The eye has many parts and functions and it can take forever to list all of them. Some parts and their functions include (refer to Appendix A to see more about the structures of the eye):

Suspensory Ligaments

Suspensory ligaments are also known as the zonular fibres that attach the lens to the rest of the eye. They grip the lens in place and function to change the focusing power of the eye. They do this by changing the tension of the fibres by contraction and relaxation of the ciliary muscle.

Fovea

The fovea, a tiny part of our retina responsible for our topmost visual acuity; it is the core of our central vision and is responsible for sharp central vision. It is located in the mascula of the retina.

Ciliary Muscle

The ciliary muscle changes the shape of the lens called accommodation. The muscle is a smooth muscle affecting the zonular fibres in the eye. The ciliary muscle relaxes to flatten the lens for long-range focus. For short-range focus the ciliary muscle contracts making the lens more spherical. This releases the tension on the lens caused by the zonular fibres.

Lens

The lens also known as the crystalline lens is transparent and is located just behind the iris. The lens is encased in a capsular-like bag and suspended within the eye by tiny zonules. The lens along with the cornea helps to focus light onto the retina.

Aqueous Humour

The Aqueous humour is a water like fluid, produced by the ciliary body; the aqueous humour fills the front of the eye between the lens and cornea. The aqueous humour also provides the cornea and lens with oxygen and nutrients. It drains back into the blood stream through the canals of Schlemm.

Pupil

The pupil is the hole or slit shaped opening in the centre of the iris that light passes through. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. The iris muscles control its size.

Vitreous Humour

The vitreous humour is a gel like liquid that fills most of the eye (from the lens back). The gel like liquid is a clear aqueous solution that fills the space between the lens and the retina of the vertebrate eyeball. The solution is 99% water, but has a gelatinous viscosity two to four times that of water. The remaining solutes include salts, sugars, phagocytes, and a network of collagen fibres. The vitreous humours purpose is to cushion the eye. As we age it changes from a gel to a liquid and gradually shrinks separating from the retina.

Optic Nerve

The Optic nerve is the nerve that transmits visual information from the retina to the brain. Each optic nerve has about 1.2 million nerve fibres. This is the cable connecting the eye to the brain.

Iris

The iris is the coloured part of the eye: brown, green, blue, etc. It is a ring of muscle fibres located behind the cornea and in front of the lens. It contracts and expands opening and closing the pupil, in response to the brightness of surrounding light. The iris of the eye helps protect the sensitive retina.

Cornea

The cornea is a dome-shaped surface that covers the front of the eye. It is the first and most powerful lens in the eye’s optical system. Together with the lens, the cornea refracts light, and as a result helps the eye to focus, accounting for approximately 80% of its production to 20% of the lens focusing power. To keep it transparent the cornea contains no blood vessels. Tears that flow over it and aqueous humour in the chamber behind it keep it nourished.

Conjunctiva

The conjunctiva is a thin, clear membrane covering the sclera (white part of eye), front of the eye and inner eyelids. It helps lubricate the eye by producing mucus and tears. This is the eyes first layer of protection against infection. Inflammation of this membrane is called conjunctivitis, or pink eye.

Sclerotic Coat

Also known as the sclera, it is a whitish fibrous membrane that with the cornea forms the outer covering of the eyeball. It is the tough, opaque tissue that serves as the eye’s protective outer coat. The optic nerve is attached to the sclera at the very back of the eye. It along with internal fluid pressure keeps the eye shape and protects its delicate internal parts.

Choroid Layer

The choroid is a layer of blood vessels between the retina and sclera; it supplies blood to the retina. The choroid provides oxygen and nourishment to the outer layers of the retina.

Retina

The retina is part of out central nervous system (CNS). It is only a thin layer of neural cells that lines the back of the eyeball of vertebrates. It converts light rays into electrical signals and sends them to the brain through the optic nerve. The sides of the retina are responsible for our peripheral vision. The centre area, called the macula, is used for our fine central vision and colour vision. The retina is where most the problems leading to vision loss occur.

