The third ventricle is one of the four CSF-filled cavities that together comprise the ventricular system.
The third ventricle is a median cleft between the two thalami, which make up the superior aspect of the lateral walls. The thalami are separated from the hypothalamus (anteriorly) and subthalamus (posteriorly) by the hypothalamic sulcus.
The roof is formed by the choroidal fissure and the bodies of the fornix.
There are two small anterior recesses and two small posterior recesses:
The third ventricle contains choroid plexus along its roof along the tela choroidea continuous with the choroid plexus from the lateral ventricles.
It communicates with the:
- lateral ventricles via the foramina of Monro (interventricular foramina) lying just posterior to the columns of the fornix
- fourth ventricle via the aqueduct of Sylvius (cerebral aqueduct) just below the posterior commissure
In some patients, the two thalami meet forming the interthalamic adhesion and when casts are made of the ventricular system, a hole is seen through the third ventricle.
The infundibular recess can vary in its invagination into the infundibulum, most commonly extending the entire length, less commonly 50% or <50% of the length of the infundibular stalk.