The Eye

(Bud Helisson)

The eye is an organ made of many layers of specialised cells which work together to detect light, colour and depth. It consists of many cells and tissues to carry out specific roles;

• Sclera. The Sclera is the white outer layer of the eye. It is tough and strong to prevent damage
• Cornea. The cornea is the transport section of the sclera which allows light to be passed into the eye. It is curved to change the direction of light rays entering in order to focus light rays onto the retina. 
• Optic Nerve. The Optic nerve carries impulses from the retina and into the brain to generate visual information.
• Retina. The retina is a light sensitive layer made up of light receptor cells (rod cells) and colour receptor cells (cone cells). When the rod and cone cells are stimulated, the retina generates impulses which are sent along the sensory neurons in the optic nerve.
• Lens. The lens is a transparent disk that focuses light rays onto the retina. It is held in place by the ciliary muscles and suspensory ligaments.
• Iris. The iris controls the size of the pupil and subsequently the amount of light able to enter the eye. It is made of circular and radial muscles.
• Blind Spot. The blind spot is a part of the retina with no receptor cells as it is where the optic nerve reaches the retina. We cannot see from this part as there are no cells to detect stimuli!

My wonderful artists impression of the eye

The receptor cells allow our eye to function as they detect the very things we see. The rod and cone cells are spread among the back of the eye on the retina, which generates the impulses from the light/colour stimulation and passes these impulses down the optic nerve and into the brain. Cone cells are responsible for colour vision and function best in bright light (this is why we struggle to see colour at night). Rod cells are responsible for detecting light intensity They function best in less intense light, they are more involved in peripheral vision and more sensitive than cone cells. The rod cells are almost entirely responsible for night vision and the way we see when it is dark.

So how do we respond to light and dark? We respond to light and dark by deciding (subconsciously) how much light we need to let into the eye. If it is light (high light intensity), more light will enter the eyes so our eyes will contract (shrink). If it is dark (low light intensity) our eyes will need more light to enter the eyes so the pupils will contract (widen) to allow more light in. It is important to know this so summarised;

• DARK. More light needs to enter eye, pupil will dilate. Radial muscles will contract and ciliary muscles will relax
• LIGHT. Less light needs to enter eye, pupil will contract, Radial muscles will relax and ciliary muscles will contract.

As well as reacting to light intensity, the eye must also be able to focus on objects that are near, far (wherever they are!) and moving. To focus, we alter the shape of the lens using the ciliary muscles and suspensory ligaments. 

• FAR AWAY. If an object is far away, the ciliary muscles relax and the suspensory ligaments are pulled tight. This flattens the lens which causes less light to be refracted onto the retina
• CLOSE UP. If an object is close up, the ciliary muscles tighten and the suspensory ligaments are loose. This causes the lens to bulge and more light to be refracted onto the retina.

Typical problems of the eye are hyperopia (long-sightedness) and myopia (short-sightedness) which are often caused by a misshapen lens. Hyperopia is often caused by a flatter lens which means less light is refracted causing a person so struggle to see objects close up. This lack of refraction when needed means that the images aren't reflected onto the retina soon enough. To treat this we can use a convex lens which increases the refraction. 

Alternatively, myopia is often caused by a more bulged lens causing too much light to be refracted onto the retina and the light rays are crossed/refracted too soon. To treat this, we can use a concave lens to reduce refraction so that the light rays are reflected onto the retina correctly.

There are three main ways of treating myopia and hyperopia;

• Contact lenses. The lenses are placed onto the eye to increase or decrease amounts of refracted light onto the retina. Soft lenses can be worn every day for a month and should be taken out at night and sterilised. 
• Laser eye surgery. This is only available to adults when their eye has fully grown and stopped changing- vision becomes more stable. The lasers can reduce the thickness of the cornea to treat myopia (short-sighted) so that less light is refracted.
• Replacement lenses. Most recent treatments of hyperopia and myopia are to replace the lens by either adding a permanent lens inside the eye to correct lens refraction or to replace the lens completely with an artificial lens. The problem with adding a permanent lens on top of the original cornea is that is can cause cataracts in the natural lens and also cause infections.