How human eye works?

The eye plays an essential role in the body to move around.

This organ helps to offer support to our vision that brings out the awareness of our surroundings.

About 8 out of 10 learning cues come from our sight.

The function of a camera carries a resemblance with the human eye.

When light from the outside world falls on an object, it reflects in our eyes and enters through the pupil into the eye through the optical elements.

In this article, we will study some of the principal portions and applications of the eye.

The components of the human eye

The anterior or front-end of the eye is composed of the lens, cornea, iris, and pupil.

The image is focused into the retina - an optical membrane covering the posterior or hind-end of the eye.

The optical membrane or retina comprises numerous (millions in number) nerve cells collected together at the hind end of the eye as a large nerve, also known as the optic nerve.

As light enters the eye, it then reaches the macula, which is situated at the posterior portion of the eye.

The macula is a tiny portion or area that lies in the middle of the retina.

This macula helps provide visual details that enable one to identify letters, colors, or faces with more clarity.

As light falls on the retina, it stimulates the nerve cells as it sends neural impulses from the optic nerve into the brain.

These “pair” of optic nerves from both the eyes connect within the brain.

Visual perception occurs when the brain processes the combined information from both the optic nerves, resulting in vision.

Cornea

The cornea is the front, clear and transparent portion of the eye that lies before the pupil and the iris.

Light can enter the eye through this cornea.

The cornea consists of five layers which are –

1. Corneal Epithelium 

Measuring about 50 microns and with a cell thickness ranging between 5 to 7 cells, the epithelial cells on the outer corneal epithelium are formed and destroyed continuously.

It takes around a week for the complete corneal epithelium to turnover.

2. Bowman’s layer 

The change between the underlying stroma and the corneal epithelium is formed by the thick sheet of connective fiber tissue called the "Bowman's layer," which has a thickness of 8 to 14 microns.

3. Corneal Stroma 

Connective tissue strands of collagen fibrils make the central cornea layer of corneal stroma about 500 microns thick.

The individual fibrils carry a uniform size with a parallel arrangement to the cornea's surface, known as lamellae.

There are about 200 to 300 of these lamellae.

The cornea remains completely clear, with the lamellae having a standardized structure with perfect spacing. 

4. Descemet’s Membrane 

The underlying endothelial corneal layer is separated from the stroma by the extremely thin Descemet’s Membrane.

The membrane begins by being 5 microns thick among children to about 15 microns thick among elders.

The Descemet’s Membrane keeps thickening with time and age.

5. The corneal endothelium 

Bearing a thickness of a single cell layer with a measure of about 5 microns, the corneal endothelium forms the innermost layer of the cornea.

The rear portion of the endothelium contains clear aqueous humor, which fills up the empty gap between the iris and pupil and the cornea.

The majority of these endothelial cells appear in a six-sided or hexagonal shape.

However, some of these cells might even carry five or seven sides.

Pupil

The central opening at the iris is called the pupil, which lets light pass through to reach the retina.

The pupils come in complete round shapes, and their sizes remain similar.

The pupil appears "black" in color since the incoming light that enters it does not escape it.

Both the pupil and the iris manages the concentration of incoming light.

The muscles that lie inside the iris control the size of the pupil.

These iris muscles either dilate (or opens) the pupil to widen its size or close the opening and shortens its size.

The change in the size of the pupil helps to deal with the amount of light that reaches the eye.

In normal cases, on a bright light, a pupil size varies about 2 to 4 millimeters in diameter, and in dark conditions, the diameter can increase to about 4 mm to 8 mm.

Retina

The sensory membrane lining the back portion of the eyeball is the retina.

This retina consists of multiple layers along with the specialized photoreceptor cells.

The human eye comprises a pair of photoreceptor cells which are, rods and cones.

The rod photoreceptors perform well in reduced light conditions and also provide black-and-white vision.

The cones provide color vision and central vision as they work well in medium or bright light.

While the retina consists of rods spread across its surface, the cones remain concentrated at the core of the retina.

The depression in the center of the macula is the fovea, which carries cone receptors responsible for colored vision.

Choroid

The mid-tissue layer on the eyewall is called the choroid that appears between the retina and sclera.

The primary constituent of the choroid is blood vessels which help to supply nutrients and oxygen on the external portion of the retina.

The layers of the choroid start from the border edge of the eyeball that runs along its back.

The choroid layer is compressed between the retina and the sclera.

The choroid thickness depends according to its lining area.

In general, the choroid is the thickest at the posterior end at about 0.2 mm and is thinnest toward the peripheral end at around 0.1 mm.

Conclusion

The eye is a crucial organ as it helps the body to visualize its surroundings and circumstances.

Coupled with the brain's cognitive capacity, the eye enables the individual to make better and informed decisions.

The scientific facts and photogenic principles of the eye have inspired scientists towards the invention of the photographic camera.