The human eye sits safely within the bony orbit of the skull, cushioned by adipose tissue which protects it from physical shock. Several accessory structures work continuously to shield, lubricate, and move the eyeball.
The wall of the eyeball consists of three distinct layers. The outer fibrous tunic provides structural shape and protects delicate internal parts. It includes the opaque, white sclera, which covers most of the eye, and the transparent cornea at the very front, which bends incoming light rays.
Beneath the fibrous layer lies the middle vascular tunic, or uvea. This layer contains the choroid, a heavily pigmented membrane supplying blood to the retina. Toward the front, the choroid thickens into the ciliary body, which holds the lens and controls its shape. The colored part of the eye, the iris, regulates the size of the central opening known as the pupil, thereby controlling the amount of light entering the eye.
The lens and ciliary body divide the interior of the eye into two main cavities. The anterior cavity sits in front of the lens and contains a watery fluid called aqueous humor. The ciliary body continuously secretes this fluid, which nourishes the cornea and lens before draining through the scleral venous sinus, also known as the canal of Schlemm.
Blockage in the drainage of aqueous humor leads to a dangerous buildup of pressure inside the eyeball, a condition called glaucoma. Behind the lens lies the much larger posterior cavity, filled with a transparent, jelly-like substance named the vitreous body. This gel maintains the spherical shape of the eyeball and presses the retina firmly against the choroid layer.
The innermost layer of the eye is the retina, which contains the photoreceptor cells responsible for converting light energy into electrical nerve signals.
When light hits photopigments inside these receptor cells, it triggers a chemical change that hyperpolarizes the cell, altering neurotransmitter release. This signal passes to bipolar cells and then to ganglion cells, whose axons bundle together to form the optic nerve leaving the eye at the optic disc, commonly known as the blind spot.
For a clear image to form on the retina, light rays entering the eye must be bent, or refracted, so they converge at a single focal point. The cornea performs the majority of this bending, while the adjustable lens fine-tunes the focus for objects at varying distances.
When focusing on close objects, the ciliary muscle contracts, releasing tension on the suspensory ligaments. This allows the elastic lens to bulge into a more convex shape, increasing its refractive power, a process called accommodation. When viewing distant objects, the ciliary muscle relaxes, pulling the ligaments taut and flattening the lens.
Nerve impulses generated in the retina travel along the axons of ganglion cells, which converge to form the optic nerve (Cranial Nerve II). At the base of the brain, the two optic nerves meet at the optic chiasm, where fibers from the medial half of each retina cross over to the opposite side, while lateral fibers stay on the same side.
Beyond the chiasm, the fibers continue as the optic tracts, carrying visual information to the lateral geniculate nucleus of the thalamus. From there, neurons project through the optic radiations to the primary visual cortex located in the occipital lobe, where conscious perception of sight finally occurs.