How well do you know about the anatomy of the eyeball?
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An understanding of eye anatomy provides a better insight of just how amazing the eye is. The eye is a small organ, measuring around 24 millimeters in diameter and length at its full adult size. Despite the small size, many cells in the eye work together to enable you to see in different ways.
Your eye can discern objects at distant and near locations, differentiate hundreds of different colors, appreciate the perception of depth and detect motion. So far, despite all the advances in science and technology, there is nothing man made that that even remotely comes close to matching how the eye works.
In general, the camera provides the best analogy for how the eye works. The eye is a camera, where the eyelid is the shutter, the crystalline lens is the lens, and the retina is the film. In addition, the optic nerve is the cable that connects the eye (camera) to the brain (computer).
To help with eye anatomy, it is useful to differentiate the eye into 3 parts:
- Front section (anterior segment)
- Back section (posterior segment)
- Surrounding supporting tissues
(Image reproduced with permission from Dave Carlson - Carlson Stock Art)
Diagram demonstrating the front and back eye anatomy
EYE ANATOMY - FRONT SECTION (ANTERIOR SEGMENT)
The eye anatomy at the front comprises the following tissues: conjunctiva, sclera, cornea, iris, ciliary body and crystalline lens.
The conjunctiva is the thin layer that lines the front of the sclera and also the inner surfaces of the eyelids. It contains glands that secrete lubricating fluids as well as lymphoid tissue. The conjunctiva is therefore important in keeping the eye moist and in protecting the eye from infection. Dry eye syndrome can result when there are problems with the conjunctiva.
The sclera is the tough outer coat of the eye. It is the white part of the eye. It maintains the structural integrity of the eye and provides support for all the other eye tissues. The extraocular muscles are attached to the sclera - the anchor points on the sclera allow the eyeball to move when the extraocular muscles contract.
The cornea is the clear, front window of the eye. It is transparent; this allows light to enter the eyeball. As light passes through the cornea, it becomes refracted. Together with the lens, the cornea is responsible for focusing light onto the retina. The space in between the cornea and lens is called the anterior chamber, which is filled by aqueous humor. The aqueous humor provides nutrients and oxygen to the cornea.
The iris is the colored part of the eye. It is a membrane that sits between the cornea and the lens. The iris color depends on the amount of pigment in the iris. Those with dark brown irides have a lot of iris pigment. The pupil is the round opening in the center of the iris. The iris is responsible for controlling the amount of light that enters the eye through the pupil. When there is too much light, the iris muscles constrict the pupil to restrict the amount of light entering the eye. Conversely, when it is dark, the iris muscles dilate the pupil to allow as much light to enter the eye as possible.
The crystalline lens is located just behind the iris. It is held in place in the eye by zonules (also known as suspensory ligaments) that attach it to the ciliary body. Along with the cornea, it is responsible for focusing light onto the retina. It is able to change its thickness, and thus can change the focus from far to near objects and vice versa (i.e. accommodate). As you grow older, this ability to change the focusing distance diminishes and you end up needing to wear reading glasses - this is called presbyopia. Age also causes the lens to become progressively more cloudy, resulting in cataract formation.
The ciliary body is positioned just behind the iris. Its two main functions are to produce aqueous humor and to change the thickness of the lens by stretching or relaxing the zonules that are attached to it.
EYE ANATOMY - BACK SECTION (POSTERIOR SEGMENT)
The eye anatomy at the back comprises the following: vitreous humor, retina, choroid and optic nerve.
The vitreous humor is the jelly-like substance that fills the vitreous cavity between the lens and the retina. It is transparent and thus allows light to be focused onto the retina. The vitreous humor also helps the eye to maintain its shape and turgidity. Vitreous loss into the anterior chamber is one of the
complications that can occur during cataract
The retina is the light-sensitive innermost lining of the eyeball. It comprises the retinal pigment epithelial layer and neurosensory retina. The retinal pigment epithelium helps to maintain and support the neurosensory retina. The neurosensory retina detects, absorbs and processes the light that enters the eye. Retinal photoreceptor cells in the neurosensory retina collect light signals and convert them to bio-electrical signals, which are then sent to the optic nerve. There are two types of photoreceptor cells: rods and cones.
Rod photoreceptors are located in the peripheral retina, and are responsible for night vision and detection of motion.
Cone photoreceptors are mostly concentrated at the macula (center of the retina) and are important for fine detailed visual acuity and color vision. Any damage to the macula, such as in
macular degeneration, will cause blurring of the central vision.
(Image adapted from the internet)
The optic nerve is composed of over one million nerve fibers. It transmits bio-electrical information from the neurosensory retina to the brain, so that the brain can interpret what it is that you are seeing. Your vision can be affected by damage to the optic nerve, such as in glaucoma and optic neuritis.
The choroid is the layer between the retina and sclera. It is full of blood vessels and is responsible for supplying nutrients to the retina. The dark melanin pigment in the choroid absorbs any excess light in the eye.
EYE ANATOMY - SUPPORTING TISSUES
Eye anatomy is not limited to the eyeball itself. There are also important supporting structures, and these include the eyelids, extraocular muscles and lacrimal drainage system.
The eyelids protect the eyes from any external trauma. Blinking the eyelids maintains the tear film, which keeps the eye moist. The upper and lower eyelids have a tough fibrous plate called the tarsal plate, which provides support and rigidity to the lid. These plates also contain meibomian glands, which produce oil that helps to stabilize the tear film. Dysfunction of the meibomian glands can result in inflammation of the eyelids and blepharitis.
The extraocular muscles move the eye. There are six muscles that are attached to the sclera at one end and the eye socket at the other end. The six muscles are the superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique and inferior oblique muscles. These muscles work together so that both eyes will view the object at the same time.
(Image adapted from the internet)
The lacrimal system produces, distributes and drains the tears that are important to keep the surface of the eye moist. The lacrimal gland located under the upper eyelid produces tears. When you blink, the tears are distributed throughout the eye surface. The tears are then drained through the punctum, lacrimal canaliculi, nasolacrimal sac and nasolacrimal duct to eventually enter the nose.
(Image adapted from the internet)
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