The Optic Chiasma (chiasma opticum), somewhat quadrilateral in form, rests upon the tuberculum sellæ and on the anterior part of the diaphragma sellæ.
It is in relation, above,with the lamina terminalis; behind, with the tuber cinereum; on either side, with the anterior perforated substance.
Within the chiasma, the optic nerves undergo a partial decussation. The fibers forming the medial part of each tract and posterior part of the chiasma have no connection with the optic nerves. They simply cross in the chiasma, and connect the medial geniculate bodies of the two sides; they form the commissure of Gudden. The remaining and principal part of the chiasma consists of two sets of fibers, crossed and uncrossed. The crossed fibers which are the more numerous, occupy the central part of the chiasma, and pass from the optic nerve of one side to the optic tract of the other, decussating in the chiasma with similar fibers of the opposite optic nerve. The uncrossed fibers occupy the lateral part of the chiasma, and pass from the nerve of one side into the tract of the same side.
The crossed fibers of the optic nerve tend to occupy the medial side of the nerve and the uncrossed fibers the lateral side. In the optic tract, however, the fibers are much more intermingled.