Eye Structure
The human eye is a spherical, hollow, sensory organ specialized to receive light, focus it, convert it into electrical signals, and send those signals to the brain. Its structure can be understood by dividing it into three main layers and then looking at the structures inside.
OUTER LAYER (Protective Layer)
Cornea
The cornea is the transparent, curved front portion of the eye that allows light to enter. Its curvature bends (refracts) incoming light sharply, making it responsible for most of the eye’s focusing power. Because it has no blood vessels, it receives oxygen directly from the air and nutrients from the tear fluid.
Sclera
The sclera is the opaque, white, tough outer coat that maintains the shape of the eyeball and gives attachment points to eye muscles. It protects delicate inner tissues from injury and maintains internal pressure.
MIDDLE LAYER (Vascular Layer)
Choroid
The choroid is a dark, pigmented layer rich in blood vessels. It nourishes the retina and absorbs stray light inside the eye so that images do not appear blurred from internal reflections.
Ciliary Body
The ciliary body contains smooth muscle fibers arranged in a circular fashion. These muscles change the shape of the lens during accommodation (focusing on near and distant objects). The ciliary body also secretes aqueous humor, the fluid filling the front chamber of the eye.
Iris
The iris is the colored diaphragm of the eye. It has circular and radial muscles that adjust the pupil’s diameter. It controls how much light enters the eye, narrowing in bright light and widening in dim light.
Pupil
The pupil is the central opening of the iris. It is not a structure but a variable-sized aperture that regulates the intensity of light reaching the retina.
INNER LAYER (Sensory Layer)
Retina
The retina is a multilayered, photosensitive membrane lining the back of the eye. It contains rods (for dim light and black-and-white vision) and cones (for bright light and color vision). These photoreceptors convert light energy into nerve impulses. The retina also contains supporting neurons such as bipolar cells and ganglion cells that process visual information before sending it to the brain.
Macula and Fovea
The macula is a small, central, yellowish area of the retina responsible for high-resolution vision. At its center lies the fovea, which contains only cones and gives the sharpest, most detailed visual perception required for reading, recognizing faces, and fine tasks.
Optic Nerve
The optic nerve is the bundle of axons from the retinal ganglion cells. It carries visual information to the brain’s visual cortex. The point where it exits the eye is the optic disc, which lacks photoreceptors and forms the natural blind spot.
INTERNAL MEDIA OF THE EYE
Lens
The lens is a transparent, biconvex, flexible structure located behind the iris. It changes its curvature to focus light on the retina. In young eyes, it is elastic and can easily adjust shape; with age, it stiffens and loses some ability to focus.
Aqueous Humor
The aqueous humor is a clear, watery fluid in the anterior and posterior chambers (between cornea and lens). It maintains eye pressure, nourishes the cornea and lens, and carries away metabolic waste.
Vitreous Humor
The vitreous humor is a transparent, gel-like substance filling the space behind the lens. It maintains the eyeball’s shape, keeps the retina in place, and allows light to pass through to the retina without distortion.
ACCESSORY STRUCTURES
Eyelids
The eyelids protect the eye, spread tear fluid, and reduce light entry. They close reflexively to shield the eye from threats.
Eyelashes
Eyelashes detect approaching objects through sensitive touch receptors and help trap dust.
Conjunctiva
The conjunctiva is a thin, transparent mucous membrane lining the inner eyelids and covering the front sclera. It lubricates and protects the eye surface.
Lacrimal Apparatus
This system includes the lacrimal gland and ducts that produce and drain tears. Tears moisten the cornea, wash away particles, and contain enzymes that kill microbes.