Quarter 4 Portfolio

My eye

Our Eyes and brain divide what we see into a right and left half. In the drawing above, light gray represents the left half; dark gray represents the right half. The eyes invert the image and the left side of what we see ends up in the right side of our brain and visa versa. This all works out because the right side of our brain controls the left side of our bodies and visa versa.

Aqueous Humor

A water like fluid, produced by the ciliary body, it fills the front of the eye between the lens and cornea and provides the cornea and lens with oxygen and nutrients. It drains back into the blood stream through the canals of schlemm.

Ciliary Muscle

The eye can bring the fine print in a phone book into focus, or focus in on the moon over ¼ million miles away. The ciliary muscle changes the shape of the lens - (this is called accommodation). It relaxes to flatten the lens for distance vision; for close work it contracts rounding out the lens. Everyone will develop an eye condition called presbyopia. As we age, the ciliary muscle and crystalline lens lose their elasticity. This is why most people need reading glasses by their 40's.

Cornea

The cornea is a clear, dome-shaped surface that covers the front of the eye. It is the first and most powerful lens in the eye's optical system. To keep it transparent the cornea contains no blood vessels. Tears that flow over it and aqueous humor in the chamber behind it keep it nourished. When you hear of eye banks and eye transplants, it is the cornea that is being replaced. The cornea can be damaged from: accidents, infections, and genetic defects.

Fovea Centralist

The fovea is an indentation in the center of the macula. Its diameter is only 1.5 mm or about 1/16 inch. This small part of our retina is responsible for our highest visual acuity. It is the center of our central vision.  

Iris

This is the colored part of the eye: brown, green, blue, etc. It is a ring of muscle fibers located behind the cornea and in front of the lens. It contracts and expands, opening and closing the pupil, in response to the brightness of surrounding light. Just as the aperture in a camera protects the film from over exposure, the iris of the eye helps protect the sensitive retina. 

Lens

The eye's lens works like the adjustable lens in a camera. Positioned just behind the cornea; it is responsible for keeping images in focus on the retina. It is adjustable for distance and close work. A cataract is the lens clouding up. This happens to most people as they age. A few people are even born with cataracts. Modern surgery has all but eliminated cataracts as a cause of blindness in the developed world.

Muscles

Six muscles are in charge of eye movement. Four of these move the eye up, down, left and right. The other two control the twisting motion of the eye when we tilt our head. Defects in these muscles and the nerves that control them lead to conditions like Nystagmus and Amblyopia (Lazy Eye).

Optic Disk

The optic disk is the spot on the retina where the optic nerve leaves the eye. There are no sensory cells here, creating a blind spot. Each eye covers for the blind spot of the other eye and the brain fills in the missing information.

Optic Nerve

Each optic nerve has about 1.2 million nerve fibers. This is the cable connecting the eye to the brain.

Muscles

Six muscles are in charge of eye movement. Four of these move the eye up, down, left and right. The other two control the twisting motion of the eye when we tilt our head. Defects in these muscles and the nerves that control them lead to conditions like Nystagmus and Amblyopia (Lazy Eye).

Photoreceptor Cells

The retina is composed of two types of photoreceptor cells. When light falls on one of these cells, it causes a chemical reaction that sends an electrical signal to the brain. Cone cells give us our detailed color daytime vision. There are 6 million of them in each human eye. Most of them are located in the central retina - macula fovea area. There are three types of cone cells: one sensitive to red light, another to green light, and the third sensitive to blue light.
Rod cells are about 500 times more sensitive to light then cone cells; they give us our dim light or night vision. They are also more sensitive to motion then cone cells. There are 120 million rod cells in the human eye. Most rod cells are located in our peripheral or side vision.

Pupil

The pupil is the hole in the center of the iris that light passes through. The iris muscles control its size.

Retina

The retina is the film of the eye. It converts light rays into electrical signals and sends them to the brain through the optic nerve. The sides of the retina are responsible for our peripheral vision. The center area, called the macula, is used for our fine central vision and color vision. The retina is where most the problems leading to vision loss Occur. Three of the leading causes of blindness, from retina damage, are Retinitis Pigmentosa, Macular Degeneration and Diabetic Retinopathy.

Retinal Blood Vessels

A doctor can see the blood vessels that supply the retina when he looks into your eyes. These vessels are in the choroid just beneath the retina. Abnormal blood vessel growth and leaking blood vessels are the cause of vision loss in eye conditions like, Diabetic Retinopathy, ROP, and Macular Degeneration.

Sclera

The sclera is the white, tough wall of the eye. It along with internal fluid pressure keeps the eyes shape and protects its delicate internal parts.

Visual Axis

The Visual Axis is an imaginary line drawn through the center of the pupil to the center of the Fovea. The orbital muscles keep the visual axis of both eyes aligned on the center of what you are looking at (fixation point). An eye condition called Strabismus - (misaligned eyes) results when the orbital muscles fail to keep the eyes in alignment. Any damage to eye structures along this axis leads to severe vision loss.

Vitreous Humor

The vitreous humor is a jelly like liquid that fills most of the eye (from the lens back). As we age it changes from a gel to a liquid and gradually shrinks separating from the retina. This is when people start seeing floaters, dark specs in their vision. This is a normal sign of aging, but in a few cases the retina can become detached as the vitreous separates.