By Sanjana Malkareddy
Introduction
Your eyes are necessary to everyday life as they are sensory organs that allow you to see. Since your brain cannot have sensory skills, it relies on your eyes for information about your surroundings. For example, your eyes take in your surroundings (color, shape, size, etc.) and then your brain translates it into something it can utilize and comprehend. This is also known as sight and vision.
Anatomy
Parts located at the front of the eye:
The Sclera is a strong membrane that gives the eyes their shape and protects their internal structures. The sclera is also known as the “whites” of the eye. It consists of three main parts:
Episclera - connective tissue beneath the conjunctiva
Sclera proper - think tissue responsible for the white color
Lamina fusca - innermost region composed of elastic fibers
The Conjunctiva is a clear membrane that covers the sclera, cornea, and the lining of the eyelids. It lubricates the eye and makes blinking easier as it allows the eyelids to move smoothly over the eye.
The Cornea is a clear dome-shaped structure that covers the front of the eyes and helps protect the eyes from foreign objects (germs, debris, etc.). The cornea doesn’t have any blood vessels as it gets oxygen from its surroundings. The five layers associated with the cornea include:
Epithelium - the cornea’s outer layer, prevents foreign objects from entering the eye, and receives nutrients and oxygen from the tears
Bowman’s layer - helps preserve the cornea’s shape
Stroma - thickest and central layer and gives the cornea a flexible but strong composition
Descemet’s membrane - the layer where the endothelial cells are located
Endothelium - removes waste products (ex: excess water) and deposits them into the fluid located in the anterior chamber
Located in the middle of the cornea and the iris, the anterior chamber contains aqueous humor, a fluid that helps conserve the shape of the eye and nourishes other structures in the eye. Excess fluid drains through the trabecular meshwork to maintain a balanced amount of aqueous humor.
Consisting of connective tissue the trabecular meshwork helps drain the aqueous fluid from the surface of the eye into the veins and finally to the bloodstream.
The pupil is a dark circle located at the center of the iris and admits light into the eye. It constricts and dilates to control the amount of light entering the eye.
Located behind the anterior chamber, the iris controls the amount of light going into the eye by constricting or widening the pupil.
The ciliary body is a muscle that is located behind the iris and helps focus the lens, allowing us to adjust our focus between objects far away and near. It also produces aqueous humor. If the ciliary body fails to do its job accurately and produces too much, intraocular pressure can rise if the fluid doesn’t drain.
Consisting of three parts, the ciliary body, the iris, and the choroid, the uvea transports blood to the retina.
The lens is located behind the iris, it helps you see by focusing light on the retina. The lens can also change its shape so that you can see objects up close.
Parts located at the back of the eye:
The vitreous humor is a fluid that fills up the vitreous cavity or the middle of the eye. It keeps the retina in place, protects the inner components of the eye, and gives the eyeball its shape.
The sclera is found from the cornea to the optic nerve, being denser around the optic nerve to give additional protection.
Located in the space between the retina and sclera, the choroid is a layer of blood vessels. The four layers of the choroid include:
Haller’s layer - includes the larger blood vessels
Sattler’s layer - includes the moderate-sized blood vessels
Choriocapillaris - layer contains the capillaries
Bruch’s membrane - the choroid’s inner layer
The choroid contains melanin which keeps the blood vessels safe from toxic light and improves vision clarity by absorbing light that causes light reflections. It also brings nutrients and oxygen to the retina’s exterior.
The retina is a layer of cells on the back wall of the eye that sense light and communicate with the brain via signals, allowing you to see.
Photoreceptors are cells that convert light into signals that are channeled to the brain. The retina contains two types of photoreceptors; rods and cones.
Rods are located in the peripheral area of the retina. They are sensitive to light and allow us to see in low light by detecting shades of black and white. They also help detect motion and recognize shapes.
Cones are located in the retina’s center and are responsible for central vision. There are three types of cones; red-sensing, green-sensing, and blue-sensing cones.
The macula is located in the middle of the retina, this area contains numerous cone cells and is responsible for central vision.
Located in the macula is the fovea which contains a higher concentration of cone cells.
