The third ventricle (ventriculus tertius) is one of four connected fluid-filled cavities comprising the ventricular system within the human brain. It is a median cleft between the two thalami, and is filled with cerebrospinal fluid (CSF).
It is in the midline, between the left and right lateral ventricles.
It communicates with the lateral ventricles anteriorly by the interventricular foramina (of Monro).
It communicates with the fourth ventricle posteriorly by the cerebral aqueduct (of Sylvius).
The third ventricle, similarly to other parts of the ventricular system of the brain, develop from the central canal of the neural tube. Specifically, the third ventricle originates from the portion of the tube that is present in the developing prosencephalon, and subsequently in the developing encephalopathy.
It is bounded by the thalamus and hypothalamus on both the left and right sides. The lamina terminalis forms the anterior wall of the third ventricle.
There are two protrusions on the front of the third ventricle:
the supra-optic recess (above the optic chiasma)
the infundibular recess (above the pituitary stalk).
In casts of the ventricular system, a small 'hole' may be seen in the body of the third ventricle. This is formed where the two thalami are joined together at the interthalamic adhesion (not seen in all people).