The Eye and Vision
In the embryo, the eye develops as an outpocketing of the brain. It is a delicate organ, protected by a number of structures:
* The skull bones form the walls of the eye orbit (cavity) and protect more than half of the posterior part of the eyeball.
* The upper and lower eyelids aid in protecting the eye’s anterior portion (Fig. 7-1). The eyelids can be closed to keep harmful materials out of the eye, and blinking helps to lubricate the eye. A muscle, the levator palpebrae, is attached to the upper eyelid. When this muscle contracts, it keeps the eye open. If the muscle becomes weaker with age, the eyelids may droop and interfere with vision, a condition called ptosis.
* The eyelashes and eyebrow help to keep foreign matter out of the eye.
* A thin membrane, the conjunctiva, lines the inner surface of the eyelids and covers the visible portion of the white of the eye (sclera). Cells within the conjunctiva produce mucus that aids in lubricating the eye. Where the conjunctiva folds back from the eyelid to the anterior of the eye, a sac is formed. The lower portion of the conjunctival sac can be used to instill drops of medication. With age, the conjunctiva often thins and dries, resulting in inflammation and enlarged blood vessels.
Figure 7-1 Protective structures of the eye.
* Tears, produced by the lacrimal glands (Fig. 7-2), lubricate the eye and contain an enzyme that protects against infection. As tears flow across the eye from the lacrimal gland, located in the upper lateral part of the orbit, they carry away small particles that may have entered the eye. The tears then flow into ducts near the nasal corner of the eye where they drain into the nose by way of the nasolacrimal duct (see Fig. 7-2). An excess of tears causes a “runny nose”; a greater overproduction of them results in the spilling of tears onto the cheeks. With age, the lacrimal glands produce less secretion, but tears still may overflow onto the cheek if the nasolacrimal ducts become plugged.
Figure 7-2 The lacrimal apparatus. The lacrimal (tear) gland and its associated ducts are shown.
Coats of the Eyeball
The eyeball has three separate coats, or tunics (Fig. 7-3). The outermost tunic, called the sclera, is made of tough connective tissue. It is commonly referred to as the white of the eye. It appears white because of the collagen it contains and because it has no blood vessels to add color. (Reddened or “bloodshot” eyes result from inflammation and swelling of blood vessels in the conjunctiva). The second tunic of the eyeball is the choroid. This coat is composed of a delicate network of connective tissue interlaced with many blood vessels. It also contains much dark brown pigment. The choroid may be compared to the dull black lining of a camera in that it prevents incoming light rays from scattering and reflecting off the inner surface of the eye. The blood vessels at the posterior, or fundus, of the eye can reveal signs of disease, and visualization of these vessels with an ophthalmoscope is an important part of a medical examination. The innermost tunic, the retina, is the actual receptor layer of the eye. It contains light-sensitive cells known as rods and cones, which generate the nerve impulses associated with vision.
Figure 7-3 The eye. Note the three tunics, the refractive parts of the eye (cornea, aqueous humor, lens, vitreous body), and other structures involved in vision.
Pathway of Light Rays and Refraction
As light rays pass through the eye toward the retina, they travel through a series of transparent, colorless parts described below and seen in Figure 7-3. On the way, they undergo a process known as refraction, which is the bending of light rays as they pass from one substance to another substance of different density. (For a simple demonstration of refraction, place a spoon into a glass of water and observe how the handle appears to bend at the surface of the water.) Because of refraction, light from a very large area can be focused on
a very small area of the retina. The eye’s transparent refracting parts are listed here, in order from exterior to interior:
* The cornea is an anterior continuation of the sclera, but it is transparent and colorless, whereas the rest of the sclera is opaque and white. The cornea is referred to frequently as the window of the eye. It bulges forward slightly and is the main refracting structure of the eye. The cornea has no blood vessels; it is nourished by the fluids that constantly wash over it.
* The aqueous humor, a watery fluid that fills much of the eyeball anterior to the lens, helps maintain the slight forward curve of the cornea. The aqueous humor is constantly produced and drained from the eye.
* The lens, technically called the crystalline lens, is a clear, circular structure made of a firm, elastic material. The lens has two bulging surfaces and is thus described as biconvex. The lens is important in light refraction because it is elastic and its thickness can be adjusted to focus light for near or far vision.
* The vitreous body is a soft jellylike substance that fills the entire space posterior to the lens (the adjective vitreous means “glasslike”). Like the aqueous humor, it is important in maintaining the shape of the eyeball as well as in aiding in refraction.