Collinson Library: Health &amp; PE10 Human Movement (2015): Eyes

Anatomy Of The Eye

How The Human Eye Works

The human eye can be compared to a camera which gathers, focuses, and transmits light through a lens to create an image of the environment. In a camera, the image is created on film; in the eye, the image is created on the retina, a thin layer of light sensitive cells at the back of the eye. The lens of the eye bends, or refracts, light that enters the eye.

The cornea, which is a clear, transparent covering in the front portion of the eye also contributes to focusing light on the retina. Nerve fibres extending back from the retina's nerve cells come together behind the retina to form the optic nerve, a "cable" of nerve fibres connecting the eye with the brain.

The optic nerve transmits messages about what we see from the eye to the brain. Like a camera, the human eye controls the amount of light that enters the eye through the lens under various lighting conditions.

Parts Of The Human Eye And Its Functions

Eye Parts	Description and Functions
Cornea	The cornea is the outer covering of the eye. This dome-shaped layer protects your eye from elements that could cause damage to the inner parts of the eye. There are several layers of the cornea, creating a tough layer that provides additional protection. These layers regenerate very quickly, helping the eye to eliminate damage more easily. The cornea also allows the eye to properly focus on light more effectively. Those who are having trouble focusing their eyes properly can have their corneas surgically reshaped to eliminate this problem.
Sclera	The sclera is commonly referred to as the "whites" of the eye. This is a smooth, white layer on the outside, but the inside is brown and contains grooves that help the tendons of the eye attach properly. The sclera provides structure and safety for the inner workings of the eye, but is also flexible so that the eye can move to seek out objects as necessary.
Pupil	The pupil appears as a black dot in the middle of the eye. This black area is actually a hole that takes in light so the eye can focus on the objects in front of it.
Iris	The iris is the area of the eye that contains the pigment which gives the eye its color. This area surrounds the pupil, and uses the dilator pupillae muscles to widen or close the pupil. This allows the eye to take in more or less light depending on how bright it is around you. If it is too bright, the iris will shrink the pupil so that they eye can focus more effectively.
Conjunctiva Glands	These are layers of mucus which help keep the outside of the eye moist. If the eye dries out it can become itchy and painful. It can also become more susceptible to damage or infection. If the conjunctiva glands become infected the patient will develop "pink eye."
Lacrimal Glands	These glands are located on the outer corner of each eye. They produce tears which help moisten the eye when it becomes dry, and flush out particles which irritate the eye. As tears flush out potentially dangerous irritants, it becomes easier to focus properly.
Lens	The lens sits directly behind the pupil. This is a clear layer that focuses the light the pupil takes in. It is held in place by the ciliary muscles, which allow the lens to change shape depending on the amount of light that hits it so it can be properly focused.
Retina	The light focuses by the lens will be transmitted onto the retina. This is made of rods and cones arranged in layers, which will transmit light into chemicals and electrical pulses. The retina is located in the back of the eye, and is connected to the optic nerves that will transmit the images the eye sees to the brain so they can be interpreted. The back of the retina, known as the macula, will help interpret the details of the object the eye is working to interpret. The center of the macula, known as the fova will increase the detail of these images to a perceivable point.
Ciliary Body	Ciliary body is a ring-shaped tissue which holds and controls the movement of the eye lens, and thus, it helps to control the shape of the lens.
Choroid	The choroid lies between the retina and the sclera, which provides blood supply to the eye. Just like any other portion of the body, the blood supply gives nutrition to the various parts of the eye.
Vitreous Humor	The vitreous humor is the gel located in the back of the eye which helps it hold its shape. This gel takes in nutrients from the ciliary body, aqueous humor and the retinal vessels so the eye can remain healthy. When debris finds its way into the vitreous humor, it causes the eye to perceive "floaters," or spots that move across the vision area that cannot be attributed to objects in the environment.
Aqueous Humor	The aqueous humor is a watery substance that fills the eye. It is split into two chambers. The anterior chamber is located in front of the iris, and the posterior chamber is directly behind it. These layers allow the eye to maintain its shape. This liquid is drained through the Schlemm canal so that any buildup in the eye can be removed. If the patient's aqueous humor is not draining properly, they can develop glaucoma.

Why People Need Glasses

Most often people need to wear eyeglasses to correct blurred or distorted vision caused by imperfections in the eyes' focusing mechanism. These imperfections, which occur because light entering the eye is not brought into sharp focus on the retina, are known as common errors of refraction or refractive errors.

