Vision, Hearing, and Balance

The eyes and ears provide the body with its greatest protection. Because these organs are sensitive to distant stimuli, they can give early warnings about possible dangers.

The Eyes

The eye is often compared to a camera, but it is more compli­cated than the most sophisticated camera. Humans have binocu­lar vision, the ability to view objects with two eyes. People also have stereoscopic vision, the ability to see objects in three di­mensions—height, width, and depth. With stereoscopic vision a person can assess the speed of a moving object and determine the distance of an object in space.

Structure of the Eye

The eye, or eyeball, is an almost per­fect sphere with a diameter of about 2.5 cm (1 in.). The eye is protected in a number of ways. A fatty layer within the orbit, a socket in the skull, cushions the eyeball. Eyelids and eyelashes also provide protection by preventing foreign particles from en­tering the front of the eye. If something touches the eyelashes or moves suddenly in front of the eye, the eyelid closes and reopens rapidly in a blinking reflex.

Each eye is moved by three sets of muscles and is lubricated by mucus and tears. The mucus is secreted by the conjunctiva, a delicate, blood-rich membrane that lines the inner eyelid and covers the front of the eye. Tears are produced by the lacrimal gland near the outer corner of the eye. When the eye closes, the eyelid spreads the mucus and tears, which moisten the eye and help remove foreign particles.

The eye has an outer wall that consists of three layers of tissue: the sclera, the choroid, and the retina. These three layers surround a jellylike substance, the vitreous humor that makes up two-thirds of the eyeball.

The sclera is tough, white connective tissue that forms the outermost layer. About 80 percent of the sclera, including the ''white'' of the eye, is opaque. The remainder is a transparent layer called the cornea at the front of the eye.

The choroid is the middle, darkly pigmented layer of tissue. It absorbs light and so prevents reflection, which would result in fuzzy images. The choroid contains many blood vessels that nourish the eye. Toward the front of the eye, the choroid forms a colored ring, the iris, which gives the eye its color. In the center of the iris is an opening called the pupil. In bright light, one set of muscles in the iris contracts and causes the pupil to become smaller. In dim light, a different set of iris muscles contracts, making the pupil larger.

Behind the pupil is the lens, a transparent, curved structure. By changing shape, the lens helps focus images onto receptor cells at the rear of the eye. The curvature of the lens is con­trolled by ciliary muscles attached to the choroid. A clear, wa­tery fluid called the aqueous humor fills the space between the lens and cornea. The vitreous humor fills the space behind the lens.

The innermost layer of the eye is the light-sensitive retina. The retina contains about 125 million receptors called rods and cones. The rods and cones are stimulated by light to generate nerve impulses. The rods are extremely light-sensitive and can detect various shades of gray even in dim light. However, they cannot distinguish colors, and they produce poorly defined images. The cones detect color, produce sharp images, and are important for seeing in bright light. In a tiny pit at the center of the retina is a concentration of cones. This area, the fovea, produces the sharpest image.

How You See

Light passes through the cornea, aqueous humor, pupil, lens, and vitreous humor on its way to the retina. Impulses generated by the rods and cones travel to the visual center in the occipital lobe of the brain by means of the optic nerve. The optic nerve from each eye consists of about 1 million nerve fibers. No rods or cones exist at the point where the optic nerve enters the retina. This area, called the optic disc, or blind spot, does not transmit impulses. Near the base of the brain, half of the nerve fibers from the left eye cross over and join half of the nerve fibers from the right eye. All these fibers go to the right side of the brain. Likewise, half the nerve fibers from the right eye join half from the left eye and go to the left side of the brain. Each side of the brain thus receives images from both eyes. The point at which the partial crossing-over of the fibers occurs is the optic chiasma.