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Ventral part of the diencephalon extending from the region of the optic chiasm to the caudal border of the mammillary bodies and forming the inferior and lateral walls of the third ventricle.

Subthalamic region, divided into the pars mamillaris, comprising the corpue mamillare, and the pars optica, comprising the tuber cinereum, infundibulum, hypophysis (pituitary body), and the lamina cinerea.

The subthalamic tegmental region and the structures forming the greater part of the floor of the third ventricle.