Your human eye is an anatomical masterpiece

With an axial length of about 24 millimetres, the main purpose is to be an interface between the outside world and your inner world.

The physical eye captures images of objects around you, after which the cornea, pupil, and lens reproduce a precise copy on your retina.

The crystalline lens automatically deforms itself so that the image is defined, an objective to which the refractive capacity also contributes. Images enter your eye in the form of light rays that strike a focus on the retina. This phenomenon is called refraction.

It is very similar of what happens in a video camera, and in fact there are some similarities, although the human eye is still superior to any device created so far. For example, it is much more sensitive to light. This is the reason we can orient ourselves in a room almost completely dark as in a beach illuminated by a blinding sun.

Compared to your eye, a video camera has a much lower performance.

The muscles of your eye

In your eye, there are six muscles, which work in pairs and allow you to rotate the eyeball in all directions. They are extraordinary muscles, because they can move your eye quickly and precisely, directing your gaze to the object you want to observe.

They adapt in real time and allow you to follow the tennis ball in its flight from one side of the court to the other.

Four of these muscles are called rectus. They insert into the sclera in a fairly advanced position. They originate in fono the orbital cavity.

The muscle that is at the top is called the superior rectus muscle. It serves to move your eye upward while the inferior rectus moves your eye downward.

Horizontal movements are governed by the internal rectus muscle and the external rectus muscle, which are located on either side of each eye.

Together, these four muscles make it possible for your eye to move in all directions.

At the back of your eye is another pair of muscles called the superior oblique muscle and the inferior oblique muscle. They form a sort of belt that surrounds almost the entire middle bulb and allow your eyes to move closer together or farther apart.

In this way, you can direct them to a definite point or follow objects that are coming towards you or moving away. Your upper oblique muscle is connected to the bone next to your nose by a long tendon that comes into action when you look at the tip of your nose with both eyes.

In myopia the back of your eye is constantly pressed outward and the bulb is elongated, which prevents clear vision.

To get an idea of the order of magnitude of these changes, know that each millimetre of bulb lengthening is equivalent to about three diopters of myopia.

The ability to see clearly and properly is thus reduced to about thirty centimetres, more or less the typical reading distance.

The physical changes in your eyes then are tiny, but the consequences are enormous.

In hyperopia, on the other hand, the four rectus muscles are so tight that the bulb becomes shorter.

There are also two intrinsic ring-shaped muscles. One determines the size of the iris and regulates the incidence of light in your eye. The other, the ciliary muscle, is placed around the lens and can bend it, modifying your refractive power.

Cornea

It is a transparent membrane that possesses about 75% of the ability to focus images. Through the cornea, light enters your eye and is refracted.

At the center of the pupil, the cornea is about half a millimetre thick and consists of several layers. The outermost is the tear layer. Its function is nutritive as well as being one of the refractive elements of your eye.

Perhaps, you have already noticed that repeatedly blinking helps you to see better. The surface of the cornea is called the epithelium and is made up of a protective layer of relatively hard surface cells. Their function is to protect your eye from injury.

Wearing contact lenses all the time, especially hard contact lenses, leads to a gradual wear and tear of the corneal epithelium, which can eventually make it impossible to continue wearing lenses.

Just below this is Bowman’s lamina, a layer of collagen cells that is responsible for maintaining the shape of the cornea. If it is damaged during surgery, Bowman’s lamina does not heal never again.

The stroma is the thickest part of the cornea. This is where the laser intervenes: during the operation, part of this layer is vaporized to thin the cornea and modify the refractive power of your eye.

Because the cornea has no blood vessels, it can take up to six months to heal after surgery. Refractive surgery may produce an inevitable weakening of this part of your eye.

The crystalline lens

Your eye’s ability to see, is based largely on your cornea. The crystalline lens also plays a very important role in your eye system.

It is a lens with a diameter of about 10 millimetres, made of transparent proteins.

Through a system of tiny fibers, called Zinn’s ciliary zone, the lens is connected to the ciliary body. The latter helps to achieve a correct accommodation, a process that regulates vision at different distances.

