The term Telencephalon medium refers to a composite substructure of the forebrain that includes parts of the cerebrum and of the diencephalon. It lies rostral to an imaginary plane from the base of the interventricular foramen to the upper border of the optic chiasm. It includes the preoptic area, the anterior commissure, the lamina terminalis and rostral portions of the third ventricle and fornix ( Crosby-1962 ).