the pineal and suprapineal recesses behind.

Location: Midline diencephalon, between the right and left thalami and hypothalami.

Parts & subdivisions

Lateral walls
Thalamus (upper) and hypothalamus (lower); interthalamic adhesion.
Floor
Optic chiasm, infundibulum (pituitary stalk), tuber cinereum, mammillary bodies.
Roof
Tela choroidea with choroid plexus; fornix above.
Recesses
Optic, infundibular, pineal and suprapineal recesses.

Function

Conducts CSF from the lateral ventricles to the cerebral aqueduct
Produces some CSF (roof choroid plexus)
Its walls house hypothalamic nuclei that sense CSF and blood composition (circumventricular organs at the lamina terminalis)

Clinical significance

Colloid cyst

A benign cyst in the anterior roof of the third ventricle at the foramina of Monro can intermittently block both foramina. The result is acute obstructive hydrocephalus with positional headache, drop attacks and, rarely, sudden death.

Endoscopic third ventriculostomy

For obstructive hydrocephalus distal to the third ventricle (e.g. aqueductal stenosis), an endoscope passed through the foramen of Monro perforates the thin floor between the mammillary bodies and the infundibular recess. This lets CSF bypass the block into the interpeduncular and prepontine cisterns.

Test yourself

CSF passes from the third ventricle to the fourth ventricle through the:

Interventricular foramen (of Monro)
Central canal
Median aperture (of Magendie)
Cerebral aqueduct (of Sylvius)

Answer: Cerebral aqueduct (of Sylvius)

The pathway is lateral ventricles → interventricular foramina → third ventricle → cerebral aqueduct (in the midbrain) → fourth ventricle → median and lateral apertures → subarachnoid space. The aqueduct is the narrowest point and the commonest site of congenital obstruction.