The eyes are the most complex 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 visualise 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 specialised cell layer of the eye called retina acts as a film that helps produce an image.
External anatomy of eye
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 pupil by enlarging in dim light and contracting in bright light, much like the shutter of a camera. The central aperture of iris is known as 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 brown colour whereas more of pheomelanin is seen in blue- or green- eyed people.
- Pupil: This is an opening within the centre of the iris, and not itself a physical structure. It acts as a control of 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, sometimes referred to as the window of the eye. 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.
Internal anatomy of 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 cornea.
- The intermediate layer: The intermediate layer is divided into two parts namely anterior and posterior. Anterior part consists of iris and ciliary body and posterior part consists of choroid. 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 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.
- Anterior chamber: Present between the cornea and iris.
- Posterior chamber: Present between iris, zonule fibres and lens.
- Vitreous chamber: Located between the lens and the retina.
The anterior and posterior chambers are filled with aqueous humour, whereas the vitreous chamber is filled with the 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 known 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: This 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 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: The optic disc or optic nerve head is a circular area at the back of the eye where the retinal fibres join to form the optic nerve. 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. The extraocular muscles work to move and align the eyes.
- 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 behind the eyeball to a fibrous ring surrounding the optic nerve. 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 within the orbits.
Normal vision takes place when the light rays entering the eyes are focused on the retina. As light rays enter the eye they are refracted or bent as they pass through the cornea and lens, to form a clear focussed image on the retina at the macula. 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.
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 farsightedness in which the focused image falls behind the retina. In children, the visual system is still developing and treatment is fundamental for good adult vision to develop. Corrective lenses in the form of glasses or contact lenses are used to cause the image to fall on the retina, creating a clear image. Once stable, laser vision correction can circumvent the need for glasses.
When the vision is refined for distance, the lens, with it’s softness and flexibility, changes it’s shape to give a continuous range of near focus. As the eye ages, the lens loses it’s softness and flexibility. The ciliary muscles that adjust the shape of the lens may also harden with aging. The deterioration of near vision, on average in people’s mid-40s , is called presbyopia. At this stage reading glasses are needed. Some patients are suitable for surgery to correct this age process.
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.