Blind Spot

A blind spot is the place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on the optic disc of the retina where the optic nerve passes through it. Since there are no cells to detect light on the optic disc, a part of the field of vision is not perceived. The brain fills in with surrounding detail and with information from the other eye, so the blind spot is not normally perceived.

Eye Focus

How does an eye focus so sharply and allow the right amount of light to enter? It’s not very simple and must go through a complicated process.

Firstly, on the back of your eye is a layer of cells known as the retina. The retina reacts to light and conveys that information to the brain. The brain, in turn, translates all that activity into an image.

When you look at something, three things must happen:

• The image must be reduced in size to fit onto the retina
• The scattered light must come together. That is, it must focus at the surface of the retina.
• The image must be curved to match the curve of the retina

Light

When light rays pass through your pupil, the muscle called the iris (coloured ring) makes the size of the pupil change depending on the amount of light that’s available. You may have noticed this with your own eye if you have looked at it closely in a mirror. If there is too much light, your pupil will shrink to limit the number of light rays that enter.

Not Enough Light

If there is very little light available, the pupil will enlarge to let in as many light rays as it can. Just behind the pupil is the lens and it focuses the image through a jelly-like substance called the vitreous humour onto the back surface of the eyeball, called the retina.

Sometimes the eye doesn’t focus quite right:

The surfaces of the lens or cornea may not be smooth, causing an aberration those results in a streak of distortion called astigmatism.

• The lens may not be able to change its curve to properly match the image (called accommodation)
• The cornea may not be shaped properly, resulting in blurred vision

Most vision problems occur when the eye cannot focus the image onto the retina. Here are a few of the most common problems:

• Myopia (nearsightedness) happens when a distant object looks blurred because the image comes into focus before it reaches the retina. Myopia can be corrected with a minus lens, which moves the focus farther back.
• Hyperopia (farsightedness) occurs when a close object looks blurred because the image doesn’t come into focus before it gets to the retina. Hyperopia, which can also occur as we age, can be corrected with a plus lens. Bifocal lenses, which have a small plus segment, can help a farsighted person read or do close work, such as sewing.
• Astigmatism is caused by a distortion that results in a second focal point. It can be corrected with a cylinder curve.

Eye Shine

Red eyes? We have all seen these in photographs where people have spooky red eyes. Where do they come from? The red colour comes from light that reflects off of the retinas in our eyes. If you shine a flashlight or headlights into animal eyes at night, their eyes shine back with bright, white light.

In many nocturnal vertebrates the white compound guanine is found in the epithelium or retina of the eye. This provides a mirror like surface, the tapetum lucidum, which reflects light outward and thereby allows a second chance for its absorption by visual pigments at very low light intensities. Tapeta Lucida produce the familiar eye shine of nocturnal animals.

The Tapetum lucidum is a reflecting layer just behind the retina of the eye of many vertebrates and it serves to reflect light back to the retina, increasing the quantity of light caught by the retina. This improves vision in low light conditions, but can cause the perceived image to be blurry from the interference of the reflected light. It is therefore primarily found in nocturnal animals with good night vision.

Humans don’t have this tapetum lucidum layer in their retinas. If you shine a flashlight in a person’s eyes at night, you don’t see any sort of reflection. The flash on a camera is bright enough; however, to cause a reflection off of the retina. What is seen is the red color from the blood vessels nourishing the eye.

“Tapetum lucidum” literally means bright carpet. 

Animals’ Eye Shine Colors Include:

• Alligators: red
• Cats with green/yellow eyes: green
• Cats with blue eyes: red
• Cows: yellow/red
• Deer: white/yellow/red
• Dogs: green/blue
• Foxes: blue/red
• Horses: white
• Opossums: pink
• Owls: red
• Prairie dog: orange
• Rabbits: orange/red
• Racoon:green/white
• Rats: orange/red
• Sheep: orange
• White-tailed deer: silver-white

Blepharitis

Blepharitis is an inflammation condition of the eyelids, particularly at the outer edge of the lid. Many people who suffer blepharitis will also have oil gland that are unable to function regularly through thickening of the oil and perhaps, plugging of the glands.