Due to its high water content, the lens is very flexible, which means that it can change shape with the contraction of the suspensory tendons. When your ciliary muscle, with its ring, relaxes, this tendons stretch. As a result, the lens flattens and your ability to focus is reduced. If, on the other hand, they contract, your crystalline lens curves, giving you a better focusing.

During your eye exam, your doctor will sometimes pour a few drops of atropine into your eye to paralyze the ciliary muscle. His idea is that by knocking out these muscles, he may be able to figure out what is the true condition of your eyesight.

According to some, this is the only valid way to get exact results…

Other doctors say that it doesn’t work, because when you have to measure a person’s height you don’t paralyze the spinal column, preventing it from stretching so you can’t pretend to be taller than you really are.

The proteins of the crystalline lens remain unchanged throughout your life. Every year a new layer grows (like onions), so between your twenties and eighties the thickness almost doubles.

The crystalline lens is not full of blood vessels, so it is nourished only by the fluid continuously produced by the ciliary body.

The most important nutrient is vitamin C, in fact in this part of your eye there is a concentration seven times higher than in any other part of the body.

Oxidation processes caused by free radicals dull the lens of the eye, resulting in cataracts. Since your crystalline lens is only a small part of your visual system, if it is surgically removed, you do not lose your sight. Your vision is reduced by 10 percent, but after the removal of the lens you can drive a car perfectly.

Retina

Your retina is one of the most important elements of your eye. It is made up of a thin layer of interconnected, light-sensitive cells that receive light stimuli that the optic nerve transmits to the brain.

Damage to your retina results in permanent vision loss. The most serious diseases that affect it, destroying it, are macular degeneration and diabetic retinopathy.

Another problem is retinal detachment, which can occur in individuals with a high degree of myopia.

Photosensitive cells

You have two different types of cells sensitive to light: the cones and rods.

The stimulus received by these cells is transmitted to the cerebral cortex by means of the optic nerve.

Your rods work when there is little light, in what is called: night vision.

Your cones, on the other hand, are responsible for clear vision and color perception.

The rods are the most numerous: in total, we have about 120 million rods per eye. They are extremely sensitive to weak light and movement. They react to light/dark variations, but do not perceive colors.

Your rods contain a photosensitive pigment: rhodopsin. Each rod is so called because of its elongated shape. This cell is made up of a thousand tiny discs each containing about ten thousand molecules of rhodopsin.

Each molecule can capture one photon of light. Given the enormous number of rhodopsin molecules, the ability to capture light is enormous.

If light hits a rod, the rhodopsin splits. It only takes a single amount of light to split a rhodopsin molecule. The sensitivity of your eye is therefore closely related to the properties of rhodopsin.

The rods are arranged mainly around the fovea centralis and therefore when you look at an object in the dark you have to look at it from the side of your eye to focus it. In the central part, there are no rods and your cones are neutralized when there is an absence of light.

The cones cluster in the center of the fovea centralis, they are located immediately behind your iris. The perception by the cones is called photopic vision.

There are three types of cones: those that react to low frequencies, those that react to medium frequencies, and those that react to high frequencies. Depending on the type, the photosensitive pigments in these cells react to the long waves of red light, the medium waves of green light, or the long waves of blue light.

The three primary colors red, green and blue allow you to see all the colors of the spectrum. Why? Because by mixing them, you can produce any shade of color.

In each eye, there are about six million cones, distributed throughout the retina. Although they are concentrated in the center of the fovea, you will only find four percent of the total cones.

Curiously, the cones that react to blue light are not present in your central fovea, but all around it.

This is why you cannot perceive small blue objects if you stare directly at them.

The macula lutea

The central area of your retina is called the macula lutea. It is covered by a yellow pigment formed by carotenoids.

In the center of the macula is the fovea centralis, the part of your eye responsible for the sharpest vision and color perception.

In the fovea centralis the neurons move to the side, in this way the light directly hits the cones below. Precisely in this area they reach the highest concentration, amounting to about 150,000 cones per square millimetre.

These cells are also connected with a much larger area of the visual cortex, which allows you to have very sharp vision.

Photo-sensitivity

The values for resolution acuity and photosensitivity depend largely on how your rods and cones are connected to the retina.

The connection of the rods with the retina makes them, compared to the cones, extremely sensitive to light. However, their resolution capacity is much lower.

