The Human Eye Colin Istvan

Eye It is estimated that the eye took more than 30 million years to evolve. The human eye is not the largest, most sensitive, or most complex eye in the animal kingdom. The human eye differs from others in the animal kingdom in many ways, including its basic design.

Eye Layout The histology of the eye is very complex, and still not completely understood. All of the parts of the eye are essential to vision, and a problem with any of them can lead to vision impairment. The human eye is also one of the fastest healing parts of the human anatomy, but also one of the most delicate.

Histology of the eye The layers of the eye, as light passes through them is as follows:  The Cornea This is the clear covering of the eye that contact lenses come in contact with This is the second fastest tissue in the human body, second only to lips PRK (Lasek) uses a laser to reshape this tissue and correct vision, reducing the dependency on corrective lenses.

Histology Cont'd The next layer of the eye is the aqueous humor  This layer is actually a clear fluid separating the cornea and the lens  This fluid is important to keep the lens of the eye clean, and to help carry nutrients to the poorly vasculated areas of the eye The pupil  The pupil the black dot at the center of an eye.  It is the hole that light passes to reach the lens  The pupil is surrounded by the iris, which contains muscles used to focus the lens of the eye

Histology Cont'd The lens  The lens is the part of the eye that focuses the entering light on the light sensitive retina  The lens is the source of many eye problems, as a small change can cause large issue with the focus of an image The vitreous humor  The vitreous humor is another clear fluid in the eye  It maintains the eye's round shape by applying outward pressure  The vitreous humor can be surgically replaced to help clear clouding that can occur with old age

Histology Cont'd The Retina  The retina is the light sensitive portion of the eye  It contains two main types of cells: Rods  Rods are the cells in the eye that allow night vision.  They allow greater clarity at night, but are unable to resolve colors Cones  Cones are less sensitive to light, but allow for color vision  Colorblindness is usually caused by a problem with these cells, but can be neural in nature

Histology Cont'd The eye is poorly vasculated in the interior, but well vasculated around the circumference The retina has nerves that run along the top, and culminate at the optic disc The nerves that run along the top of the retina cause visual clarity to be diminished  Hawks have excellent vision because the optical nerves run behind the retina The optic disc, where all the nerve coalesce, is the cause of the “blind spot”

The Optic Nerve The optic nerve runs from the back of the eye, through an opening in the orbital bone, to the occipital lobe at the back of the brain The optic nerve contains more than 12 million nerve fibers, yet is less than 1cm wide By implanting electrodes that are attached to cameras to the optic nerve, it is possible to restore crude vision to those that are blind

Sight Problems Hyperopia  Also called hypermytropia, or farsightedness  Caused by the lens focusing light behind the retina  Easily fixed using convex corrective lenses Myopia  Also known as nearsightedness or shortsightedness  Caused by the lens focusing light in front of the retina  Easily fixed using biconcave corrective lenses

Sight Problems Cont'd Cataracts  Caused by a clouding of the lens  Most common in the elderly, but can occur at any age  Easily treated surgically bu removal and replacement of the lens Colorblindness  First discovered by John Dalton, a British chemist  Can be caused by heredity, brain or nerve damage, or exposure to certain chemicals  Affects 8.5 percent of population

Eyes of the Animal kingdom The largest eye in the animal kingdom is that of the giant squid: up to ten inches across The sharpest eye in the animal kingdom is that of the birds of prey: more than 8 times sharper than the best human eye The best night vision in the animal kingdom is a tie between the barn owl, and the common opossum The most complex eye in the animal kingdom belongs to the stomatopod, a species of shrimp

Stomatopod eyes The eye of the stomatopod has many characteristics that set it apart form all others  It is sensitive to all of the colors that human eyes are, but also to infrared and ultraviolet  It can determine distance with just one eye, whereas other animals require binocular vision  It is extremely sensitive to light, and can control the amount of light entering by using several lenses, not just a pupil

Eyesight An experiment conducted in the 1960's and 70's by Colin Blakemore radically changed perception of how vision is developed By keeping a kitten's eyes shut during development, he discovered that when the eye is not used, vision is not developed Another experiment by Blakemore used kittens that wore goggles that only allowed vertical lines to pass through. The kittens were then placed in a tunnel with only horizontal bars impeding the way, but the kittens could not navigate. These experiments demonstrated that much of perceived reality is based upon what we observe during early childhood.