June 5, 2025

Alina Khalil

11th Grade

The Blue Coat Cofe School

“The eye is like a mirror, and the visible object is like the thing reflected in the mirror.” 

- Avicenna

The eye is one of many sensory organs that help us navigate our surroundings, allowing individuals to see their environment through the capture of light and its conversion into a signal that the brain can detect and decipher. The eye, despite being such a deceptively simple word, is incredibly complex. The eye is made up of many subcomponents, all of which enable us to perceive our environment and navigate through life.

The eye has long been the topic of conflicting interpretations and theories, with many ancient physicians and philosophers proposing that the eye is an active organ. For example, Plato described the eye as “emitting light,” allowing objects to be seen. An alternate theory: Theophrastus described the eye as having “fire.” In the second century A.D., the Greek and Roman physician Galen was able to identify key features of the eye, including the retina, cornea, iris, uvea, tear ducts, and eyelids; additionally, he was responsible for the discovery of the vitreous and aqueous humors, two fluids located within the eye. 

Light enters the eye through the pupil, and with the help of structures like the iris and cornea, a suitable amount of light is directed towards the lens. Just like a lens in a camera, the lens in the eye refracts the light onto the retina, where messages are then encoded. The retina reconfigures the image into electrical signals, which are sent to the optic disk on the retina. Lastly, the signals are transported along the optic nerve via electrical impulses to be eventually processed by the brain.

The eyeball consists of three layers:

1.  The outer layer, formed by the cornea and sclera.

2. The middle layer, which holds the primary blood supply for the eye, contains the pupil and iris. 

3. The inner layer, containing the retina.

The eyeball additionally contains three fluid-filled chambers:

1. The anterior chamber, which is between the cornea and iris

2. The posterior chamber, which is between the iris and pupil

3. The vitreous chamber, which is between the lens and the retina

The anterior and posterior chambers contain aqueous humor, a watery fluid that nourishes the interior eye structures and helps to keep the eyeball inflated. The vitreous chamber is filled with a thicker fluid called the vitreous humor—a transparent gel consisting of 99% water, which also serves to keep the eye inflated. 

The choroid is a spongy middle layer of the eye located between the sclera and the retina. It is filled with blood vessels, functioning to nourish the outer layers of the retina. It contains a pigment that absorbs excess light, serving to prevent one’s sight from blurring. The ciliary body is located behind the iris, nestled near the crystalline lens. It has two functions: one is the production of the aqueous fluid that fills the front of your eye; the other is that the ciliary body is made up of muscles, which allows the eye to focus at different distances.

Cone cells are a type of light-sensitive cell found in the retina. These are specialized cells, and there are millions of cones that function best in bright light and are essential for sharp vision.

The cornea is the transparent, circular part of the front of the eyeball. The function of the cornea is to refract the light entering the eye onto the lens, which focuses it onto the retina. The cornea has no blood vessels and is very sensitive to pain. The conjunctiva lines the eye socket, protecting and lubricating the eyeball. The conjunctiva is a thin, moist, and clear membrane that covers the sclera.

The crystalline lens is the transparent structure inside the eye, located directly behind the iris. It focuses rays of light onto the retina. The iris, the colored visible portion of the eye, regulates the amount of light that enters the eye. It is directly in front of the lens.

The difference between the eyes of humans and animals lies in the shape and size of the eye. Human eyes are round, like those of bears, foxes, and birds. Larger mammals, such as cows, horses, and moose, have horizontally oval-shaped eyes. Additionally, most animals have a greater range of peripheral vision than humans do, usually 285 degrees of vision to the human's 190 degrees. Most animals also possess a third eyelid; however, snakes have none whatsoever, only having a thin membrane over the eye. This is because snakes shed their skin and have evolved to not need eyelids. In dogs, two eyelids are visible, while the third layer is more hidden, there only to provide extra protection for the cornea. In cats, the third eyelid is used to prevent injuries to the eyes.

The eye is an important aspect of our body and contributes significantly to our sensory system. Without it, carrying out daily activities can be difficult, especially if blindness is caused by a disease or eye damage.