How the Eye Works

How the Human Eye Functions

The eye is a biological marvel – it allows us to sense objects at vast distances at nearly theoretical resolution. We rely on this sense organ more than any other to live our everyday lives. Its structure and function is truly a testament to evolution. In this article, we will survey and explore the various structures of the human eye and their functions.

The eye is a fluid-filled ball that is contained within the eye sockets of the skull. The eye sockets are lined with pockets of adipose tissue which help to lubricate the eye and absorb sudden impacts. Surrounding the eye are various muscles, which serve to rotate the eye in its socket – vital for tracking objects. The optic nerve exits at the dorsal side of the eye, and carries nerve impulses from the eye to the occipital lobe of the brain.

The eye is coated with rigid, white protein, known as the sclera. The sclera maintains the structural integrity of the eyeball and serves as an attachment site for muscle.

Covering the sclera at the exposed part of the eyeball is a thin layer of cells called the conjunctiva. This layer protects the eye from invading pathogens. Underneath the conjunctiva is a transparent extension of the sclera known as the cornea. The cornea allows outside light to enter the eye, and its convex shape helps to focus incident light into the eye. During laser eye surgery, this part of the eye is reshaped to form a corrective lens.

Behind the cornea is a layer of low-viscosity fluid known as the aqueous humor. This layer separates the cornea from the iris. The iris is a layer of pigmented cells that controls the size of the pupil. When the iris dilates, the pupil enlarges, allowing more light to enter the eye during low-light conditions or during the fight-or-flight response. Dilation of the pupil is associated with the sympathetic nervous system.

Behind the iris is the lens of the eye, whose role is to focus incident light onto the retina. The lens is attached via ligaments to the ciliary muscle of the eye. When the ciliary muscle relaxes, the ligaments stretch the lens, decreasing its curvature. This allows for long-distance viewing. For close vision, the muscle contracts, allowing the lens to assume a more convex shape.

Between the lens and the retina is a viscous fluid known as the vitreous humor. This fluid comprises the greatest part of the eye’s volume. Its main purpose is to maintain the shape of the eye, while still allowing light to reach the retina.

The retina is comprised of photosensitive cells: rod and cone cells. Rod cells are very sensitive to light, but cannot sense color. These are primarily responsible for low-light vision, as they are useless in high light intensity environments. This explains why we cannot sense color at night – everything is monochrome. Cone cells are each sensitive to one of the three primary colors: red, green or blue. These cells are most concentrated at the center of the retina – the fovea centralis – where the image is usually formed by the lens. These cells allow for detailed, color vision, but are less sensitive than rod cells and are thus useless in low-light environments.

The rod and cone cells in the retina are associated with bipolar nerve cells, which lie above the photosensitive cells. These cells synapse with ganglion cells which carry visual information to the occipital lobe of the brain.

Behind the retina and surrounding the inside of the eye is a black light-absorbing pigmented layer called the choroid. The choroid prevents any excess light from being reflected inside of the eye, which improves quality of vision.