The Human Eye

Anatomy of the Eye

Human vision is a wonderful and complex process that is not yet completely understood. It involves the eye, nerves and brain all working together to provide the visual information we see.

The Eye

The outer shell of the eye, called the sclera, is the white rigid spherical shell that gives the eye is structure. The sclera itself is opaque. In order to allow light into the eye, it merges in the front with the transparent cornea, which is the window of the eye. The cornea has an index of refraction of about 1.37. Immediately behind the cornea is the aqueous humor, a clear watery liquid which supplies the cornea with the nutrients it needs since blood vessels in the cornea would affect the optical clarity.

The Pupil

The pupil is the opening in the center of the iris that controls the amount of light entering the eye. The iris merges with colored connective tissue called the choroid which lines the inside of the sclera. In humans, the pupil is circular, whereas horses and goats have a horizontal slit, and snakes, alligators and cats have a vertical slit.

Tiny muscles on the iris automatically adjust the size of the pupil within tenths of a second depending on the light level. It is interesting to note that the pupils of both eyes will open and close in unison, even if only one is stimulated with light due to the consensual pupillary reflex. In addition, our attitude about what we are seeing also influences the size of the pupil. This effect, common when viewing attractive pictures of the opposite sex, can effect the pupil size by up to 30 percent.

Research even illustrated that we are subconsciously aware of pupil size. Men found a picture of a woman more attractive when the photograph was retouched to make her pupils larger. None of the men studied consciously noticed the difference. Conversely, a more sinister, cold hateful look can be achieved with smaller pinpoint pupils.

Read about the effect of blue light on pupil contraction.

The Lens

The lens, which is immediately behind the iris, provides fine focusing adjustment, compensating for the distance from an object. This process is called accommodation and is accomplished by a ring of muscles around the lens. When the muscles are relaxed for viewing distant objects, the lens is relatively flat. When the muscles constrict to view objects close-up, the lens changes shape, becoming more curved.

The near-point is the closest the eye can still focus. This distance increases with age as the lens gradually looses elasticity. This distance usually surpasses your arm length between the ages of 50 and 60, which then calls for corrective lenses. Cataracts, or a loss of transparency of the lens, also affects many elderly people.

Vitreous Humor

The inner chamber of the eye is filled with a clear jellylike substance known as the vitreous humor. This structureless substance has an index of refraction close to that of water. Sometimes when you look carefully, you can see bits of cellular debris in the vitreous humor called floaters that give a faint shadow to the image you see.

See how color can cause our eyes to see optical illusions.

Retina

Click here for a discussion of the retina.