The Eye – Parts Of The Eye And Their Functions
Table Of Contents
The eye is one of our most precious senses, and thanks to the eye, we can move in our environment the way we do. But do we ever wonder how our eyes are built? Do we know all its parts and functions?
The human eye has two parts: an external and visible one, which is made up of elements such as the eyelids, eyelashes, or the iris, among others, and an internal one, which is possibly the one we know the least about, where we find structures that are essential for our vision, such as the fovea, the ciliary body or the crystalline lens.
In turn, the eye has 3 layers: an outer layer, an intermediate layer, and an inner layer.
What Is The Eye?
The eye is our organ of sight, a hollow, almost spherical globe filled with humors (liquids or fluids).
The eye is also called the eyeball and is the organ in charge of capturing light energy to send it to the brain in the form of nerve impulses so that it can analyze the information.
The eye weighs 7 to 7.5 grams, measures 2.5 cm in diameter and has an angle of vision of 200 degrees, and is composed of 3 distinct layers:
The Three Layers of the Eye
The Outer Layer: Cornea and Sclera
The outer layer is protective and fibrous; it is composed of the following:
The sclera (the white part of the eye) has an opaque part where the muscles are located that makes eye movements possible.
Surrounding the sclera is the conjunctiva, a transparent membrane or layer of skin that keeps the eye from drying out.
And the cornea is the transparent surface of the front part of the eye that transmits light to the retina in the back of the eye.
The Middle or Intermediate Layer: Úvea
The middle layer is vascular and consists of the choroid, iris, and ciliary body. The blood vessels allow the distribution of oxygen and nutrients to the inner parts of the eyeball.
The Inner Layer: Retina
The inner layer or retina is sensory (or nervous), soft, and sensitive to light. It is located from the ciliary muscles to the back of the eye, where we find the photoreceptors and nerve structures.
The retina sends signals to the brain through the optic nerve for the brain to interpret them as visual images.
The fluids (humors) that fill the eye are separated by the crystalline lens.
The vitreous humor is located behind the lens and is a transparent gel that gives volume to the eye.
The aqueous humor is in front of the lens. It covers the anterior chamber between the cornea and the iris and the posterior chamber between the iris and the lens.
The lens is suspended and supported by ligaments that make it possible for it to change its shape and allow light to focus on the retina, which is made up of sensory neurons.
What Are The Parts Of A Human Eye?
The human eye has a rather complex anatomy, so let’s take a look at each of its parts and what they are made of.
Parts of the External Eye
The parts of the external eye are the structure that makes up the eye in the visible area. In the external or outer part of the eye, we find the eyebrows, the eyelashes, the eyelids, the orbit, the sclera, the conjunctiva, the cornea, the iris, the pupil, and the lacrimal gland.
The eyebrows are at the top of the eye socket and are full of hairs that protect the eye from sweat.
The eyelashes around the eye are stiff hairs that protect the eye from the elements.
The eyelids are folds that cover and protect the cornea and conjunctiva from excessive light and foreign agents.
In turn, the eyelids produce blinking, which creates tears to lubricate the eye and facilitate the elimination of foreign agents.
The orbit is the bony cavity where the eyeball is located and whose function is to protect the eye.
Inside the orbit, besides having the eye, there are a series of annexed structures such as the extraocular muscles, the lacrimal gland, the optic nerve, the suspensory ligament of the eyeball, etc.
The sclera, or white part of the eye, protects the internal parts of the eye and is a hard, thick, white layer.
The conjunctiva allows the eye to stay moist and is a thin, transparent membrane that covers the sclera.
The cornea is transparent and has a convex shape; it has no blood vessels. Its function is to protect the iris, the crystalline lens, and other structures and focus the retinal images together with the crystalline lens.
The cornea is of great importance to the visual system since the lack of transparency or any deformity in the cornea causes a bad image on the retina.
The iris is located between the cornea and the crystalline lens. It is a colored membrane with a central opening, the pupil, and it controls its size.
The color of the iris will depend on the number of color granules. The amount of pigment is usually related to genes, skin type, and hair color.
The pupil is located in the center of the iris and protects the eyeball from excessive light. The function of the pupil resembles that of a camera diaphragm, as it can contract or dilate (miosis and mydriasis) depending on the light in the environment.
The lacrimal gland produces tears to lubricate the conjunctiva and cleanse the eye to remove foreign matter.
Parts of the Inner Eye
The internal structure of the eye is composed of: the anterior chamber, the lens or crystalline lens, the ciliary body, the vitreous, the retina, the macula, the optic nerve, the optic chiasm, the visual cortex, the muscles (the main ones are: superior rectus, inferior rectus, medial rectus, superior oblique and inferior oblique).
The interior of the eye is divided into three compartments:
The anterior chamber (between the cornea and iris contains aqueous humor); the posterior chamber (between the iris, ciliary body, and lens contains aqueous humor); and the vitreous chamber (between the lens and retina contains humor or vitreous body).
The anterior chamber is the space at the front of the interior of the eye, between the cornea and the iris, and in front of the lens.
The anterior chamber is filled with a fluid called aqueous humor that nourishes the eye and its tissues.
The ciliary body is located in the middle layer of the eye. It is an extension of the iris that is attached to the conjunctiva. It is vital for the process of accommodation.
The ciliary body is formed by: the ciliary processes, responsible for producing the aqueous humor, and the ciliary muscle, responsible for modifying the curvature of the crystalline lens so that we can focus at different distances.
The crystalline lens is a natural lens, elastic and transparent. It is located behind the iris, has no blood vessels, and is held in place by the ciliary muscles.
The vitreous humor is a clear, jelly-like fluid that lies behind the crystalline lens to the retina and forms 80% of the eyeball.
The retina is the innermost part of the eye and is located at the back of the eye. It is where light is transformed into nerve impulses sent to the brain through the optic nerve.
Did you know that...
The retina is made up of 10 layers, and its function is to allow us to visualize images:
The 10 Layers of the Retina:
- Pigmented epithelium
- Layer of cones and rods
- Outer limiting membrane
- Outer nuclear layer
- Outer plexiform layer
- Inner nuclear layer
- Inner plexiform layer
- Ganglion cell layer
- Nerve fiber layer
- Inner limiting membrane
The anterior part of the retina is blind, and as we move away from this area, its sensitivity increases; being the point of maximum sensitivity, the fovea, a small indentation where there is a higher concentration of cones and rods, the cells responsible for the sensitivity of the retina, the photoreceptors.
The posterior area of the retina is the papilla that connects with the optic nerve to transfer the nerve impulse to the brain.
The macula is the central area of the retina that allows central vision and fine vision.
The optic nerve transmits visual information from the retina to the brain.
The optic chiasm is a structure where the optic nerve of each eye joins and is located at the base of the brain.
The visual cortex is the part of the cerebral cortex located in the occipital lobe responsible for processing visual information.
Finally, the main muscles involved in the internal structure of the eye that allow it strength and mobility are:
The superior rectus muscle is an extraocular muscle and allows upward mobility of the eye. It is located in the upper part of the eye.
The inferior rectus muscle allows downward mobility of the eye and is located in the inferior or lower part of the eye.
The medial rectus muscle is in charge of moving the eye inward and is located near the nose on the lateral part of the eye.
The superior oblique muscle causes the eye to rotate inward and around the axis. It allows downward movement and is located at the back of the orbit.
And the inferior oblique muscle is responsible for the eye to rotate outward along the axis and move upward. It is located at the front of the orbit.