Eye

About the Eye

The eye is a sphere approximately 24 mm in diameter with three layers. The outer fibrous layer includes the cornea transparent, avascular, heavily innervated most sensitive tissue in the body for light refraction and sclera white, tough for structure. The middle vascular layer uvea includes the choroid blood supply, ciliary body lens accommodation via zonular fibers, and iris pupil regulation. The inner neural layer is the retina containing photoreceptors: approximately 120 million rods peripheral, night vision, achromatic and 6 million cones central, fovea, color vision, daylight. Three cone types respond to blue S, green M, and red L wavelengths. The lens is biconvex, avascular, and accommodates changes shape for near vision via ciliary muscle contraction reducing zonular tension, allowing lens to round up. Presbyopia, which typically becomes noticeable in the forties, results from lens stiffening. Aqueous humor is produced by the ciliary body and drains through the trabecular meshwork and Schlemm canal. Glaucoma involves elevated intraocular pressure damaging the optic nerve. Vitreous humor fills the posterior chamber. The optic nerve carries 1.2 million axons to the brain. Visual pathway: retina, optic nerve, optic chiasm nasal fibers cross, optic tract, lateral geniculate nucleus, optic radiations, visual cortex occipital lobe.