Eyes have ONE Language Everywhere
All About Eye
The ability to see is dependent on the actions of several structures in and around the eyeball. The graphic below lists many of the essential components of the eye’s optical system.
When you look at an object, light rays are reflected from the object to the cornea, which is where the miracle begins. The light rays are bent, refracted and focused by the cornea, lens, and vitreous. The lens’ job is to make sure the rays come to a sharp focus on the retina. The resulting image on the retina is upside-down. Here at the retina, the light rays are converted to electrical impulses which are then transmitted through the optic nerve, to the brain, where the image is translated and perceived in an upright position!
Think of the eye as a camera. A camera needs a lens and a film to produce an image. In the same way, the eyeball needs a lens (cornea, crystalline lens, vitreous) to refract or focus the light and a film (retina) on which to focus the rays. If anyone or more of these components is not functioning correctly, the result is a poor picture. The retina represents the film in our camera. It captures the image and sends it to the brain to be developed. The macula is the highly sensitive area of the retina. The macula is responsible for our critical focusing vision. It is the part of the retina most used. We use our macula to read or to stare intently at an object.
Pupil
This is the black circle in the middle of the eye. The primary function of the pupil is to control the amount of light entering the eye. When you’re in a bright environment, the pupil becomes smaller to allow less light through. When it’s dark, the pupil expands to allow more light to reach the back of the eye.
Cornea
The cornea is sometimes referred to as the “window of the eye”. It is composed of 5 layers of tissue. Its outer layer (the epithelium) provides protection for the eye. The epithelium is made up of highly regenerative cells that have the ability to grow back within 3 days, allowing for fast healing of superficial injuries. Most of the inner layers of the cornea provide strength to the eye.
Vitreous Body
This is the clear gel-like substance located inside the eye’s cavity. Its purpose is to provide a spherical shape to the eye. The vitreous body may develop small clumps known as ‘floaters’, which are especially common in nearsighted people.
Iris
This is the colored part of the eye surrounding the pupil. The primary function of the iris is to control the size of the pupil. This is acheived through contraction or expansion of the muscles of the iris.
Sclera
The sclera is the white part surrounding the iris. The sclera’s purpose is to provide structure, strength, and protection to the eye.
Retina
The retina consists of fine nerve tissue which lines the inside wall of the eyes and acts like the film in a camera. Its primary function is to transmit images to the brain.
Angle Structures
The area in the anterior chamber where the cornea and iris join is known as the angle. This is comprised of several structures that make up the eye’s drainage system. The angle structures include: the outermost part of the iris, the front of the ciliary body, the trabecular meshwork, and the Canal of Schlemm. Aqueous is formed in the ciliary body behind the iris. It flows through the pupillary space into the anterior chamber. From there, the fluid travels into the angle structures and drains from the eye.
As the aqueous fluid leaves the angle, it passes through a filter called the trabecular meshwork. After leaving the trabecular meshwork, the aqueous travels through a tiny channel in the sclera called the Canal of Schlemm. The aqueous flows into other tiny channels and eventually into the eye’s blood vessels. The production and drainage of aqueous fluid determines the eye’s intraocular pressure (IOP).
Layers of the Cornea
The epithelium is layer of cells that cover the surface of the cornea. It is only about 5-6 cell layers thick and quickly regenerates when the cornea is injured. If the injury penetrates more deeply into the cornea, it may leave a scar. Scars leave opaque areas, causing the corneal to lose its clarity and luster. Boman’s membrane lies just beneath the epithelium. Because this layer is very tough and difficult to penetrate, it protects the cornea from injury. The stroma is the thickest layer and lies just beneath Bowman’s. It is composed of tiny collagen fibrils that run parallel to each other. This special formation of the collagen fibrils gives the cornea its clarity.
Descemet’s membrane lies between the stroma and the endothelium. The endothelium is just underneath Descemet’s and is only one cell layer thick. This layer pumps water from the cornea, keeping it clear. If damaged or disease, these cells will not regenerate. Tiny vessels at the outermost edge of the cornea provide nourishment, along with the aqueous and tear film.
Tear Production System
The eye’s tears are composed of three layers: oil, water and mucous. The outermost oily layer is produced by the meibomian glands which line the edge of the eyelids. The watery portion of the tear film is produced by the lacrimal gland. This gland lies underneath the outer orbital rim bone, just below the eyebrow. The mucous layer comes from microscopic goblet cells in the conjunctiva.
With each blink, the eyelids sweep across the eye, spreading the tear film evenly across the surface. The blinking motion of the eyelids forces the tears into tiny drains found at the inner corners of the upper and lower eyelids. These drains are called puncta (plural for punctum).
The tear film travels from the puncta into the upper and lower canaliculus, which empty into the lacrimal sac. The lacrimal sac drains into the nasolacrimal duct which connects to the nasal passage. This connection between the tear production system and the nose is the reason your nose runs when you cry. Some patients can actually taste eye drops as
Tear Film
Tears are formed by tiny glands that surround the eye. The tear film is comprised of three layers: oil, water, and mucous. The lower mucous layer serves as an anchor for the tear film and helps it adhere to the eye. The middle layer is comprised of water. The upper oil layer seals the tear film and prevents evaporation. The tear film serves several purposes: it keeps the eye moist, creates a smooth surface for light to pass through the eye, nourishes the front of the eye, and provides protection from injury and infection.
Optic Nerve
The optic nerve transmits electrical impulses from the retina to the brain. It connects to the back of the eye near the macula. When examining the back of the eye, a portion of the optic nerve called the optic disc can be seen. The retina’s sensory receptor cells of retina are absent from the optic nerve. Because of this, everyone has a normal blind spot. This is not normally noticeable because the vision of both eyes overlaps.
