Posterior commissure

The posterior commissure lies in the inferior wall of the base of the pineal stalk. It contains efferent neurons of the pretectal nucleus which cross the midline to synapse with the parasympathetic accessory nucleus of oculomotor nerve–a.k.a. Edingar-Westphal nucleus (EWN)–on the opposite side.

Anatomy of the posterior commissure can be best understood in context of knowing some details about the pretectal nucleus and the pineal stalk:

Pretectal nucleus is situated in the mid-brain. It receives afferents from the optic tract and conveys efferent neural signals to the EWN of the contralateral side, via the posterior commissure. EWN is a parasympathetic nucleus and helps in constricting the pupil (a.k.a. direct light reflex). The posterior commissure helps to bring about constriction of pupil of the opposite side eye as well (a.k.a. consensual light reflex).

The pineal gland, on the other hand, is a small structure that drops backwards and downwards from the posterior wall of third ventricle by a pineal stalk. The pulling force of the pineal gland sucks in part of the third ventricle into the base of the stalk. This gives rise to a pineal recess that is sandwiched between superior and inferior walls of the stalk. The posterior commissure comes to lie in the inferior wall of the base of the pineal stalk.