In children, the vitreous has a consistency similar to an egg white. With age it gradually thins and becomes more liquid. The vitreous is firmly attached to certain areas of the retina. As the vitreous thins, it separates from the retina, often causing floaters.
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 they drain from the nasal passage into the throat.
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.
The sclera is commonly known as "the white of the eye." It is the tough, opaque tissue that serves as the eye's protective outer coat. Six tiny muscles connect to it around the eye and control the eye's movements. The optic nerve is attached to the sclera at the very back of the eye.
In children, the sclera is thinner and more translucent, allowing the underlying tissue to show through and giving it a bluish cast. As we age, the sclera tends to become more yellow.
The retina is a multi-layered sensory tissue that lines the back of the eye. It contains millions of photoreceptors that capture light rays and convert them into electrical impulses. These impulses travel along the optic nerve to the brain where they are turned into images.
There are two types of photoreceptors in the retina: rods and cones. The retina contains approximately 6 million cones. The cones are contained in the macula, the portion of the retina responsible for central vision. They are most densely packed within the fovea, the very center portion of the macula. Cones function best in bright light and allow us to appreciate color.
This photograph shows a normal retina with blood vessels that branch from the optic nerve, cascading toward the macula.
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The pupil is the opening in the center of the iris. The size of the pupil determines the amount of light that enters the eye. The pupil size is controlled by the dilator and sphincter muscles of the iris. Doctors often evaluate the reaction of pupils to light to determine a person's neurological function.
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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.
