Vision Health – The anatomy of the eye

eye

The human visual system is comprised of many structures that are necessary to give us vision. Understanding the different parts of the eye helps give us a better understanding of how the eyes work and what happens when things go wrong. Enclosed in the protective layers of eyelashes and eyelids, the main parts of the eye include the cornea, the sclera, the lens, the iris/pupil and the retina. These functional parts  appear on the surface of the eyeball which is controls by connective eye muscles and a network of visual nerves going to the brain. Let’s take a closer look at each part individually:

The cornea. The clear, round, central window in the very front of your eyeball is called the cornea. The cornea is made up of five curved transparent layers of tissue that protect the eye while allowing light to pass through to the retina. The cornea has the important job helping the lens bend and refract light so that images can focus on the retina and be interpreted as clear images in the brain. Interestingly, the cornea is the only part of the eye that does not have a blood supply however, it does contain several tiny nerves that make it very sensitive to pain when touched or scratched.

The sclera. The whites of our eyes are made up of tightly woven fibers called the sclera. The sclera protects the eyeball from injury and helps give our eyes its spherical shape. Connected to the back of the sclera are several muscles that pull on this white ball of fibers to move the eye to what we want to see.

The iris and pupil. The colored, circular part of the eye is called the iris which is centred by the dark pupil. The pupil is not actually a physical structure but a circular space in the iris that allows light to pass through. The colour of the iris is determined by the amount of melanin in its cells; brown eyes have more melanin while blue or green eyes have less. The pupils and iris dilate and contract to adjust the amount of light that travels through to the lens, similar to the action of an old-fashion camera.

The lens. Shaped like a lentil, the lens is located behind the iris where it hangs suspended in its own clear capsular bag. Like the cornea, the crystal-like structure of the lens also bends and refracts light so that images appear sharp and clear on the retina. The lens is mainly comprised of clear proteins that allow light to pass through. As we age, these proteins continually grow making the lens larger and less clear with yellow-brown deposits. These cloudy deposits are called cataracts.

The retina and macula. The thin, complex layer of nerve tissue that lines the inside back wall of the eyeball is called the retina. The small central area of the retina is called the macula and this area is responsible for how much detail we see. The retina and macula contain rods and cones that translate light rays into electrical signals that are sent to the brain. The rods process black and white information, while the cones handle all the colours in between. The highest concentration of cones are located in macula, which is why it handles more detailed coloured images. As we age, the layers of the macula can be become damaged, weakened and scarred resulting in progressive loss of central vision and perception of details.

References:

All about your eyes: A Practical Guide in Plain English from the Physicians at Duke University Eye Center. Edited by Sharon Fekrat and Jennifer S. Weizer. Duke University Press 2006. Chapter 1, pages 3-7.

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