How Does The Eye Work?
In this blog, we will look at how does an eye work and what are the common diseases that affect them. We will discuss specific diseases in subsequent blogs.The eye is one of the most sensitive parts of the body which is why Eye Education is important. An eye works like a camera. In-camera there are focusing lenses that focus light coming from an object into a film of the camera.
Cornea
The eye has 2 focusing organs. The first is the cornea. It’s the transparent part of the eye that is visible to us from outside. It’s like the outer glass of a watch. It is the most powerful focusing surface of the eye. Loss of transparency because of scar caused by injury or infection leads to severe visual disability. Similarly change in the shape of the cornea due to certain congenital or acquired diseases can also lead to vision loss. These are generally known as ectatic corneal disorders, Keratoconus being most common.
Iris & Pupil
Inside of that is the pupil which we can see change its shape depending on the brightness of ambient light. It becomes small in size, constricts, when the light is bright and becomes larger in size, dilated, when light is dim. A pupil is like the aperture of the camera. A pupil is the central opening in Iris. It’s the color of Iris that gives color to our eyes. In darker races like Asians, Indians, and African races the iris has a lot of pigments, hence eye color is dark while in fairer races Iris lacks pigments making them appear grey, green or blue. Certain autoimmune or infectious diseases can cause inflammation of Iris leading to redness, pain, photophobia, and is known as iridocyclitis.
Anterior chamber and angle of the eye
The clear area between the back of the cornea and iris is known as anterior chamber. It is filled with transparent liquid known as aqueous fluid. This fluid gets circulated continuously and is important to maintain pressure inside the eyeball, known as intraocular pressure (IOP) and hence its shape. The angle where iris and cornea meet is known as the angle of the eye. It contains pores through which transparent fluid inside the eye gets drained into blood circulation. Glaucoma is a disease where the drainage pathway at the angle of the eye gets blocked, leading to a rise in intraocular pressure (IOP) which in turn damages the nerve of the eye. Learn more about Glaucoma disease
The Lens
The lens of the eye is behind the iris. It also helps in focusing the light onto the film of the eye, the retina. The lens is held in a suspended position behind the iris by thin fibers known as zonules. It makes it possible for the lens to change its shape because of these zonules, this in turn makes it possible for the eye to work as an autofocus camera and focus at different distances. This autofocus mechanism is known as accommodation. Accommodation starts becoming weak after the age of 40 and that’s when one requires glasses to look at near objects. This is a natural process and is known as presbyopia. In young individuals the lens of the eye is completely transparent, however as one grows older, this transparency progressively reduces and it becomes opaque. This is known as Cataract. As the cataract becomes more and more opaque patients vision progressively reduces. When it reaches a point where she is unable to do daily activities, cataract surgery with intraocular lens implantation is done. In olden days when techniques and technology for cataract surgery were primitive, doctors used to wait for Cataract to mature. However with improvements in technology and surgical techniques, surgery can be done when a patient feels the need and need not be postponed until complete blindness has developed. A patient who is homebound and has limited visual needs may choose to wait a lot longer than someone who leads an active lifestyle and even minimal visual disturbance hampers daily life.
The Retina
The retina is the film of the eye. It is present in the back of the eyeball and is at a distance of almost 17-18mm from the back of the lens. The retina is made of sensitive cells known as photoreceptors. The nerve fibers of these photoreceptors pass through the optic nerve and take vision to the brain. Depending upon the position of cell bodies and nerve fibers, 10 different layers of retina can be distinguished!!
There are 2 types of photoreceptors, cones & rods. The cones are highly sensitive and are responsible for fine vision like the color of objects, intricate details, reading vision, etc. Cones are mainly present in the central portion of the retina known as Macula. The Center of the macula is a very tiny area, known as the fovea. The fovea is almost as small as a pinhead. But it is responsible for fine vision almost entirely. Rods are more sensitive in dim light conditions and help one see in low contrast conditions like dusk time. However rods are unable to distinguish fine details. They are more in numbers in Peripheral retina.
There is a small blood vessel that carries blood supply to the retina known as the central retinal artery. It bifurcates into many smaller branches after entry into the eye. Similarly many tiny blood vessels carry impurities from cells of the retina and merge into larger branches till it forms a single vein known as a central retinal vein. The Central retinal artery and central retinal vein share a common entry/ exit point on the optic nerve head. There are no blood vessels in the Fovea which is necessary for fine visual capability, it is known as the Foveal Avascular Zone (FAZ).
