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The posterior commissure (PC) is a transversely-oriented commissural white matter tract that connects the two cerebral hemispheres along the midline. It is a very important anatomical landmark which is thought to play a role in the visual system, however its functions are still largely unknown.
The posterior commissure is a small fasciculus that decussates in the inferior pineal lamina, and corresponds to a white matter tract almost completely surrounded by gray matter that crosses the midline just dorsal to the cerebral aqueduct. The posterior commissure, along with the pineal gland, habenular commissure, and trigone habenulae comprise the epithalamus and form the posterior wall of the third ventricle.
There are a number of small nuclei associated with the PC, these lie rostral to the superior colliculi. Amongst others these include the nucleus of Darkschewitscz and Cajal. As they decussate, the fibers from these nuclei contribute to form the posterior commissure. Additionally it transmits fibers from the thalamic and pretectal nuclei, and from the superior colliculi.