The system of blood supply of the optic chiasma was studied on 85 human fresh bodies by several methods. The optic chiasma is supplied by the Willis' circle, arterioles from which, after forming anastomosis at two levels, enter the chiasma and break into capillaries. The authors found that the median portion of chiasma is a weak point of microcirculation, as demonstrated by rarefied and transverse capillaries. When the blood supply of chiasma is compromised by lesions near the sella turcica, this weak point of microcirculation is apt to be disturbed first and becomes ischemic, causing disorders of the crossing optic nerve fibers in the median portion of chiasma, and the characteristic bitemporal visual field defects appear.