ABSTRACT
The visual system is a highly complex structure, all parts of which are susceptible to disorder or disease. The cornea and lens focus the visual scene on the retina. The image is clearest in the centre (the fovea) and is less clear towards the periphery. There are many millions of photoreceptors. The cones serve day and colour vision and are most tightly packed in the centre but are present throughout the retina. The rods become sensitive under darkened conditions and serve night vision that is less clear and has no colour. The image data are passed via bipolar cells to the ganglion cell fibres that pass down the optic nerves to synapse in the lateral geniculate bodies. There are approximately 1.2 million fibres in each optic nerve. The optic nerves fibres from the nasal retina (serving temporal visual field) cross at the optic chiasm, and the optic tracts thereafter contain the information from one hemifield (the left optic tract and beyond subserves the right hemifield and the right side the left hemifield). The tracts pass to the lateral geniculate body and from there to the visual cortex in the posterior occipital lobes. The upper fibres passing over the top of the lateral ventricles serve the lower visual fields. The lower fibres, which loop into the temporal lobes around the lateral horns of the lateral ventricles, serve the upper visual fields.
