How the Human Eye Works

Sclera

The sclera is a tough tissue that forms the outer protective and structural layer of the eye.

Choroid

The choroid is a layer between the sclera and retina that provides oxygen and nutrients to the outer layers of the retina.

Retina

Macula

Optic nerve

Retinal arteries & veins

Vitreous body

The vitreous body is filled with a gel-like substance called vitreous humor, and maintains the structure of the eye.

Lens

Anterior chamber

The anterior chamber is filled with aqueous humor, a watery fluid that provides nutrition to the lens, iris, and back of cornea.

Pupil

Iris

Cornea

Focusing light

Iris

The iris controls light levels entering the eye by altering pupil size.

Iris sphincter muscle

The iris sphincter muscle is a circular muscle that contracts to shrink the pupil.

Iris dilator muscle

The iris dilator muscle has spoke-like fibers that contract to widen the pupil.

Pupil

The pupil is the opening in the iris center. It appears black because light entering the pupil is absorbed by tissues in the eye before it can escape.

Suspensory ligaments

The suspensory ligaments hold the lens in place and connect it to the ciliary muscle.

Ciliary muscle

The ciliary muscle contracts to release tension on the lens, which relaxes into a thicker, more spherical shape to focus on nearer objects. The ciliary muscle relaxes to tighten and flatten the lens to focus on farther objects.

Lens

The lens compresses and expands to focus light in a process called accommodation.

Light ray angles become more parallel with distance, so the lens becomes flat as less curvature is needed for proper focus.

Iris color (eye color)

The amount of melanin (a dark brown or black natural pigment in skin, hair and eyes) and iris structure determine eye color.

Melanin pigment gives brown eyes their color.

A “blue” iris has little or no melanin, and its physical structure reflects blue light back.

Cornea

The cornea refracts (bends) light into the pupil. The cornea accounts for about 2/3 of the eye’s optical power or ability to focus light.

Basic optics

Light rays hit a point on an object and bounce off in many directions.

Bounced light rays from specific points interfere with each other in a “blurry” tangle.

Upside down and reversed image

Seeing how light rays bounce in outward angles makes it easier to understand why rays

refocused at a distance would produce an upside-down, reversed result.

Pinhole camera example

A very small hole in a box allows a small range of light angles to enter. The reproduced image is in focus, but limiting light rays means it’s not very bright.

Focusing

A larger opening allows more light rays to pass through for a brighter image, but these rays might interfere with each other causing a blurry result.

A lens focuses or “bends” multiple rays from single origin points back into bright, singular, focused points.

The retina

Retinal tissue covers much of the inner eye, and contains special neurons called photoreceptors.

Macula & fovea

The macula is a spot near the retina’s center that contains the fovea. The fovea is a pit of densely packed cones for sharp, colorful vision in direct focus. Peripheral vision has less sharpness and color perception.

Optic disc (blind spot)

Arteries, veins, and the optic nerve interface with the eye through the optic disc, which has no photoreceptors. As such, it’s the location of our so-called visual “blind spot.”

Photoreceptor cell density map

Cones (shown in color) increase in density near the center of vision while rods (in dark grey) cover peripheral areas.

Rods

Rods function in dim light and provide black and white vision.

Cones

Cones function in brighter light and provide color vision. Cones are sensitive to particular light wavelengths such as red, green and blue light.

Optic nerve fibers

These ganglion cell roots or “axons” are intertwined throughout the optic nerve and extend into the brain.

Retina function

Light processing starts at the back of the retina. Photons are absorbed in rods or cone cells, which contain chemicals called opsins that react when absorbing photons of light.

Subsequent layers of bipolar and ganglion cells have a “receptive field”, that is, they receive input from groups of rods and cones. Some fields are stimulated by direct light while others respond to indirect light.

Signals are transmitted to the brain through the ganglion cells, which have long axons that extend from the retina into the brain through the optic nerve.

The contrasting direct and indirect signals allow the brain to find edges between distinct regions of color or brightness, providing a sharper image.

Ganglion cell

Amacrine cell

Bipolar cell

Horizontal cell

Eye movement

Six extraocular (outside the eye) muscles work in unison to control eye movement, and attach to the sclera.

Superior rectus

Primarily moves the eye upward (elevation).

Divergence

The eyes diverge, or rotate away from each other, to view far-away objects.

Convergence

The eyes converge, or rotate towards each other, to view nearby objects.

Lateral rectus

Primarily moves the eye away from the nose (abduction).

Inferior oblique

The inferior oblique originates from the maxilla (upper jaw bone). It mainly rotates the eye away from the nose (extorsion), but also moves it upward (elevation), and away from the nose (abduction).

Inferior rectus

Primarily moves the eye downward (depression).

Medial rectus

Primarily moves the eye toward the nose (adduction).

Superior oblique

The superior oblique originates from the orbit (eye socket), near the nose, and loops through the trochlea, which acts as a pulley. It mainly rotates the eye toward the nose (intorsion), but also moves it downward (depression), and away from the nose (abduction).

Common tendinous ring

Four of the eye muscles (superior, inferior, medial, and lateral rectus) originate from the common tendinous ring and control most up-and-down and side-to-side movement.