填空题 .  The Structure of the Human Eye
    Comprised of more than thirty, major parts, the human eye ranks among the body's most intricately designed and complex organs. However, for simplicity's sake, this complicated body part can be reduced to its major components—the pupil, iris, lens, cornea, retina, and optic nerve—and their main functions. Each part plays a vital role in granting sight to a person.
    The most familiar parts of the eye are the pupil and the iris because they are easily visible when looking at a person's eyes. The pupil is the central hole that allows light to enter the eye. It typically changes size to permit entry to more or less light by contracting in bright light and expanding in dim light. The iris is the colored area of the eye surrounding the pupil and is the part which controls the contracting and expanding of the pupil thanks to muscles connected to it that enable the iris to open and close.
    The cornea and lens let the eye focus light in particular ways after it enters the eye. The cornea is located in front of the pupil and iris while the lens lies behind them. The cornea is a curved, transparent structure that bends light toward the pupil. On account of this function, the cornea has no blood vessels that could potentially cloud it, so it receives nutrients from the watery tears that cover the eye whenever a person blinks. As for the lens, it takes the light entering the pupil and focuses it toward the back of the eye in the direction of the retina. The lens has the ability to change shapes to focus on objects at various distances.
    The retina is formed by sensitive photoreceptor cells called rods and cones, which form the images that a person sees through the pupil. The rods handle black and white images and are used mostly in situations involving dim light while the cones are responsible for images requiring colored light. As images form on the retina, they are instantaneously sent along the optic nerve to the brain, where they are interpreted as what a person sees.
    *component: a piece or pad of something larger
    *cloud: to make blurry7.  The author discusses the iris in paragraph 2 in order to ______