The gross anatomy of the optic nerves and chiasma has been studied, and differences in the tension in the crossed and uncrossed fibres after chiasmal displacement have been investigated. The anterior and posterior attachments of the medial and lateral fibres of the nerves have been studied. The chiasma has been dissected under low power microscopy and a three dimensional picture of it developed. Bitemporal hemianopia, as well as associated or independent hemianopic scotomata, results from stretching of the crossing fibres in the chiasma. Binasal hemianopia results from compression of the uncrossed fibres in the optic nerve or chiasma by the anterior cerebral or internal carotid arteries. The compression is effective because it is sharply localized and, probably as a result of pulsation, deeply grooves the nerve with a resulting acute distortion of fibres; it is likely that the lax lateral fibres would be less affected by a more widely spread compression. When this defect develops on top of an existing bitemporal hemianopia, it is believed that its usual cause remains the same. The crossed and uncrossed fibres of the optic chiasma differ not only anatomically in the areas of retina in which they arise but also physically. Tension is the force which occasions bitemporal hemianopia and pressure that which produces nasal field defects.
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