To make sure that this is the case, Nature has made sure that many of your rods are connected in parallel to a single nerve fiber (retinal ganglion cell).

By adapting to the dark, your rods (Night Vision) achieve a much higher sensitivity than the cone system.

Your cones (Day Vision) have greater power at the expense of light sensitivity.

Before reporting an event, these cells must absorb an amount of light equal to ten quantum of photon. In addition, the light must be present within a certain time, otherwise the stimulus is lost.

An examination of your eyes

There are several types of tests to find the necessary lens grading to compensate for refractive defects and give you an optimal visual capacity.

In the first one the optician uses a technical instrument to get an objective result.

The second system is subjective and when you undergo the test you look through various lenses to determine which ones you see best.

Usually the test is carried out with a dim light, but this method presents some problems. Very often your eyes try to adapt to the different lenses, so you usually end up with glasses that are too strong.

You may have experienced a strong discomfort the day after your eye examination when trying on your glasses again. This is due to the fact that the lenses are too strong and cause over-correction.

The visual acuity of your eye varies by up to two diopters during the course of the day. If you measure it every two hours, the result will never be the same.

Vision control

Some professionals are opposed to prescribing glasses with a reduced graduation scale. If yours is one of them, we recommend that you look for another one.

Ask the optician to measure your eyesight with his machines. It must be said that in reality, the values provided by their automatic instruments are nothing more than rough estimates.

Test results are variable and the margin of error tolerated by the machines is half a diopter over or under.

Once the measurement is complete, your optician will correct the detected problem and bring your visual acuity up to 100%.

If you feel that your eyes are hurting, ask them to reduce the correction by half a diopter or one diopter. Then go outside and look around with the new lenses to see if the correction is working.

It’s not enough to try the glasses inside the store or mall, you have to see how the world looks to you in natural sunlight.

Lenses should be chosen so that distant objects are slightly blurred. Do not reduce the magnification by more than one diopter or you risk putting too much strain on your eyes.

In order for your eyes to relax and maybe even regenerate, your eyes must be full of energy and not tired.

Glossary

Accommodation: Adjustment of your eye that allows it to see at different distances

Amblyopia: Decrease in vision not caused by organic defects

Anisometropia: Condition in which the eyes have a different refractive capacity

Antioxidants: Substances that neutralize free radicals and prevent oxidative stress

Astigmatism: Disorder due to an abnormal curvature of the cornea that causes poor clarity of lines in a particular axis

Binocular: Affecting both eyes

Keratoconus: Conical deformation of the cornea

Convergence: Positioning of the axes of your eyes that allows you to fixate on nearby objects

Macular Degeneration: Decay of nerve cells in the macula lutea leading to loss of central vision

Deprivation: Lack of stimulation

Derivative: Compound derived from another compound

Diopter: Unit of measurement of refractive power; used to describe refractive defects and their correction

Focal distance: The distance at which parallel light rays meet at a focal point behind the lens

Divergence: Deviation of the axis of your eye toward the outer part

Photoreceptors: Nerve cells in the retina (cones and rods) that serve to absorb all light stimuli

Photosensitive: Which changes when light enters your eye

Phovea Centralis: Dimple in the central part of the retina, the point of sharpest vision

Fusion: Merging the images of your right and left eye to form a single representation

Hyperopia: Long vision

Hyperopia: Lesser synonym for hyperopia

Bifocal Lenses: Spectacle lenses with two focuses (for distance and near vision)

Prismatic Lenses: Special lenses to correct the optical axis in the presence of latent strabismus.

Macula Lutea: Yellow spot in the retina, contains the most important sensory cells of your eye and makes it possible to see clearly.

Meridian: Energy channel

Monovision: Use of one eye for near vision, the other for far vision

Oculomotor: Connected with ocular motility

Ophthalmology: study of your eye

Optometry: Discipline that measures your visual acuity

Oxidation: Chemical reaction in which a substance combines with oxygen

Proximal Point: The closest point to your eye where you can still have clear vision

Remote Point: The farthest point that your eye can focus without accommodation

Free Radicals: Atoms and molecules with one or more unpaired electrons. An excess of free radicals destroys the essential components of the cell, membranes and genetic material.

Strabismus: Deviation of one eye from the normal direction of gaze.