The retina can be affected by many systemic diseases like diabetes or hypertension. Swelling or detachment of retina can lead to serious visual morbidity and often needs prolonged treatment. Other diseases like age-related macular degeneration (ARMD/AMD) can hamper reading/ writing vision significantly. Visit the best Retina Specialist in Nashik at Bapaye Eye Hospital.
Vitreous cavity: The area between the back of the lens and the retinal surface is filled with jelly-like material known as Vitreous. Vitreous is densely adherent to the retina in certain points like around optic nerve, Fovea, and around blood vessels. In young individuals vitreous is denser, while as a person’s age increases it becomes more liquified. This often leads to a feeling of small black spots moving inside the eye known as floaters. The liquified vitreous can get separated from retina known as posterior vitreous detachment (PVD) In some individuals this process can be more sudden and leads to bleeding inside the eye &/or retinal tear formation. If retinal tears are detected in time, laser treatment can be done to avoid retinal detachment. However if retinal detachment does develop, it needs surgical treatment.
The choroid: A layer of blood vessels known as choroid lies behind the retina. It supplies oxygen and nutrients to the outer layers of the retina. There is a tight barrier between retina and choroid which keeps the choroidal blood vessels from growing into the retina. When this barrier breaks down, it leads to the seepage of fluid and blood into the retina followed by the growth of abnormal blood vessels under the retinal surface, known as age-related macular degeneration (ARMD/AMD). Iris is an extension of the choroid in the front part of the eye.
The sclera: Sclera is the outermost covering of the eye. It is white in color and opaque. It is rigid and helps to maintain the shape of the eye and avoid injury to inner more sensitive organs. Various autoimmune diseases linked to arthritis may affect sclera causing its inflammation, scleritis.
Eyelids: Eyelids are the skin folds that cover the eyeballs. They protect the eye from injury as well as getting dryness. Every time one blinks the tears are spread over the surface of the eye to maintain the health of the cornea. Various diseases can develop because of changes in the position of the lids like ptosis, entropion, ectropion, etc. These problems need surgical treatment.
The Conjunctiva: The conjunctiva is a thin layer of tissue that covers the sclera and inside of lids, thus making blinking easier and also acts as the first protective layer for the eye. Conjunctivitis is a common cause of redness eye. It is usually associated with watering &/or discharge from the eye.
Orbits: The eye is placed inside the skull in a bony cavity known as Orbits. An orbit is pear-shaped, tapering from outside to inwards. The orbit has 4 walls, roof, floor, outer and inner wall. They taper into a cone towards back known as the apex of the orbit. The apex of the orbit is an exit point for optic nerve into the brain. It also houses muscles that move the eyes, known as extraocular muscles. Two orbits are placed on either side of the nose. Two eyes on either side of the nose give us the ability to have a wider field of view and greater depth of focus. The depth of focus makes it possible to differentiate distances of various objects relative to each other.
Extraocular muscles: There are 6 extraocular muscles which move the eyes in various direction. Muscles of both eyes move in co-ordinate fashion thus avoiding double vision. They also help to keep eyes in a straight direction and not allow the eye to drift outwards. Diseases causing changes in the direction of eyeballs are known as squint and surgical treatment is done on extraocular muscles to correct these problems.
The lacrimal system: The lacrimal system is concerned about tears. The tears are produced by the lacrimal gland which is present in the outer extent of the roof of the orbit. They are drained into the nose through small holes known as punctum in upper and lower lids close to the nose. A punctum is the opening of a small canal in each lid, the canaliculus. These canaliculi open into a small pear-shaped bag made of muscles, the lacrimal sac. They are then drained into the nose via a small tube-like structure called a nasolacrimal duct. Diseases causing reduction of tear production by lacrimal glands lead to dryness of the eye while blockage of drainage system causes watering of eyes. Most cases where drainage is blocked can be treated by surgery to create additional drainage channels. This surgery is known as dacryocystorhinostomy.
The health of eyes is important for our vision. Even though cataract is the most commonly known problem that affects the eye, there many other diseases that can endanger our eyesight. Some of these diseases can be present from birth and start in early childhood while many other diseases develop as the age of person increases.
A detailed eye examination on an annual basis from an early age until the old age group is absolutely necessary. A detailed eye examination should involve checking spectacle number, intraocular pressure check-up and detailed retinal examination with pupillary dilatation. A day spent in one whole year in the eye specialist clinic may go a long way in saving your eyesight
