MAIN PARTS OF THE EYE

In the following, I will discuss the main physical structures of the eye in a more detailed way.

The main components of the eye include : à

Cornea à                        Sclera à

Iris à                              Pupil à

Aqueous Humor à          Lens à

Choroid à                      Ciliary muscles à

Retina à                         Macula à

Blind Spot à                   Vitreous Humor à

Optic Nerve –

Cornea

The cornea is the outermost layer of the eye through which the light passes and is focused into the eye. It contains no blood vessels but does contain many small nerve fibres. It is comprised of several layers so that the optical properties are measured for the whole cornea rather than generalizing from a tissue sample. Convexity of a radius = 8mm

                         Refractive index = 1.3771

Sclera

The sclera is a tough white or non-transparent tissue surrounding the cornea at the outer surface of the eye.

Iris

The iris is the part which gives the "colour" to the eye and that explains why the eye colour of the Easterners is brown or black (contain large amount of pigment cells) while the eye colour of the Westerners is usually green or blue (contain least amount of pigment cells). Its main significance is to help regulate the amount of light that enters the eye. When the amount of light is efficient, the circular muscle of the iris diaphragm contracts, the radial muscle relaxes, the pupil becomes smaller and less light enters the eye thus prevents too much light from bleaching the retina and causes damage to the eye. On the other hand, the iris relaxes and the pupil becomes larger when a person is in the dark so allowing sufficient light to enter the eye.

Pupil

The pupil is the dark centre in the middle of the iris. It is not a structure, but an empty space which light passes through before reaching the lens and the retina. It determines how much light is let into the eye by changing sizes to accommodate for the amount of light that is available.

Aqueous Humor

The aqueous humor is the clear liquid between the cornea and the lens. The space that it inhabits is called the anterior chamber. For it is fairly homogenous in nature, its optical properties are easily measured. Refractive index = 1.336

Lens

The lens is the transparent structure inside the eye that focuses light rays onto the retina. It is curved on both sides and is attached to the ciliary muscle which changes the shape of the lens. The change of the shape of lens is called accommodation. Here I give two examples to see how accommodation is carried out:

When focusing on infinity – Ü

  The goal of accommodation :  to get the lens as flat as possible

  Process of accommodation: ciliary muscle relaxes, tension on the zonular fibers is maximized, ø                                              zonular fibers tug on the lens to make it flat

When focusing on your finger Ü

  The goal of accommodation :  to get the lens as round as possible Ü

  Process of accommodation :  ciliary muscle contracts, tension on the zonular fibers is minimized, ø                                                zonular fibers do not pull on the lens and it becomes flat

Choroid

Choroid is the middle layer with rich blood supply which is important to retina. It is also covered with pigment cells that prevent the reflection of light within the eye.

Ciliary muscles

Ciliary muscles are circular sheet of smooth muscle fibres forming bundles of circular and radial muscles which alter the shape of the lens during accommodation.

Retina

The retina is the inner layer and the nerve layer that lines the back of the eye, very much liken a film in camera. It senses light and creates impulses that are then sent through the optic nerve to the brain, that means it converts light energy to electrical energy. Two types of receptor cells are present in retina: rods and cones. There are approximately 100 million rods and 6 million cones in one eye. Dry and colour vision are provided by the cones while the rods provide vision for low illumination and gray scale. The retina receives nourishment by the blood supplied from the outer layer of the eye. Retina is an extremely delicate structure which has a different embryological origin than the rest of the eye. It is one of the most important structure of the eye for we cannot view any image if it is damaged.

Macular

Macular is a small area at the centre of the retina where most of the cones (a kind of light receptor cells) are concentrated there. It provides the main area of acute and accurate daylight vision and allows us to see fine details clearly.

Blind Spot

Blind spot is an area on the retina where the optic nerve leaves the eye. No image is recorded in this specific area for there is no cones and rods, the light-sensitive cells.

Vitreous Humor

Vitreous Humor is a clear, jelly-like substance that fills the middle space between the retina and lens. The space that it fills is called the vitreous body. Generally speaking, it has no particular function at all.

Optic Nerve

Optic Nerve is the nerve that connects the eye to the brain. It carries the impulses formed by the retina to the brain, which interprets them as images.