It is a slit-like cavity formed in the diencephalon between the two thalami, in the midline between the right and left lateral ventricles, and is filled with cerebrospinal fluid (CSF).

The third ventricle is a narrow, laterally flattened, vaguely rectangular region, filled with cerebrospinal fluid, and lined by ependyma.

The superior part of the posterior border constitutes the habenular commissure, while more centrally it the pineal gland, which regulates sleep and reacts to light levels.

[2] The telencephalon gradually expands laterally to a much greater extent than it does dorsally or ventrally, and its connection to the remainder of the neural tube reduces to the interventricula foramina.

The optic recess is noticeable by the end of the 6th week, by which time a bend is distinguishable in the dorsal portion of the ventricle border.

Rostral of the bend, the medial dorsal portion of the ventrical begins to flatten, and become secretory (i.e. choroid plexus), forming the roof of the ventricle.

The floor of the third ventricle is formed by hypothalamic structures and this can be opened surgically between the mamillary bodies and the pituitary gland in a procedure called an endoscopic third ventriculostomy.

[3] These observations are interpreted as indicating a loss of neural tissue in brain regions adjacent to the enlarged ventricle, leading to suggestions that cytokines and related mediators of neurodegeneration may play a role in giving rise to the disease.