Vision is a very important sense for humans. We use our vision to understand our world, and to understand ourselves. A person in a pitch dark room in unfamiliar surroundings would be quite uncomfortable. A person having to carry a very full cup of coffee without looking at the coffee cup may spill hot coffee all over themselves. But what is it that we see?
As you travel through these web pages on vision, you will be learning about the eye, light, how light affects our eyes, and then how the light information gets carried back to the brain. Some bits of this story should already be familiar to you. For example, you know that light has to pass through the eye’s pupil, and that the information from the eye gets carried back toward the brain through the optic nerve (cranial nerve II), passes through the optic chiasm, makes a pitstop in the thalamus, and then runs to the occipital lobe. This stuff you already know.
But, you do not yet know how the eye functions. What are the sensory receptors for light? How do they work? What happens to the light when it enters the eye? The answers to these questions are some of what you’ll learn here.
Anatomy of the eye
- The Sclera is a membrane of tendon in the eye, also known as the white of the eye. Rugged and robust, the sclera works to protect the inner, more sensitive parts of the eye like the retina and choroid. It is about 0.03 of an inch thick except for where the four “straight” eye muscles append, where the depth is no more than 0.01 of an inch. Four of six eye muscles on each eye are known as “straight” eye muscles (Mm. Recti) but all six control the movements of the eye. On top of the sclera, the episclera is located. The episclera contains blood vessels that nourish the sclera with oxygen and nutrients. These vessels are thus visible on the surface of the white of the eye.
- The Choroid is the layer located within the sclera. The layer consists of a network of vessels that nourish the retina with oxygen and nutrients. The macula and the anterior part of the optic nerve are dependant on blood supply from the choroid.
- The Retina refers to the light sensitive tissue in the back of the eyeball. The retina absorbs light that ultimately is sent as visual signals to the brain. The retina contains two kinds of light receptors, the cones and the rods. The cones, which are color sensitive, are located in the center of the retina and mainly absorb stronger light. The rods absorb softer light in black and white and are located peripheral to the fovea.
- The Vitreous body is a clear gel-like substance filling the eyeball, behind the lens. Aqueous fluid fills the space in front of the lens. When you get older, the vitreous body shrinks in volume. When the vitreous body decreases in size it can detach from the retina. This is referred to as vitreous body detachment and is in itself not a dangerous phenomenon. It is only when the vitreous body drags along the retina that complications can occur. This is called retinal detachment.
- The Macula is the area around the fovea. Closely-packed visual cells in the macula result in high image resolution, or high visual acuity. Around the macular area, visual cells are not as closely packed as in the macula itself, but they are good enough for our peripheral vision to work properly. A common eye disease amongst the elderly is age-related macular degeneration (AMD). This condition can be divided into two sub-categories: “dry” or “wet”. Wet AMD is treatable with FDA-approved drugs such as Lucentis, but it affects the eye worse than dry AMD, often leading to more serious cases of vision loss. Dry AMD is an incurable eye disease and, unfortunately, the most common form of AMD.
- The Fovea is the most central part of the macula. The visual cells located in the fovea (cones) are packed tightest, resulting in optimal sharpness of vision, perfect for reading or watching TV. Rods are packed just outside the fovea, and are active in low light conditions. The fovea covers about 5 degrees of the vision field.
- The Optic Disc, also known as the optic nerve head or the blind spot, is the point where the optic nerve attaches to the eye. The size of the blind spot is equivalent to the width of the optic nerve. It is the visible portion of the optic nerve during an eye examination. All visual messages from cone and rod cells leave the eyeball from this point and then are relayed through nerve threads to the optic center of the brain. The absence of visual cells in the optic disc causes the appearance of a blind spot in your field of vision.
- The Lens is where about 20 of the eye’s 60 diopters obtain diopteric power. The remainder of the eye’s diopteric power is contained in the cornea. Furthermore it is the lens that enables us to change the focus according to different distances, e.g. when we change focus from a TV screen across the living room to a newspaper in our hand. The lens is attached to a mass of threads called zonula threads. The zonula threads are then attached to the ciliary body. We can compare this optic correlation with a bicycle wheel where the lens is the hub, the threads the spokes and the ciliary body the rim. When we then want to focus on a near object, a muscle in the ciliary body contracts. This way the zonula threads can loosen up, allowing the lens to contract in diameter and thicken, thus increasing its acuity. As a person becomes older, the flexibility of the lens decreases. By the age of 45 the lens will often have stiffened to the extent that normal reading vision, without corrective glasses or lenses, is unattainable. Cataracts are an age-related deficiency in the lens that sooner or later affects everybody in some form. The most common symptom associated with cataracts is blurry vision. Cataracts can result in removal and replacement of the lens.
- The Optic nerve sends signals from the eye to the different parts of the brain where the signals are interpreted into images. The optic nerve consists of about 1,000,000 nerve threads. The optic nerves from both eyes are reconnected behind the eyes so that everything that is seen in the right field of vision is sent to the left cerebral hemisphere and vice versa.
- The Pupil is essentially just a hole in the iris. The reason why it’s black is because the layer of pigment inside the eye absorbs major parts of the light, thus resulting in a darker shade. However, when pupils appear red in photos it is actually the color of the retina that is reflected.
- The Iris is the part of the eye that regulates the amount of light that enters the eye. With strong light, the iris sphincter muscles will contract the pupil. In darkness, the iris opens the pupil using the dilator muscles. When focusing on near objects, the pupil decreases in diameter, but it will expand when focusing on distant objects, a reflex known as the Accommodation Reflex. A smaller pupil enables better focal depth. A person’s eye color actually refers to the amount and type of pigments in his/her iris. The most common color is brown, while the least common is green.
- Aqueous fluid (or aqueous humor) fills the front part of the eye, between the lens and the cornea. This fluid is produced at the back of the ciliary body, then seeps through the pupil, into the anterior chamber and ultimately is drained through the trabecular meshwork. The aqueous fluid’s main function is to supply the cornea and the lens with nutrients and oxygen. The anterior chamber is the space located between the iris and the cornea.
- The Cornea is the transparent structure located in the front of the eye, covering the iris, pupil, and anterior chamber. About 40 of the eye’s 60 diopters are present in the cornea. The cornea is supplied with oxygen and nutrients through tear-fluid and not through blood vessels. That also explains why the cornea is so clear.
- The Ciliary Body contains the ciliary muscle, the muscle that controls the vision accommodation reflex. The ciliary body also produces aqueous fluid (aqueous humor) and is a point of attachment for the zonula threads that holds the lens.
- The Conjunctiva is a mucous membrane that covers the sclera and the inside of the eyelids. Many of the glands that play a part in the production of tear-film are located in the conjunctiva.