The outer parts of the eye include the eyelids, the conjunctiva, the lacrimal glands, and the lacrimal sac. The front of our eyeball is protected by the eyelids. Eyelashes on the lids screen out some of the dust and other particles that might enter the eye. Any sudden movement in the front of the eye (or anything that touched the eyelashes) causes our lids to blink in a protective reflex action.
The conjunctiva is a membrane that lines the inside of the eyelids and extends over the front of the white part of the eye. It produces mucus, a clear, slimy fluid that lubricates the eyeball. The conjunctiva also produces some tears, which help keep the eye clean. However, most of our tears are made by the lacrimal glands. One of these glands lie at the upper outer corner of each orbit. Every time we blink, the eyelids spread a smooth layer of mucus and tears over our eyes. These fluids then flow into tiny canals in the lids. These canals lead to the lacrimal sac, a pouch at the lower inner corner of each orbit. From the lacrimal sac, the mucus and tears drain through a passage to our nose. This is why after we cry, we may have to blow our nose to clear the drainage system of excess tears!
The wall of the eyeball contains a clear, jellylike substance called the vitreous humor. This substance occupies about 80% of our eyeball. It helps maintain our eye’s shape and the pressure within the eyeball. The wall is made of three layers of tissue:
1.The Iris is the colored disc behind the cornea. The color of the iris comes from a brownish-blackish substance called melanin. The more melanin there is and the closer it is to the surface of the tissue, the darker the color of our iris. Think of someone who has brown eyes. They have more melanin, and it is closer to the surface that someone who has blue eyes. Melanin also absorbs strong light that might deprive the eye from seeing or cause blurred vision.
At the center of the iris is a round opening called the pupil, which looks likes a black circle. The size of the pupil regulates the amount of light that will be let into the eye. Two muscles in the iris automatically adjust the size of the pupil to the level of light. In dim light, the dilator muscle enlargens the pupil. As much light as possible is then allowed to enter the our eyes. In bright light the sphincter muscle makes the pupil smaller which prevents too much light from entering our eyes. The pupil also becomes smaller when looking at a nearby object, therefore bringing the object into sharp focus.
2. The Ciliary Body encircles the iris. It’s connected by strong fibers to the crystalline lens, which lies directly behind the iris. The lens is a flexible structure about the size and shape of an aspirin tablet. Like the cornea, the lens is transparent because it has no blood vessels and is relatively dehydrated. The muscles of the ciliary body make constant adjustments in the shape of the lens. These adjustments produce a sharp visual image at all times as the eye shifts focus between nearby and distant objects. The ciliary body also produces a clear, watery fluid called aqueous humor. This fluid nourishes and lubricates the cornea and the lens, and it fills the area between them. The ciliary body produces this continuously. The old fluid flows into a drainage system at a spongy, circular groove where the cornea and the sclera meet. It then travels through the veins of the eyeball into the veins of the neck.
3. The Choroid forms the back of the uveal tract. It looks and feels like a blotter soaked with blick ink. The choroid has many blood vessels. Blood from the choroids nourishes the outer part of the retina.
Bits of pigment (colored material) in the rods and cones absorb even the smallest particle of light that strikes the retina. The pigment in the rods is called rhodopsin or visual purple. It enables the eye to see shades of gray and to see in dim light. There are 3 types of pigment in the cones. They enable our eyes to see colors and to see sharp images in bright light. Cyanolabe absorbs blue light, chlorolabe absorbs green light, and erythrolabe absorbs red light. These pigments enable us to distinguish more than 200 colors.
Near the center of the retina is a round area called the macula lutea. The macula consists mainly of cones. It produces a sharp image of scenes at which the eyes are directly aimed, especially in bright light. The rest of the retina provides peripheral vision, it enables us to see objects to the side while looking straight ahead (the old sly ‘I can see what you’re doing but you don’t know I’m watching you’ trick). Most of the rods lie in this part of the retina. Because rods are more sensitive in the dark than cones, faint objects often can be seen more clearly if the eyes aren’t aimed directly at them. For example, looking at the side of a dim star makes its image fall on the part of the retina that has the most rods and provides the best vision in dim light.
Nerve fibers attached to the rods and cones join at the center of our retina and form the optic nerve. This nerve consists of about a million fibers. It serves as a flexible cable that connects the eyeball to the brain. In fact, the optic nerve and the retina are actually extensions of the brain. The optic nerve carries the electrical signals produced in the retina to the brain, which interprets them as visual images.
What is your blind spot?
Well, it's the point where the optic nerves enters the eye. It doesn't have any rods or cones and therefore cannot respond to light. Normally, we don't notice our blind spot because it covers such a small area and our eyes make so many quick movements. However, anything our blind spot of one of our eyes can't see, is seen by our other eye.