Located around the posterior part of the third ventricle, it is the phase-shifting part of thalamus and midbrain anterior parietal region, including pineal gland, habenular commissure and posterior commissure.

① Pineal gland: located between two superior colliculus, about 1cm long, pyramidal, with its base attached to the junction of habenular nucleus; ② Junction of habenular nucleus: located between two superior colliculus, in front of pineal gland, composed of transverse fiber bundles; ③ Posterior commissure: Located below the pineal gland, it also consists of transverse fiber bundles.

Pineal gland tumors are common with lesions of the upper thalamus, and Parinaud syndrome caused by tumor compression of the midbrain tetrad can occur, which is manifested as follows: ① the reflection of the pupil to light disappears (the superior colliculus is damaged); ② Dyskinesia of eyeball in the same vertical direction, especially upward gaze paralysis (damage to superior colliculus); ③ Neurodeafness (inferior colliculus damage); ④ Cerebellar ataxia (injury of commissural arm). Symptoms are mostly bilateral.

The subthalamic nucleus is the transitional area between diencephalon and midbrain, located between thalamus and the lower part of internal capsule.

The subthalamic nucleus is close to the inside of the internal capsule and has a round-trip fiber connection with the globus pallidus. This kind of fiber bundle passes through the internal capsule and is called subthalamic bundle.

Subthalamic floor is an important structure of extrapyramidal system, and its main function is to inhibit globus pallidus. Unilateral lesions can cause eccentric throwing movement, which is manifested as continuous uncontrollable throwing movement.