noun
: a small body arising from the roof of the third ventricle and enclosed by the pia mater in all craniate vertebrates that in man and most existing vertebrates is conical, reddish gray, and suggestive of a gland or in larval lampreys and a few reptiles is raised on a stalk toward the upper surface of the head and has the essential structure of an eye, that functions in some birds as part of a time-measuring system, and that is variously postulated to be a vestigial third eye, an endocrine organ, or the seat of the soul — called also pineal gland, pineal organ ; see brain illustration