(_δ_) The centre for visual impressions is now the best known of any of the sensory cortical areas. The experimental studies and pathological results of the last few years have indicated that the occipital lobe was probably the seat of higher, organized vision (for form and color). More recent autopsies and re-examination[10] of the subject point to the cuneus and adjacent gray matter as the visual centre. The anatomical arrangement is, however, peculiar and complex, in that each cortical visual area receives impressions from one lateral half of both retinæ, through the fasciculus opticus. [Footnote 10: E. C. Seguin, “A Contribution to the Pathology of Hemianopsia of Central Origin (Cortex-hemianopsia),” _Journal of Nervous and Mental Diseases_, 1886, No. 1.] Destructive lesion of one visual centre is therefore indicated during {84} life by the symptom lateral hemianopsia alone (the dark half-fields on the side opposite the lesion). The accompanying diagram illustrates the course of the visual paths from the eyes to the cortical centres, and the mechanism of production of various forms of hemianopsia: [Illustration: FIG. 8. Diagram of Visual Paths, designed to illustrate specially Left Lateral Hemianopsia from any lesion. _L. T. F._, left temporal half-field; _R. N. F._, right nasal half-field; _O. S._, oculus sin.; _O. D._, oculus dexter; _N. T._, nasal and temporal halves of retinæ; _N. O. S._, nervus opticus sin.; _N. O. D._, nervus opticus dext.; _F. C. S._, fasciculus cruciatus sin.; _F. L. D._, fasciculus lateralis dext.; _C._, chiasma, or decussation of fasciculi cruciati; _T. O. D._, tractus opticus dext.; _C. G. L._, corpus geniculatum laterale; _L. O._, lobi optici (corpus quad.); _P. O. C._, primary optic centres, including lobus opticus, corp. genic. lat., and pulvinar of one side; _F. O._, fasciculus opticus (Gratiolet) in the internal capsule; _C. P._, cornu posterior; _G. A._, region of gyrus angularis; _L. O. S._, lobus occip. sin.; _L. O. D._, lobus occip. dext.; _Cu._, cuneus and subjacent gyri, constituting the cortical visual centre in man. The heavy or shaded lines represent parts connected with the right halves of both retinæ. The reader may place the lesion as he pleases.] {85} The following diagnostic propositions are applicable to cases presenting the symptom lateral hemianopsia: “1. Lateral hemianopsia always indicates an intracranial lesion on the opposite side from the dark half-fields. “2. Lateral hemianopsia, with pupillary immobility, optic neuritis, or atrophy, especially if joined with symptoms of basal disease, is due to lesion of the tractus opticus or of the primary optic centres on one side.