Refractive errors occur as a result of irregularities in the shape of the cornea, the actual size or shape of the eyeball itself, or the focusing capacity of the lens. Common refractive errors that are fully corrected with eyeglasses or contact lenses are not visual impairments because sight can be corrected to normal. Nearly every person is likely to have a refractive error at some point in life, especially after age 40, and perhaps need to wear eyeglasses or contact lenses. The common refractive errors are:

Myopia (Nearsightedness)
Myopia is blurred vision that occurs when the eye's focusing mechanism brings light to a focus in front of the retina, usually because the eyeball is very elongated in shape. Eyeglasses or contact lenses correct myopia but do not slow or alter its progression.
Hyperopia (Farsightedness)
Hyperopia is blurred vision that occurs when light is focused behind the retina, usually because the eyeball is short or small. Eyeglasses or contact lenses correct hyperopia but do not slow or alter its progression.
Astigmatism
Astigmatism refers to an irregularly curved cornea that distorts the focus of light entering the eye. Generally corrective lenses restore clear vision.
Presbyopia
Presbyopia refers to the eye's loss of accommodation, the eye's focusing power and ability to adjust the focus of the eye on the distance between the individual and the object. People with presbyopia, typically those age 40 and older, experience a progressive inability to focus for near vision viewing as the lens becomes less elastic with age. Lenses with magnification are used to provide the correction needed. These lenses are commonly referred to as "reading glasses," or necessary magnification can be added to a person's regular eyeglasses as bifocals, or trifocals. Variable focus lenses are also available to correct presbyopia.

Some Eye Conditions - Diseases

CATARACTS

A cataract is a cloudiness in the lens of the eye.

Normally, the lens is completely clear and acts to adjust the focus of light rays entering the eye onto the retina to form a clear image.

The proteins which form the lens are normally very precisely layered and arranged and it is this feature which allows the proteins to be completely transparent. If this internal structure deteriorates and the proteins become damaged or disorganised, the lens itself starts to become cloudy or brownish and this interferes with the transmission of light back onto the retina.

There are several patterns of cataract formation but the end result is the same. The greater the cloudiness, the worse the vision becomes until finally only light and dark can be distinguished.

GLAUCOMA

Glaucoma is a condition where the pressure within the eye (intraocular pressure or IOP) is enough to cause damage to the optic nerve. This damage occurs at the point at which the nerve passes through the wall of the eye.

The damage to the nerve causes a loss of its function which may be mild, moderate or severe. In its severe forms, glaucoma can cause total blindness.

The eye has a statistically normal range of pressure but this can be elevated from various causes.

In some cases of glaucoma, damage to the optic nerve can happen even without apparent elevation of the IOP. In these cases it seems that the optic nerve is predisposed or abnormally vulnerable to damage for some reason.

There are 3 main types of glaucoma :

Open-angle glaucoma - this usually causes a gradual, but sometimes rapid, loss of vision. The filter mechanism in the anterior chamber of the eye has increased internal resistance to the outflow of aqueous humour.

Closed-angle glaucoma - onset can be sudden, and can cause painful, rapid loss of vision. It can also be more gradual. Access to the filter mechanism is blocked bya an abnormal configuration of the iris.

Secondary glaucoma - the progress of this depends on the cause. The filter mechanism is blocked by a variety of mechanisms related to some other disease process within the eye

MACHULAR DEGENERATION

Macular degeneration (also called age-related macular degeneration) is a set of conditions in which the central part of the retina, called the macula, develops degenerative changes due to damage caused by the accumulation of metabolic waste products.

Waste products accumulate in this area because the density of cells is highest here, and because of their high levels of metabolic activity.

This process caused a reduction in function of the macula so that central or reading vision is reduced, sometimes severely.

Macular degeneration usually affects both eyes.

There are two main forms of macular degeneration, the so-called dry form and the wet form.

Dry form macular degeneration - The accumulation of waste products in the macula damages critical layers of cells. This leads to areas of atrophy (thinning and disappearance) which can gradually combine until the whole central area of the retina is affected.

Wet form macular degeneration - The process which occurs in the dry form is complicated by the growth of abnormal new blood vessels which leak fluid, protein, fat and blood into the delicate tissues of the retina. This causes damage and loss of function but, more importantly, leads to destructive scar formation at the macula. Once a scar has formed, the damage to the macula is irreversible.