Author: Dr Anitha Arvind
This article will help understand the parts of the eye, different types of refractive errors of eyes and the vision care system.
Eyes are windows to the soul as they speak the truth and cannot hide one’s emotions. The eyes of a person reveal a person’s emotional state and give a lot of information about their health in general. Eyes speak about the physical and emotional well being of a person. This article will provide you with all the essential information about how our eyes see and what makes up the eye.
The eyeball and its parts: A complex machine!
The eyeball is a sphere-like structure present in a socket called the orbit in the skull. An adult eyeball weighs roughly around 7.5 to 9 milligrams, and its length keeps increasing from birth till the age of 20 or 21 years. Eyes undergo changes throughout one’s life and also undergo age-related changes. The health of the eye is affected by the body’s health; hence, it is essential to maintain good health and take good care of the eyes.
How do we see?
Light coming from an object carries information about that object. Under right circumstances our visual system uses this information to decide the objects colour, size, shape and position. So, in order to see an object our eyes have to receive light from it and convert the information stored in the light to signals by the light detectors that passes this to the brain.
The brain processes this information and that is how we see! If there are defects in the eye, then the information reaching the eye’s light detectors do not get the right information and hence the object cannot be seen accurately. At times even if the information is passed in correct form to our brain it is possible that it will not be interpreted correctly. Human eyes are very complex!
The eye as a camera
It is often said that the eye is like a camera and there are similarities between the two in terms of its functioning. The diagram (Figure 1) compares the eye with a digital camera.
Figure 1: Comparison of the digital camera with the eye
(PC – An online book of common-sense photography by Rhett Stuart)
Camera vs Eye
The eye and its parts
Figure 1: Anatomy of the eye
Photo URL: https://www.shutterstock.com/image-illustration/3d-illustration-human-eye-parts-1932451070
Cornea: It is a transparent structure situated at the front of the eye. It contributes the maximum in giving power to the eyes to bend the light rays.
Iris – It is the coloured part of the eye that acts like a shutter of the camera.
Pupil – It is an aperture present in the centre of the iris. The muscles in the iris control the size of the aperture so that the pupil is large in low light levels and small when the light is bright.
Crystalline lens: The crystalline lens is a transparent structure present inside the eye behind the iris that helps to focus images within the eye. Surrounding muscles in the ciliary body control the lens’s shape and power. The change in power of the lens is called accommodation. With age, the lens becomes stiffer and loses its ability to change shape.
Sclera: The sclera is the white part of the eye. It forms the tough outer coat of the eyeball and protects the eye.
Choroid: This is the middle coat of the eyeball that lines the sclera interiorly. It stops the unwanted light from the outside and absorbs any light that the light detectors have not captured.
Retina: The retina is the third coat and the eyeball’s innermost layer. The retina contains a light detection system that catches the light falling on it.
Rods & Cones – These are the light receptors named after their shape present in the retina. When light falls on these receptors, the processing system of the retina converts this information to signals. These signals travel via the optic nerve to the brain.
- Rods – For night vision or low light conditions
- Cones – For day vision or bright light conditions
Macula: It is an oval pigmented area near the centre of the retina and is specialized for high acuity vision and colour vision.
Fovea: It is located in the centre of the macula. The fovea has the highest concentration of cones in the retina and is a very important area in the retina.
Optic nerve: The optic nerve is the nerve at the back of the eye. It carries the light signals reaching the retina to the brain. The brain perceives the image and processes it the way we see it.
Peripheral Vision
- The retina lines the inside of the choroid and extends forward in front of the middle of the eye hence we not only see objects situated in our line of sight but also away from them.
- With both eyes we get a panoramic vision of 180 degrees peripheral vision.
- Because the eye is roughly spherical, we can also detect objects above and below our line of sight. How far above and below depends on how forward our cheekbones and eyebrows project.
How is our eye round in shape?
- The eye is almost spherical, and to keep the eye this way, it is slightly pressurized
- The pressure comes from the production of fluid called aqueous humour located between the cornea and iris and around the periphery of the lens
- The space behind the lens is a clear jelly called the vitreous humour. The aqueous presses on the vitreous. This helps keep the globe in shape and holds the retina against the choroid.
Eye muscles:
- The iris and ciliary body contain muscles (intraocular) that change the pupil diameter and adjust the strength of the lens. We do not have direct control over these muscles.
- There are six extraocular muscles attached to each eye that rotate the eyeball, and we can control these muscles.
- Weakness or spasm of a muscle could create problems.
What is normal vision?
A person is said to have normal vision when light entering the cornea pupil; the lens focuses on the retina.
The optic nerve (situated at the back of the eye) carries the light signals and information from the retina to the area of the brain (visual cortex or the sight centre of the brain). This area perceives and processes the image for us to see, and the image appears re-inverted. In reality, we see through the brain, and the eyes act as mediators to send information to the brain!
Defects in the eye’s focusing ability to form clear images result in refractive errors, which require correction through lenses in the form of spectacles or contact lenses.
