Structure of Human Eye

                              Structure of human eyes

Structure of the eye is an important topic to understand as it is one of the important sensory organs in the human body.

It is mainly responsible for vision, differentiation of color and maintaining the biological clock of the human body.

The human eye can be compared to a camera as both works by gathering, focusing, and transmitting the light through the lens for creating an image of an object.

To understand more in detail about our eye and how our eye functions, we need to look into the structure of the human eye.

So let’s see the structure in detail…………………………….

The human eyes are the most complicated sense organs in the human body.

The human eyes are situated in bony orbital sockets on the anterior aspect of the head and cushioned in fatty connective tissue.

They are directed forward and adapted for binocular or stereoscopic vision. Eyes are spherical hence each eye is often referred to as eyeball.

 

First, let’s see the outer protective structure of eyes:

·        Eyebrows: These are hairy arched ridges present above the orbits. Eyebrows protect the eyes from dust, and sweat.

·        Eye Lids: The upper and lower eyelids has eyelashes and blink at regular intervals to protect eyes from injury, dust and germs.

·        Conjunctiva:  The conjunctiva is a thin, transparent layer of tissues covering the front of the eye. It keeps our eyes moist and clear and provides lubrication by secreting mucus and tears.

 

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·        Eye muscle: Each eye ball contains 4 rectus, 2 oblique and 1 levator muscles.

·        Meibomian Glands: The secretion of Meibomian gland helps in frictionless blinking.

·        Lacrimal Apparatus: The lacrimal apparatus of eye consists of a lacrimal gland and various ducts. The lacrimal gland secrets an alkaline watery fluids called tears. Tears keep the eyes moist, it clean cornea and it kills bacteria as it contain Lysozyme.

 

 

Now let’s see the parts of the eye that are visible externally.

The External Structure of an Eye

Sclera: It is a white visible portion. It is made up of dense connective tissue and protects the inner parts.

Choroid: It is a layer of connective tissue and blood vessels. It provides nourishment to the outer layers of the retina.

Cornea:    The cornea is the transparent, clear layer at the front and center of the eye. In fact, the cornea is so clear that one may not even realize it is there. The cornea is located just in front of the iris, which is the colored part of the eye. The main purpose of the cornea is to help focus light as it enters the eye

Iris: It is the pigmented, colored portion of the eye, visible externally. The main function of the iris is to control the diameter of the pupil according to the light source.

Pupil: It is the small aperture located in the centre of the Iris. It allows light to enter and focus on the retina.

The Internal Structure of an Eye

The internal components of an eye are:

Lens: It is a transparent, clear, flexible biconvex, structure of an eye. The lens is attached to the ciliary body by ligaments. The lens along with cornea refracts light so that it focuses on the retina.

Ciliary body: The ciliary body contain a smooth ciliary muscle. It provide attachment to suspensory ligaments and iris. The ciliary body controls the shape of the lens, and contributes to the formation of aqueous humor.

Retina: It is the innermost layer of the eye. It is light sensitive and acts as a film of a camera. Three layers of neural cells are present in them, they are ganglion, bipolar and photoreceptor cells. It converts the image into electrical nerve impulses for the visual perception by the brain.

The most sensitive part of the retina is a small area called the macula, which has millions of tightly packed photoreceptors.

The photoreceptors in the retina convert the image into electrical signals, which are carried to the brain by the optic nerve. There are two main types of photoreceptors: cones and rods.

Cones are responsible for sharp, detailed central vision and color vision and are clustered mainly in the macula. The rod cells are elongated composed of photosensitive pigments called rhodopsin.

Rods are responsible for night and peripheral vision. Rods are more numerous than cones and much more sensitive to light, but they do not register color or contribute to detailed central vision as the cones do. Rods are grouped mainly in the peripheral areas of the retina.

 

Optic nerve: The optic nerve, a bundle of over 1 million nerve fibers, is responsible for transmitting nerve signals from the eye to the brain. These nerve signals contain information for processing by the brain. The front surface of the optic nerve, which is visible on the retina, is called the optic disk or optic nerve head.

Aqueous Humour: It is a watery fluid present between the cornea and the lens. This fluid contain glucose, amino acid and respiratory gases. It nourishes the eye and keeps it inflated.

Vitreous Humour: it is a transparent, jelly-like substance present between the lens and the retina. It contains water (99%), some salts, proteins, etc. The main function of vitreous humour is to protect eyes and maintain its spherical shape.

How our eyes work

The process of seeing begins when light waves enter the front of the eye. Brightness and distinct colours are first interpreted by structure in the back of the eye, and then sent as stimulus signals that the brain interprets as vision.

On the exterior of eye there is an area, called the cornea that includes pupil, a hole through light enters the eye.

Inside the eye light is refracted by lens and focused on to the retina.

The retina include two types of nervous system cells- cones that interpret colour of light waves, and rod cells that interpret the intensity of light.

These photoreceptors process information into nerve signals that travel through the optic nerve into the occipital lobes of the brain, where signals are interpreted to represent an image.

 

Types of vision defect

The shape of the eyeball affect vision. In normal vision the lens focus light on to the retina and an image is perceived.

In case of nearsightedness, the point of focus does not reach the retina and the image appears blurry.

Farsightedness results when the eye is too shallow, causing the image to be focus on an area beyond the retina.