The tuber cinereum is the portion of hypothalamus[1] forming the floor of the third ventricle situated between the optic chiasm, and the mammillary bodies.

[1] The tuber cinereum is a convex mass of grey matter,[3]: 495  a ventral/inferior distention of the hypothalamus forming the floor of the third ventricle.

[3]: 1464.e62 Capillaries of the tuber cinereum are specialized and confluent to enable rapid communication via brain- or blood-borne factors between compartments of the tuber, a capillary system described as the hypophyseal portal system.

The subregion of the hypothalamic arcuate nucleus closest to the median eminence contains specialized permeable capillaries surrounded by wide pericapillary spaces, enabling moment-to-moment sensing of circulating blood.

[5][non-primary source needed] This article incorporates text in the public domain from page 813 of the 20th edition of Gray's Anatomy (1918)