Overview
A person’s sense of sight is very important to humans. Vision is arguably the most used of the 5 senses and is one of the primary means that we use to gather information from our surroundings. The human eye is the organ which gives us the sense of sight, allowing us to observed and learn more about the surrounding world than we do with any of the other four senses.
People use their eyes in almost every activity they perform, whether reading, working, watching television, writing a letter, driving a car, and in countless other ways. Most people probably would agree that sight is the sense they value more than all the rest. The eyes are at work from the moment a person is wake up to the moment he or she closes them to go to sleep.
This special organ takes in tons of information about the world around you — shapes, colors, movements, and more. Then they send the information to your brain for processing so the brain knows what’s going on outside of your body.
Anatomy of the Eye
External and Accessory Structures
The adult eye is a sphere-shaped organ that measures about 1 inch or 2.5 cm in diameter. However, only one sixth (1/6) of the eye’s surface can normally be seen and the rest is enclosed and protected by a cushion of fat and the walls of the bony orbit.
The accessory structures of the eye are the following:
- Extrinsic eye muscles. The extrinsic muscles of the eye come from the bones of the orbit and are movable due to broad tendons in the eye’s tough outer surface. There are six extrinsic eye muscles that function to MOVE the eye in various directions:
- Superior rectus muscle – rotates the eye upward and toward the midline
- Inferior rectus muscle – rotates the eye downward and toward the midline
- Medial rectus – rotates the eye toward the midline
- Lateral rectus – rotates the eye away from the midline
- Superior oblique – rotates the eye downward and away from the midline
- Inferior oblique – rotates the eye upward and away from the midline
- Eyelids. The eyelids protect the eyes anteriorly which meet at the medial and the lateral corners of the eye. From the border of each eyelid are the EYELASHES. The eyelashes help filter out foreign matter, including dust and debris, and prevent it from getting into the eye. Eyelid edges associate with modified sebaceous glands make up the TARSAL GLANDs. These glands produce an oily secretion that lubricates the eye. Between the eyelashes, modified sweat glands called ciliary glands are found.
- Conjunctiva. The conjunctiva is a mucous membrane that begins at the edge of the cornea and lines the inside surface of the eyelids and sclera, which serves to lubricate the eye. It is the thin, transparent tissue that covers the outer surface of the eye. This structure is nourished by tiny blood vessels that are nearly invisible to the naked eye. The conjunctiva is composed of 3 sections:
- Palpebral Conjuctiva – covers the posterior surface of the eyelids
- Bulbar Conjuctiva – coats the anterior portion of the eyeball
- Fornix – the transition portion, forming the junction between the posterior eyelid and the eyeball
NOTE: Although the palpebral conjunctiva is moderately thick, the bulbar conjunctiva is very thin. The latter also is very movable, easily sliding back and forth over the front of the eyeball it covers. Since it is clear, blood vessels are easily visible underneath it.
Within the bulbar conjunctiva are “goblet cells,” which secrete “mucin.” This is an important component of the pre-corneal tear layer that protects and nourishes the cornea.
- Lacrimal Apparatus. The lacrimal apparatus consists of the lacrimal gland and a number of ducts that drain the lacrimal secretions into the nasal cavity. Located above the lateral end of each eye are the lacrimal glabds that continually release a dilute salt solution, known as tears, onto the anterior surface of the eyeball through several small ducts. The flow of tears flush across the following structures orderly:
- Eyeball
- Lacrimal canals medially
- Lacrimal sac
- Nasolacrimal duct that empties into the nasal cavity.
Lacrimal secretion contains antibodies and an enzyme that destroys bacteria known as lysozyme. Hence, tears cleanse and protect the eye surface as it moistens and lubricates it. in cases when lacrimal secretion substantially increases, tears spill over the eyelids and fill the nasal cavities. This causes congestion and the “sniffles.” When eyes are irritated by foreign objects or chemicals and when a person is emotionally upset lacrimal secretion is stimulated and increased.
Internal Eye structures
The Eyeball
The eye, commonly called the eyeball, is a hollow sphere and is composed of:
- 3 tunics or coats
- Humors – the interior filled with fluids that help maintain the shape of the eye
- Lens – the main focusing apparatus of the eye. This structure is supported within the eye cavity dividing it into two chambers.
