You may have wondered how your eyes actually allow you to see. There are many parts in the eyes and brain responsible for this, and each one has a unique role. Some are directly used for seeing, some are for support, and others are for protection of the other structures. Read on to learn about each one.
At the front of the eye is the cornea, a clear layer of tissue that transmits light further to the back of the eye. Extending outwards from the cornea is the sclera, the white protective part of the eye, and the conjunctiva, which overlies the sclera and supports the tissues around it.
The cornea both protects the inner structures of the eye by being tough and also provides most of the refractive or focusing power of the eye, around two thirds of the total refraction. The cornea is covered completely on the outside by the tears, composed of water, oils, mucous, and various nutrients and ions.
These tears are produced by small glands in the eyelids and by a larger gland, called the lacrimal gland, each one located in the upper outer corner of its corresponding eye. The eyelids also function to spread the tears over the cornea, keeping it lubricated and healthy, and protect it and the inner structures.
On the interior side of the cornea is the anterior chamber, filled with aqueous humour. This nourishes the cells at the back side of the cornea and the lens while filling up this part of the eye like a balloon so that it has a rigid and functional structure.
At the back border of the anterior chamber lies the iris, the colored part of the eye that controls the size of the pupil, the dark circle in the center of it. The iris responds to light hitting the back of the eye to shrink the pupil in the light and widen it in the dark.
Further into the eye lies the lens, attached by rope-like fibers to the ciliary body that surrounds it. The ciliary body secretes the aqueous humour and pulls on the lens to enable the changing of focus from distance to near. It is this ability that is lost with age in everyone and leads many of us to require reading glasses or progressive lenses.
The lens provides the other one third of the total refractive power of the eye. These structures lie in the posterior chamber near the midpoint of the eye.
Immediately behind the lens and posterior chamber is the vitreous humour, making up the vitreous chamber. This is a gelatinous mass that fills the eye up so that it is round and can fulfill its optical purpose. This entire space, from the lens in the middle to the retina at the very back of the eye, is filled with the vitreous.
Next comes the retina, the part of the eye that actually does the sensing of light. The neuron cells here transmit the light into nervous signals that are then sent to various parts of the brain in several steps for processing and the ultimate formation of vision.
Since the retina is at the back, all structures ahead of it need to be transparent and kept in perfect condition for light to reach it: the tears, cornea, aqueous humour, pupil, lens, and vitreous humour. Behind the retina is the choroid, full of blood vessels that nourish the hard working retinal cells, and behind that is the back side of the sclera.