The eyes are the most complex and largest of the sensory organs in our body. Their function is to record the images of our surroundings that the brain will interpret. Although the eye measures only about an inch in diameter, it helps to visualize and collect information of what is going on around us.
Our eyes work similar to a camera as they too have lenses that focus the incoming light and transform it into a “picture”. A specialized cell layer of the eye called retina acts as a film that helps produce an image.
Usually, whenever we look into our eyes, we can see the following structures:
- Iris: A circular muscle which is pigmented gives the eye colour and controls the size of the pupil by enlarging in dim light and contracting in bright light. The central aperture of the iris is known as the pupil. The colour of the iris varies depending on the amounts of eumelanin (brown/black pigment) and pheomelanin (red/yellow pigment) produced by melanocytes. More amount of eumelanin gives a brown colour whereas more of pheomelanin is seen in blue or green- eyed people.
- Pupil: It is an opening within the centre of the iris. It acts to control the amount of light entering the eye by adjusting its size. It appears black because of the presence of absorbing pigments in the retina.
- Cornea: The cornea is the clear, transparent front layer of the eye through which light passes. It covers both the pupil and the iris.
- Sclera: The sclera is the opaque white outer layer which forms the supporting wall of the eyeball. This layer is in continuity with the cornea and the dura of the central nervous system.
- Conjunctiva: The conjunctiva is a thin, transparent mucous membrane which lines the inner eyelid surface as well as the outer surface of the eyeball except for the cornea. It prevents the bacteria and other microbes entering the eye.
If we take a cross-section of the eyeball, the following structures can be identified:
- Three different layers:
- The external layer: It is made up of the sclera and the cornea.
- The intermediate layer: The intermediate layer is divided into two parts, namely anterior and posterior. Anterior part consists of the iris and the ciliary body and posterior part consists of the choroid. The ciliary body is a structure that is made up of the ciliary muscle. It alters the shape of the lens when the eye focuses on something. The choroid layer present between the sclera and the retina consists of blood vessels and connective tissue. It provides nourishment to the inner parts of the eye.
- The internal layer or the sensory part: It is formed by the retina.
- Three chambers of fluid:
- Anterior chamber – It is located between the cornea and the iris.
- Posterior chamber – It is located between the iris, zonule fibres and lens.
- Vitreous chamber - It is located between the lens and the retina.
The anterior and posterior chambers are filled with aqueous humour, whereas, the vitreous chamber is filled with vitreous humour, a more viscous fluid.
- The lens: It is transparent and is located behind the iris. The lens helps to focus light on the retina. It is supported by ligaments called as zonule fibres that are attached to the anterior portion of the ciliary body. The contraction or relaxation of these ligaments, as a result of ciliary muscle action, changes the shape of the lens. This process called ‘accommodation’ allows formation of a sharp image on the retina.
- Retina: It is the innermost layer of the back of the eye. This layer of light-sensitive nerve endings converts light into electrical impulses which are sent to the brain from the optic nerve. The macula is the central zone of the retina which provides most central and distinct vision. Further, in the centre of the macula is a region called fovea which provides the sharpest vision.
- Nerves and Blood vessels: The short and long ciliary nerves that are branches of the nasociliary nerve, carry sensory impulses from the cornea and uvea (middle layer consisting of iris, ciliary body and choroid). Blood reaches the eye from the ophthalmic artery through its branches– central artery of the retina, short and long posterior ciliary arteries, and the anterior ciliary arteries. The veins of the eye accompany the arteries and drain into the cavernous sinus through ophthalmic veins.
- Optic nerve: The optic nerve is made up of several million nerve fibres. It carries visual impulses from the retina to the brain.
- Optic disc or optic nerve head: Optic disc is a circular area at the back of the eye where the optic nerve joins the retina. This area does not respond to light stimulation because of the absence of photoreceptors and is known as “blind spot”.
- Extraocular Muscles: The eyeball is held in its position inside the orbit by various ligaments and muscles surrounding it.
There are three pairs of extraocular muscles. Two pairs are the rectus muscles which include the superior rectus, the inferior rectus, the lateral rectus, and the medial rectus muscles. They run straight along the eyeball and insert into the bony orbit of the skull.
The other pair of muscles is oblique muscles which include the superior and inferior oblique muscles. These muscles help in rotation of the eyeball in the orbit and also allow the image to be focused on the fovea of the central retina.
Normal vision takes place when the light rays entering the eyes get focused on the retina. Usually, the light rays that enter the eyes may get bent or refracted when they pass through the lens.
Sometimes, in conditions where the eyeball is too long, the focused image will fall short of the retina, which means the image forms in front of the retina. This is known as myopia or near-sightedness where farther objects appear unclear.
When the eyeball is too short, it results in hyperopia or far-sightedness in which the focused image falls behind the retina. These conditions also occur if the eye muscles do not change the shape of the lens such that the light rays get focused accurately. Our eyes have a unique capability of being able to focus on a single object at the same time. This type of vision helps us to sense the depth and is called stereoscopic vision.
The light rays that reflect back from an object we see enter our eye and get focused by the optical structures of the eye such as the cornea, iris, pupil, and lens. These rays finally enter the retinal layer at the back of the eye.
The retina comprises of two nerve cell types: cones and rods. The cones are sensitive to light, colour and visual details such as words in a newspaper and street sign. Rods are responsible for night vision and they detect movement and objects.