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Describe the Structure of Human Eye

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The Human Eye: Structure and Functions

The eye is one of the five sensory organs of the human body the other four being nose, ear, tongue, and skin. Because of the bilateral symmetry, humans have two eyes located on either side of the body’s longitudinal axis in two separate eye sockets, on the anterior part of the skull. The human eye is one of the most complex organs in the human body. 

Functions of the Human Eye

The functions of the human eye are:

  1. Reading External Visual Stimuli:

The main function of the human eye is to read the external stimulus of light and form meaningful information. This means that the rays from the external objects are reflected on to the eye which then forms an image of the object on retina lying behind the lens.

  1. Colour Identification

Special cells- rod and cone cells in the eye are responsible for identifying different colours. Colour identification helps in recognising the objects for use.

  1. Colour Differentiation

As mentioned earlier, cells in the eye are responsible for differentiating colours.

  1. Depth Analysis

The depth of an object can be roughly estimated through visual inspection.

  1. Distance Analysis

Visual inspection can help analyse the distance between two objects. 

  1. Biological Clock

The biological clock is dependent on the presence and absence of light to function properly.

Structure of the Human Eye

Each human eye is a perfect sphere made up of muscles and nerves. The internal cavities are filled with fluids. The structure of an eye can be divided into two parts: external eye and internal eye.

The External Portion of the Eye which is Visible From the Outside Consists of the Following Parts: 

  • The Sclera: The sclera is an opaque tough sheath. It is the outermost layer of the eye. It is white in colour.

  • Functions of Sclera: the functions of sclera are:

    1. Protection:  It is a tough sheath which protects the inner layers of the eye

    2. It helps hold the shape of the eye.

    3. It is opaque so it helps in preventing the entered light to escape from the eye.

  • Conjunctiva: it is a thin and transparent membrane that is found spread across the sclera

  • Functions of Conjunctiva:

    1. Helps keep the eyes moist: Conjunctiva secrets a little mucus and tears. It helps the eyes remain moist.

    2. Helps to keep eyes sterile: Any foreign particle getting stuck in the outer surface is washed out through the secretions of the conjunctiva.

    3. Helps keep eyes clear: Moisture in the eye helps the eyes to function better. The sight is improved and the eyes are kept clean and clear.

  • Cornea: The cornea is the transparent membrane that is continuous with the sclera. It is spread over the other two parts (iris and pupil) 

  • Functions of the Cornea :

    1. Refracts light

The primary function of the cornea is to refract the light rays so that it can reach the retina.

  • Iris: This is a pigmented layer of tissue responsible for making up the coloured portion of the eye. The iris can be of a number of colours depending on the genetic constitution of the person

  • Functions of Iris

    1. The primary function of the iris is to control and regulate the opening and closing of the pupil.

  • Pupil: It is the small opening located in the middle of the Iris.

Function of the pupil: It allows the light to enter the eye which is then transmitted through the optic nerve to the brain for completing visual sensation. 

The Internal Structure of the Eye Consists of the Following Parts:

  • Retina

The retina is like a screen at the end of the eye. The retina is the layer on which the images are formed. It is a photosensitive layer. There are a number of receptors in this layer which are sensitive to light. They help to differentiate coloured and dim lights.

Functions of Retina

    • The retina forms images.

  • Lens

The lens in the human eye is a biconvex, transparent and adjustable structure that help in converging the light rays so that they can converge at one point(on the retina) to form an image.

Functions of Lens:

    • Helps in converging light

    • Helps to focus on objects

  • Aqueous Humor: A watery fluid that is found in the area between the cornea and the lens

Functions of Aqueous Humor:

    • Helps maintain the osmotic balance of the eye. 

    • Provides nourishment to the lens and the cornea

  • Vitreous Humor: It is a transparent semi-solid or jelly-like substance that is found between the retina and the lens.

Functions of Vitreous Humor:

    • It helps maintain the shape of the eye.

    • Causes refraction of light inside the eye

  • Optic Nerve: The main sensory nerve responsible for carrying the signal from the eye to the brain. It is located behind the retina.

  • Functions of the Optic Nerve: Takes visual impulses from the eye to the brain.

The area on the retina where the head of the optic nerve meets is called the blind spot or optic disk. There are no photoreceptor cells on the optic disk. Therefore, no image is formed in this area.