Located at the back of each eye and connected to the brain, the optic nerve allows you to see by sending visual data to the brain where it is analyzed.
Coming from the optic nerve the central retinal artery spreads across the retina and supplies oxygenated blood to the retina.
The central retinal vein also also spreads across the retina, except it exports the deoxygenated blood out of the eye and into the heart and lungs where it becomes oxygenated.
Parts located outside the eye:
The eyelids help lubricate the eye by spreading tears over the surface of the eye. They also eliminate tears by pushing them through the tear duct. Tears are made up of three layers; a mucous layer, a watery layer, and an oil layer.
By sending signals to the eyelids when foreign objects are near the eye and by protecting the eye from outside particles (ex: dust and debris), the eyelashes keep the eye safe.
The meibomian glands produce the oil that makes up the oil layer in a tear. The oil is a necessary component in lubricating the eye as it prevents the tear from evaporating rapidly.
Located away from the nose under the eyebrow, the lacrimal gland is responsible for the water layer of the tear.
The tear duct helps drain tears with the help of the eyelids pushing the tears away from the eye through the tear duct, behind the nose, and finally down the throat
The extraocular muscles are attached to each eye and allow the eye to move. These muscles are the reason the eyes can rotate and move vertically, and horizontally. The six extraocular muscles attached to each eye include:
Medial Rectus - responsible for inward movement
Lateral Rectus - responsible for outward movement
Superior Rectus - responsible for rotating the upper part of the eye towards the nose and upward and inward movement
Inferior Rectus - responsible for rotating the upper part of the eye towards the ears and downward and inward movement
Superior Oblique - responsible for rotating the upper part of the eye towards the nose and downwards and outward movement
Inferior Oblique - responsible for rotating the upper part of the eye towards the ears and upward and outward movement
Physiology
In order to see, light first reflects off an object and enters through the cornea, where the light is bent. The light then goes through the aqueous humor and into the pupil. The pupil adjusts depending on the light intensity. For example, in dim settings, the pupil dilates and in areas with higher light intensity, the pupil constricts. After it enters the pupil, it goes through the lens where the light is bent and focused a second time. The light then goes through the lens to the vitreous until it arrives at the retina. The photoreceptors located in the retina then transform the light into electrical signals. The retina’s nerves gather the electrical signals, which then travel along the optic nerve where they eventually reach the brain.
Conclusion
The eyes are complicated organs and have many parts working together to achieve their function. They help us understand our surroundings, aid in our interactions, and most importantly, let us see. Since they play a major role in our lives, maintaining good eye health is crucial. Understanding the anatomy and physiology of our eyes is a good start.
Citations
Anatomy of the eye | kellogg eye center | michigan medicine. (n.d.). https://www.umkelloggeye.org/conditions-treatments/anatomy-eye
Boyd, K., & Turbert, D. (2023, May 2). Eye anatomy: Parts of the eye and how we see. American Academy of Ophthalmology. https://www.aao.org/eye-health/anatomy/parts-of-eye
professional, C. C. medical. (n.d.). Eyes: A window to the world for your brain. Cleveland Clinic. https://my.clevelandclinic.org/health/body/21823-eyes
Russ. (2021b, July 15). Eye anatomy: External parts of the eye. Optometrists.org. https://www.optometrists.org/general-practice-optometry/guide-to-eye-health/how-does-the-eye-work/eye-anatomy-external-parts-of-the-eye/
Russ. (2021c, July 15). Eye anatomy: The back of the eye. Optometrists.org. https://www.optometrists.org/general-practice-optometry/guide-to-eye-health/how-does-the-eye-work/eye-anatomy-the-back-of-the-eye/
Russ. (2021d, July 15). Eye anatomy: The front of the eye. Optometrists.org. https://www.optometrists.org/general-practice-optometry/guide-to-eye-health/how-does-the-eye-work/ocular-anatomy-the-front-of-the-eye/
Russ. (2021e, August 10). How does the eye work?. Optometrists.org. https://www.optometrists.org/general-practice-optometry/guide-to-eye-health/how-does-the-eye-work/