TUNICS of the EYEBALL
- Fibrous Tunic – this is the outermost tunic and is also known as the sclera, the thick and white connective tissue. The fibrous tunic (sclera) is seen anteriorly as the “white part of the eye.” The central portion of this tunic is modified so that it is crystal clear. The cornea is the transparent “window” through which light enters the eye and is well supplied with nerve endings. This is the main reason why blinking and increased tearing occur when the cornea is touched because most nerve endings found here are pain fibers.
- Vascular tunic – this is the middle coat of the eyeball and has three distinguishable regions namely:
- Choroid – this is located posteriorly and lies between the sclera and the retina. It contains the blood vessels that provide nourishment to the outer layers of the retina. It is composed of layers of blood vessels that nourish the back of the eye. The choroid is opaque and deeply pigmented with melanin to absorb excessive light; else internal reflection would form multiple images on the retina. It is less vascular where the retina is thin. The choroid connects with the ciliary body toward the front ofthe eye and is attached to edges of the optic nerve at the back of the eye.
2. Ciliary Body – the ciliary body is made up of ciliary muscles and ciliary processes. It lies just behind the iris. This is the structure to which lens are attached by a suspensory ligament called the ciliary zonule and then the iris. The pigmented iris has a rounded opening, the pupil, through which light passes. Nourishment for the ciliary body comes from blood vessels which also supply the iris. Ciliary processes are short, black tissues arranged radially. They secrete aqueous humour.
One function of the ciliary body is the production of aqueous, the clear fluid that fills the front of the eye. It also controls accomodation by changing the shape of the crystalline lens. When the ciliary body contracts, the zonules relax. This allows the lens to thicken, increasing the eye’s ability to focus up close. When looking at a distant object, the ciliary body relaxes, causing the zonules to contract.
3. Sensory Tunic – this is the innermost tunic of the eye and is called the retina. This structure extends anteriorly only to the ciliary body. It converts images into electrical impulses that are sent along the optic nerve to the brain where the images are interpreted. The retina can be compared to the film of a camera. It contains millions of receptor cells, the rods and cones. Rods andcones are called photoreceptors because they respond to light.
The rods and cones are not distributed evenly in the retina. The rods are most dense at the periphery or edge of the retina and decrease in number as the center of the retina is approached. It is more numerous, some 120 million, and are more sensitive than the cones. However, they are not sensitive to color. The 6 to 7 million cones provide the eye’s color sensitivity and they are much more concentrated in the central yellow spot known as the macula. In the center of that region is the fovea centralis a 0.3 mm diameter rod-free area with very thin, densely packed cones. Consequently, this is the area of greatest visual acuity or the point of sharpest vision and anything we wish to view critically is focused on the fovea centralis.
The photoreceptor cells are distributed over the entire retina, except where the optic nerve leaves the eyeball. This is the site called the optic disc or blind spot. When light from an object is focused on the optic disc, it disappears from our point of view and we cannot see it.
LENS
The crystalline lens is located just behind the iris. Light enteringthe eye is focused on the retina by the lens. The nucleus, the innermost part of the lens, is surrounded by softer material called the cortex. The lens is encased in a capsular-like bag. It is held upright in the eye by a suspensory ligament, the ciliary zonule, attached to the ciliary body. Together, the lens and the ciliary body help control fine focusing of light as it passes through the eye. The lens is divided into two segments namely:
- Anterior (aqueous) segment – located anterior to the lens and contains a clear wayetry fluid called aqueous humor. The aqueous humor helps to nourish the cornea and the lens. It is continually produced by the ciliary body.
- Posterior (vitreous) segment – located posterior to the lens and is filled with a gel-like substance called the vitreous humor or the vitreous body. The vitreous is a thick, transparent substance that fills the center of the eye. It is composed mainly of water and comprises about 2/3 of the eye’s volume, giving it form and shape. The viscous properties of the vitreous allowthe eye to return to its normal shape if compressed. The vitreous humor helps maintain the shape of the eye.
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