The eye makes up one of our five sensory organs and it’s structure begins with the curved clear, bubble-shaped cornea at the front and the optic nerve extending from the back. The white part of your eye is known as the sclera and contains collagen and elastic fibers. Located inside the eye is the coloured iris, this is a thin layer of muscles which expand and contract to allow varying amounts of light to pass into the surface of the eye. The size of the opening in the iris, the pupil, is controlled by muscles located in the iris. And when faced with an excess of light, the muscles make the pupil smaller and when there is less light it expands to widen the pupil. Directly behind the iris is the ciliary body and the lens is attached to this by thin thread-like structures. A fluid is secreted by the ciliary body and circulates around the pupil and into the anterior chamber before returning out through the chamber angle. The lens is a small clear disk which is very dry and the lenses of younger people the are highly flexible and are able to change form through becoming thicker or thinner. As the lens becomes thin you are able to see far and similarly, when it grows thicker you can see close-up objects . This is the reason why young people can see equally well at both long and short distances. As we age the lens becomes bigger and harder and can’t change form as easily.
Vitreous is a jelly like fluid located behind the eyes which contains tiny threads and particles which we can’t usually see. In some cases the threads (known as vitreous floaters) can become visible in the images we see. For instance they may appear when gazing at a clear sky or snow. It can appear as if there were a network of fine threads floating in the air. These vitreous floaters can appear randomly, they may disappear for years and then suddenly reappear. As we age the supporting threads of the vitreous can become detached from the surface of the retina and sometimes a tiny piece of retina may also become detached. This small hole which is left in the retina often heals of its own accord but in some cases will need to be surgically sealed back. Rays of light pass through the cornea, due to the curve in it’s surface it refracts the light rays which are, in turn, squeezed closer together in order to pass through the pupil. They then pass through the lens which has two curved surfaces at the front and rear, these bend the light rays two more times as they journey to the back of the eye. Inside the cells in the retina (photosensitive cells) is an appendage of the cell called the axon. This joins with other axons to make up the optic nerve which joins all the way to the brain stem at the top of the spinal cord. Each axon connects there with cells and the axon of the receiving cells travels to the occipital lobe where is connects with brain cells to produce vision.
Eyes which give the best vision tend have a decent balance between the length of the eye and the curvatures of the refracting surfaces, a clear path from the front of the eye to the back and properly functioning cells in the brain and retina which allow the light to be converted to electrical energy and interpreted into vision. Eyes that are too long or contain too much refracting power are nearsighted. The opposite occurs when eyes are too short or contain too little refracting power, creating farsighted eyes. When light rays which are oriented vertically aren’t refracted the same amount as the rays that are horizontally oriented, this condition is known as astigmatism. If the eye is unable to adjust to near images by increasing its refractive powers then a condition known as presbyopia occurs even if the eye possesses no refractive error. A loss of clarity can occur when the structures through which light rays pass interfere with their interpretation within the brain. Disorders which may cause this absorption of light rays are swelling of the cornea, opacification of the lens (or cataract) and a cloudiness of the vitreous. Similarly, the retina must also function properly. In age related macular degeneration there is a deterioration of the macula which is used for reading and other types of vision. Several diseases can cause grey patches, known as opacities, to appear in your cornea. This can cause your vision to become blurred as the cornea clouds.