Third ventricle

At a glance

English

Third ventricle

Latin (TA2)

Ventriculus tertius

System

Brain and spinal cord

Region

Head

Side

Single, on the midline

About the third ventricle

The third ventricle is a thin, slit-like space in the middle of the forebrain. It sits in the median plane between the right and left halves of the diencephalon, below the bodies of the two lateral ventricles, and like the rest of the ventricular system it is lined by ependyma and holds cerebrospinal fluid (CSF). Seen from the side it is roughly four-sided, tall from roof to floor but only a few millimetres wide. In many brains a bridge of grey matter, the interthalamic adhesion, crosses the cavity and joins the two thalami; it can be missing or doubled, and it is not clear that it carries any real fibre connection.

Fluid enters at the front and upper corner through the paired interventricular foramina (of Monro), which open from the lateral ventricles, and leaves at the back and lower corner through the cerebral aqueduct, which runs down through the midbrain to the fourth ventricle. The side walls are thalamus above and hypothalamus below, with the hypothalamic sulcus running between the foramen and the aqueduct to mark the boundary. The roof is a thin layer of ependyma and pia, the tela choroidea, with choroid plexus hanging into the cavity; the fornix lies just above it.

The front wall is the lamina terminalis, the original front end of the neural tube, with the anterior commissure in its upper part and the vascular organ that senses blood osmolality. Below it the floor carries, from front to back, the optic chiasm, the tuber cinereum and infundibulum leading to the posterior pituitary, the mammillary bodies, and finally midbrain tegmentum. The back wall holds the habenular commissure, the stalk of the pineal gland and the posterior commissure. These bulges and gaps create small recesses: optic, infundibular, pineal and suprapineal.

Because the floor is so thin between the mammillary bodies and the infundibulum, a neurosurgeon can pass an endoscope through the lateral ventricle and foramen and make a hole there. This endoscopic third ventriculostomy lets CSF bypass a blocked aqueduct in obstructive hydrocephalus. A narrowed aqueduct or a mass in the posterior third ventricle, such as a pineal tumour, dilates both the third and lateral ventricles. A colloid cyst near the foramina can block outflow suddenly.

Function and supply

Function

Passes CSF from the lateral ventricles to the cerebral aqueduct; its choroid plexus adds fluid, and its walls carry the hypothalamus and circumventricular organs that monitor the blood.

Blood supply

Its choroid plexus is supplied by the medial posterior choroidal arteries from the posterior cerebral artery and drained by the internal cerebral veins.