massa intermedia

noun

mas·​sa in·​ter·​me·​dia ˈmas-ə-ˌin-tər-ˈmē-dē-ə 

: an apparently functionless mass of gray matter in the midline of the third ventricle that is found in many but not all human brains and is formed when the surfaces of the thalami protruding inward from opposite sides of the third ventricle make contact and fuse