Anatomy and Histology of the Eye and Visual Pathway
Anatomy and histology of the eye and visual pathway
This domain tests structure-function relationships from cornea to cortex, including laminar organization, blood supply, and lesion localization. Histology questions often hinge on cell layers and orientation.
Core rules
- Cornea has five layers: epithelium, Bowman layer, stroma, Descemet membrane, endothelium. Endothelial pump maintains deturgescence.
- Trabecular meshwork lies at the iridocorneal angle; Schlemm canal drains into episcleral veins. Uveoscleral outflow is a minor alternative.
- Retina has ten layers; photoreceptors lie outermost, ganglion cells innermost. The fovea has a cone-only centre and no inner retinal layers at the foveola.
- Optic nerve fibers nasal to the fovea cross at the chiasm; temporal fibers stay ipsilateral. Postchiasmal lesions produce homonymous defects.
- LGN has six layers; magnocellular, parvocellular, and koniocellular streams project to V1.
Worked example
A patient has a bitemporal hemianopia. Where is the lesion? The crossing nasal retinal fibers carry temporal visual fields. A chiasmal lesion, such as a pituitary adenoma, damages these crossing fibers and produces bitemporal loss. A lesion behind the chiasm would produce a homonymous hemianopia instead, because both eyes' nasal and temporal fibers from the same hemifield travel together. This reasoning separates chiasmal from retrochiasmal disease.
Traps
- Reversing nasal retina and temporal visual field.
- Placing the fovea at the optic disc.
- Confusing corneal layers and their regenerative capacity.
- Assuming all postchiasmal lesions spare the macula; macular sparing is possible with occipital lesions but not a general rule.