Optic chiasma (chiasma opticum) Pars Optica Hypothalami Tuber cinereum Infundibulum and Hypophysis.
Pars Mammillaris Hypothalami{ Corpora mammillaria.
The lamina cinerea terminalis (Fig. 33) is the most superior of the median structures. It is a thin lamina of ash-colored (cinereum) gray matter closing the end of the neural tube. It extends from the anterior superior surface of the optic chiasma upward and backward to the anterior commissure, just in front of which it becomes continuous with the lamina rostralis of the corpus callosum. Laterally, it is continuous with the cortex of the cerebral hemisphere. Behind it is the third ventricle; in front of it, a part of the longitudinal fissure of the cerebrum.
Optic Chiasma (Chiasma opticum).—The optic chiasma is a quadrilateral sheet of nerve fibers whose anterior angles receive the optic nerves and whose posterior angles give off the optic tracts (Fig. 21). With the nerves and tracts attached, it is x-shaped. The chiasma is a median structure and is situated beneath the lamina cinerea, in the optic groove of the sphenoid bone. The fibers of the optic nerves and tracts compose it. There are three sets of these fibers, namely, the intercerebral, the direct, and the decussating. A fourth group of fibers, called the interretinal and said to be commissural for the retinw, has been hitherto described, but their existence is very doubtful. The intercerebral fibers are not found in the optic nerves, but form the inferior commissure (Guddeni) which joins together the medial geniculate bodies (Fig. 55). The direct (or temporal) and decussating (or nasal) fibers run through nerve and tract and join the retina with the brain on the same and the opposite sides, respectively. In most vertebrates below mammals, and in the mouse and guinea-pig, it is said that the optic fibers all decussate in the chiasma. Normally in man and the higher mammals, the temporal half of each retina contributes to the tract direct fibers and the nasal half crossed fibers (Fig. 67). The optic nerves (nervi optici) extend from the foramen sclerm of each eyeball back to the front of the chiasma, through the optic foramina; they rise in the ganglionar cells of the which are connected with the rods and cones by the bipolar neurones. The optic tracts (tractus optici) connect the chiasma with the brain. Each tract winds outward and backward
around the cerebral peduncle, and divides into a medial and a lateral root (Fig. 55). The roots wind under the thalamus and disappear at the corresponding geniculate body. The lateral root contains all the retinal fibers, the medial root has nothing to do with vision. The fibers of the lateral root (radix lateralis) may be traced to the lateral geniculate body (8o per cent. Von Monokow), to the pulvinar of the thalamus (nearly all the 20 per cent. remaining), and the rest to the superior quadrigeminal colliculus. The optic radiation of the capsule connects these centers with the medial occipital cortex. Like other sensory nerves, the optic sends a few fibers to the cerebellum, which are concerned with coordinated movements. The medial root rises and ends in the medial geniculate body and thalamus. Its fibers form the commissura inferior (Guddeni).
Tuber posterior border of the optic chiasma is contiguous to the tuber cinereum (Figs. 21 and 31). Here the gray matter is thickened and centrally prominent. The bulbous infundibulum projects downward from it to rest in the sella turcica, where it forms the posterior lobe of the hypophysis. The upper end of the infundibulum is hollow (funnel-like). Its cavity forms the lowest part of the third ventricle. In man the bulb of the infundibulum is solid at maturity, though hollow in the embryo. It is composed largely of fibrous tissue, notwith standing the fact that it is developed from the floor of the telen cephalon. From the base (superior end) of the infundibulum, the tuber cinereum extends in continuity with the anterior per forated substance on each side of it; and behind the corpora mammillaria mark the boundary between it and the posterior perforated substance of the mid-brain. In its antero-lateral part, near the optic tract, the tuber cinereum contains the supra-optic nucleus of Cajal, comprising an anterior, a posterior and a dorsal group of cells, which some consider to be the source of the intercerebral fibers of the optic chiasma. The tuber cinereum sometimes presents a second projection, behind the infundibulum, called the eminentia saccularis; this is believed to represent the saccus vasculosis of fishes.