Structure and Function of the Eye
The eye is the sensory organ of sight. It is located within the orbital cavity of the face and is surrounded by protective structures including the eyebrows, eyelashes, and eyelids, which help keep foreign objects out of the eye.
The eyeball is a globe-shaped organ that consists of three layers (Fig. 14-1). These are the sclera, the outer portion; the choroid, the middle portion; and the retina, the inner portion. Each of these layers functions to protect the eye, provide vision, or communicate vision to the brain.
The outermost layer of the eye includes the sclera and cornea. The sclera has a distinctive white color. It provides strength, structure, and shape to the eye. At the front of the eye, the sclera bulges forward to become the cornea, which is transparent and allows light into the eye. A thin mucous membrane called the conjunctiva covers the outer surface of the eye and lines the eyelids. The conjunctiva contains many tiny blood vessels and secretory glands. These glands produce a clear, watery mucus that allows the eyelid to slide smoothly over the eye when you blink. When the eye is irritated, the tiny blood vessels dilate (enlarge) and become more prominent. This makes the whites of the eyes appear bloodshot or reddened.
Over-the-counter eyedrops that “get the red out” work by constricting (narrowing) the enlarged blood vessels in the irritated eye.
The middle layer of the eyeball is the choroid layer. It is a dark-blue vascular layer between the sclera and retina that supplies blood to the entire eye. The optic nerve, which is attached to the retina, exits the posterior eye through an opening in the choroid and extends to the brain, where visual messages are delivered.
Other structures in the choroid include the iris, pupil, ciliary body, lens, and suspensory ligaments. The iris is a circular structure that surrounds the pupil and gives the eyes their typical color. The pupil functions as an adjustable window that lets light into the inner structures of the eye. The iris contracts in low lighting, thereby dilating the pupil and making it appear bigger. This allows more light into the eye. In brightly lit environments, the iris expands, causing the pupil to constrict and appear smaller. This decreases the amount of light entering the eye. The ciliary body includes several ciliary muscles, including a circular muscle that lies posterior to the iris. It is attached to the lens by the suspensory ligaments. The lens is a clear, firm, transparent disk. With the help of ciliary muscles, the lens continually changes shape, enabling us to focus clearly on objects we are viewing. For near vision, the ciliary muscles contract, causing increased rounding of the lens; ...