There may also be dandruff or infectious parts that contribute to the disorder. Blepharitis frequently occurs in people who have oily skin, dandruff or dry eyes. Some people may develop an allergy to the scales or to the bacteria that surround them. This can lead to a more serious complication with inflammation of other eye tissues, particularly the cornea.

In severe cases, it may also cause styes (develops when a gland at the edge of the eyelid becomes infected, resembling a pimple on the eyelid), irritation and inflammation of the cornea (keratitis) and conjunctiva (conjunctivitis). Some patients have no symptoms at all.

Blepharitis is usually a long-lasting problem that can be controlled with extra attention to lid hygiene. However, it is sometimes caused by an infection and may require medication.

There are two types:

Anterior Blepharitis- Acts upon the front of the eyelids near the eyelashes. The causes are Seborrheic dermatitis (similar to dandruff) and occasional infection by Staphylococcus bacteria.

• Posterior Blepharitis- Acts upon the inner part of the eyelids. This is caused by the oil glands present in this area.

What is Seborrheic Blepharitis?

The inherited most common type of Blepharitis. Seborrheic Blepharitis is usually one part of the variety of Seborrheic dermatitis seborrhea which concerns the scalp, lashes, eyebrows, folds and ears. With Seborrheic Blepharitis, the glands of the eyelid produce an abnormal quantity and quality of teat film, which normally coats, protects, and lubricates the eye. Treatment is best accomplished by a dermatologist.

What is staphylococcus Blepharitis?

Staphylococcus Blepharitis is a harsher condition that often begins in childhood and continues through adulthood. Infective in nature, staphylococcus Blepharitis continues through adulthood and is caused by bacteria (staphylococcus aureus).

Symptoms

• Foreign body in the eye sensation – Feeling like sand or grit is in the eye
• Burning eye sensation – occurring as burning or stinging eye pain
• Excessive tearing – Tearing, or having too many tears, can come from being sensitive to light, wind, or temperature changes. Protecting your eyes (by wearing sunglasses, for instance) sometimes solves the problem. Tearing may also mean that you have a more serious problem, such as an eye infection or a blocked tear duct.
• Affecting one or both eyes
• Itching
• Light sensitivity (photophobia) – Dislike, sensitivity or avoidance of bright light
• Red eyelids – Redness of one or both eyelids
• Swollen eyelids
• Warm eyelids
• Sore eyelids
• Itching eyelids
• Burning eyelids
• Scaling of the eyelids
• Red eye
• Blurred vision
• Frothy tears
• Dry eye
• Crusting of the eyelashes – usually on awakening in the morning
• Eyelids glued shut
• Matted eyelashes
• Loss of eyelashes

Causes

A germ called bacteria usually causes blepharitis. The conditions that can cause Blepharitis includes:

• Staphylococcus bacteria
• Acne rosacea
• Scalp dandruff
• Dry eyes
• Infections
• Allergies
• Diabetes

Treatment

Many medication treatments are available for blepharitis, including antibiotics and steroid (cortisone) preparations in drop or ointment form. However, medications alone are not sufficient. The daily cleansing routine described below is essential.

Most patients are instructed to scrub the eyelashes gently with cotton-tipped applicators wet with warm water, once or twice daily for a few weeks, then once every two to four days for long-term control. Warm compresses to the eyelids, artificial tears, and antibiotic or antibiotic-steroid combination ointments are often recommended as well.

Different Structures of the Eye

A transverse section of the left eyeball.

Anterior View of Lacrimal Apparatus.

Sagittal Section of Accessory Structures.

Close Up Of Sagittal Section of Accessory Structures.