n. The parts of the brain about the third ventricle developed from the hinder part of the first primary cerebral vesicle, including the thalami, the optic tracts and chiasma, the infundibulum and cerebral part of the pituitary body, the corpora albicantia, the conarium, the ependymal part of the velum interpositum, a lamina cinerea, and other structures. Also called diencephalon, interbrain, 'tween-brain. See cuts under Elasmobranchii, encephalon, Rana, Petromyzontidæ, and cerebral.