The definite geometric defects are classed under the general head of hemianopsia, by which term is meant that one horizontal or vertical half of the visual field is obscured. (1) Horizontal hemianopsia is not bounded by a very sharp or straight boundary-line, and is almost always due to intraocular disease (retinal lesions, embolism of one large branch of the retinal artery, injuries, etc.). (2) Vertical hemianopsia is usually marked by a sharply-defined vertical limit in the visual field, passing through the point of fixation, or a little to one side of it more usually, leaving central vision very acute. (_α_) Temporal hemianopsia, in which the temporal halves of the visual fields are dark, represents anæsthesia of the nasal halves of the retinæ, and is usually caused by a lesion of the chiasm of the optic nerve, so placed at its frontal or caudal edge as to injure the fasciculi cruciati. This variety is usually bilateral, but a lesion might be so situated as to affect only one fasciculus cruciatis. (_β_) Nasal hemianopsia, in which the inner (nasal) halves of the visual fields are dark, represents anæsthesia of the temporal halves of the retinæ, and is caused by a lesion injuring one fasciculus lateralis or both fasciculi. In the former case the nasal hemianopsia would be unilateral; in the second case, bilateral or symmetrical, (_γ_) Lateral or homonymous hemianopsia is that condition in which physiologically similar halves of the visual field are darkened; for example, the temporal half-field of the left eye and the nasal half-field of the right. This represents anæsthesia of the nasal half of the left retina and of the temporal half of the right. The patient can only see, with one or both eyes, the right half of any object held directly in front of him. In such cases the lesion is always caudad of the chiasm, and may consist in interruption of the right optic tract, {40} of disease of the primary optic centres (corpus geniculatum laterale and lobus opticus) on the right side, of the caudo-lateral part of the right thalamus, of the caudal extension of the internal capsule or optic fasciculus within the right occipital lobe, of the right superior parietal lobule or gyrus angularis penetrating deep enough to interrupt the optic fasciculus; or, finally, the lesion may injure the visual centre itself—viz. the cortex of the right cuneus and fifth temporal gyrus (of Ecker). Hemianopsia of any type may be incomplete or only sector-like—_i.e._ involving only a quadrant or less of one visual field or of both fields. (Vide article on Localization).