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Anatomy of the eye.

Understanding the Anatomy of your eyes

 

The human eye has been called the most complex organ in our body. It’s amazing that something so small can have so many working parts. But when you consider how difficult the task of providing vision really is, perhaps it’s no wonder after all.

Anatomy of the eye

A.  The sclera

The sclera is the white part of the eye, its protective outer layer. The optic nerve is attached to the sclera at the back of the eye. With age, the sclera becomes more yellow in colour.

 

B.  The choroid

The choroid is made up of layers of blood vessels that provide oxygen and nourishment to the outer layers of the retina at the back of the eye. It lies between the retina and sclera.

 

C.  The retina

The retina is the part of the eye which senses light. It contains cells called photoreceptors which capture light rays and convert them into electrical signals. The signals are sent via nerve cells called retinal ganglion cells (together known as the optic nerve) to the brain. There are two type of photoreceptors: rods and cones. The rods function best in dim light and are responsible for peripheral vision – the side or edges of what is seen. The cones are required to see in bright light and in detail. They are responsible for colour vision.

 

D.  The optic nerve

The optic nerve transmits visual information – what is seen – in the form of electrical impulses from the retina to the brain. The photoreceptor cells of the retina are not present in the optic nerve. This means people have a blind spot in their field of vision at the point on the retina where the optic nerve leads back into the brain. This is not normally noticeable because the vision of one eye overlaps with that of the other.

 

E.  The fovea

The fovea is a small pit near the centre of the macula. It contains cone cells. These help people see colours and see in bright light.

 

F.  The macula

The macular is found at the centre of the retina. It is responsible for central vision and the ability to see detail. When light comes into the eye, it goes to the macular. It has a diameter of approximately 1.5mm.

 

G.  The vitreous gel

The vitreous gel (also known as the vitreous humour) is a clear, thick, substance that fills the centre of the eye. It is mostly made of water. It makes up approximately 2/3 of the eye’s volume and gives it its shape. The vitreous gel helps keep the retina in place.

 

H.  The lens

The lens is behind the iris. Its job is to focus light on to the retina. The shape of the lens is curved. It is flexible and is controlled by the nerves and muscles around it. The amount the lens curves changes to enable the eye to focus on objects at different distances.

 

I.  The iris

The iris gives the eye its colour. Genes, which people inherit from their parents, determine eye colour. The main function of the iris is to control the amount of light that is let into the eye. In bright light the muscles contract. This causes the opening at the centre of the iris (the pupil) to become smaller. In dim light the muscles dilate. This makes the iris wider and allows more light into the eye.

 

J.  The cornea

The cornea is the clear surface on the front of the eye. It is about half a millimetre thick. It has two main functions. First, it acts as a barrier preventing germs, dirt and other harmful material from getting into the eye. Secondly, the cornea acts as a lens which controls how light enters the eye. The cornea helps the eye to focus on what is being seen.

 

K.  The pupil

The pupil is the black dot at the centre of the iris. It is an opening which lets light into the eye. The pupil changes size depending on how light or dark it is.

 

How The Eye Works

In a number of ways, the human eye works much like a digital camera:

  1. Light is focused primarily by the cornea — the clear front surface of the eye, which acts like a camera lens.
  2. The iris of the eye functions like the diaphragm of a camera, controlling the amount of light reaching the back of the eye by automatically adjusting the size of the pupil (aperture).
  3. The eye’s crystalline lens is located directly behind the pupil and further focuses light. Through a process called accommodation, this lens helps the eye automatically focus on near and approaching objects, like an autofocus camera lens.
  4. Light focused by the cornea and crystalline lens (and limited by the iris and pupil) then reaches the retina — the light-sensitive inner lining of the back of the eye. The retina acts like an electronic image sensor of a digital camera, converting optical images into electronic signals. The optic nerve then transmits these signals to the visual cortex — the part of the brain that controls our sense of sight.
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