How do we see objects on the side?
The retina lines the inside of the choroid and extends forward in front of the middle of the eye; hence, we see objects situated in our line of sight and away from them.
- With both eyes, we get a panoramic vision of 180 degrees peripheral vision.
- Because the eye is roughly spherical, we can also detect objects above and below our line of sight. How far above and below depends on how forward our cheekbones and eyebrows project.
Emmetropia: Power perfection!
Emmetropia is a term used to state when the eye has no refractive error and can focus the light rays coming from a distance onto the retina.
PC – www.northcincinatieyecare.com
Ametropia
Ametropia is a term used to state when the eye has a refractive error and cannot focus the light rays coming from a distance onto the retina.
Figure 3: Types of refractive errors
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Types of refractive errors:
- Myopia or short-sightedness or nearsightedness
In myopia, the eye focuses light rays in front of the retina. The eyeball length is longer than usual; hence the image focuses in front of the retina and not on it. The person with myopia notices blurring of distant objects while objects at close distances appear clear.
Figure 4: Nearsighted vision versus corrected vision
A person with myopia prefers doing near work and avoids outdoor activities. He/she will seldom complain of eyestrain or headache associated with near work.
PC – www.eyebar2squarespace.com
Figure 5: Myopia: Near work’s best friend
Photo URL: https://www.shutterstock.com/image-photo/cute-indian-child-studying-home-1689916510
How is myopia corrected?
Myopia is corrected with minus lenses or concave lenses to focus the light rays on the retina. The minus lenses can be provided in the form of spectacles or contact lenses.
PC – www.hydrogen.physik.univ
- Hyperopia or hypermetropia or farsightedness
PC – www.weyeclinic.sg
In hyperopia, the eye focuses the light rays behind the retina. The eyeball length is shorter than usual, because of which the image is focused behind the retina and not on it. A person with hyperopia notices that near vision is not as clear as distance vision.
Corrected vision Hyperopic vision
Figure 6: Hyperopic vision versus corrected vision
Corrected vision Hyperopic vision
The crystalline lens can also change shape (accommodate) to add plus power. Prolonged accommodation can lead to symptoms of eyestrain and headaches. Hyperopes may predominantly complain of the above symptoms and avoid near work.
Hyperopia or farsightedness symptoms
- Blurred vision farther near than distance
- Aching eyes
- Eyestrain
- Headaches
- Rubbing eyes
- Feel sleepy or tired with near work
How is hyperopia corrected?
Hyperopia is corrected with plus lenses or convex lenses to focus the light rays on the retina. The plus lenses can be provided in the form of spectacles or contact lenses.
- Astigmatism
In astigmatism, a point object does not form a point image, and the light rays do not come to a single focus. This is because the light rays reaching the central cornea are not spherical. Hence the person notices blurred vision at all distances.
Figure 7: Vision with astigmatism versus normal vision
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Pc – www.baeyesite.com
How is astigmatism corrected?
A cylindrical lens is prescribed to correct astigmatism. This cylindrical lens needs to be placed at the correct axis to get a sharp focus of the image on the retina. An astigmatism correction can be prescribed in isolation or with the myopic or hyperopic correction.
PC – www.hyperphysics.phy-astr.gsu.edu
- Presbyopia
Presbyopia is a condition resulting in a gradual loss of the eye’s ability to focus on near objects. This is a part of normal ageing as the crystalline lens hardens/stiffens with age.
The muscles that hold the lens in place become weaker. The lens’s ability to accommodate or change its shape and, therefore, power diminishes, resulting in presbyopia. Presbyopia sets in during the early and mid-40s and progresses with age.
Symptoms of presbyopia
Patients complain of difficulty reading the small print at the usual reading distance.
Figure 8: Uncorrected presbyopia
Photo URL: https://www.shutterstock.com/image-photo/hyperopia-woman-squinting-holding-phone-far-1324696955
How is presbyopia corrected?
Patients are prescribed plus lenses called addition or add to see objects at close distances. Presbyopia can occur in isolation, as a normal ageing process that requires reading glasses alone or along with an existing distance vision prescription, in which case requires bifocals or multifocal lenses.
Can refractive errors be prevented?
Refractive errors cannot be prevented but can be diagnosed during an eye examination and corrected using spectacles, contact lenses or refractive surgery. It is essential to diagnose the refractive errors at the earliest, so the visual development and functioning of the eye is not affected.
Summary:
The Eye is a complex organ that lets us enjoy the world as it is. The eyeball consists of several parts that play a unique role in providing vision –
- Cornea
- Conjunctiva
- Crystalline lens
- Pupil
- Iris
- Sclera
- Choroid
- Retina
- Optic nerve
- Extraocular muscles
The eye captures the image through the retina and sends the signal to the brain, which helps us perceive the image. The four types of refractive errors are myopia, hypermetropia, astigmatism, and presbyopia. These are corrected with glasses or contact lenses.
Besides presbyopia, early correction of refractive errors in children is crucial to prevent any damage to the eye’s visual development and normal functioning.