CONTINUING EDUCATION 2 CE Credits – $12.99 2 Hours, Technical, Level II Release date:  September 2008 Expiration date:  September 30, 2013		Preview Questions  TAKE THE TEST Other Available Tests
	A Tale of Ocular Anatomy

When you look at an object, light rays are reflected from the object to the cornea, which is where the miracle begins. The light rays are bent, refracted and focused by the cornea, the lens, and the vitreous. The lens’ job is to make sure the rays come to a sharp focus on the retina. The resulting image, however, is upside-down. Here at the retina, light rays are converted to electrical impulses, which are then transmitted through the optic nerve, to the brain, where the image is transmitted and perceived in an upright position.

Think of the eye as a camera. A camera needs a lens and a film to produce an image. In the same way, the eyeball needs a lens (cornea, crystalline lens, and vitreous) to refract, or focus the light, and a film (retina) on which to focus the rays. If any one or more of these components is not functioning properly, the result is a poor picture. The retina represents the film in the camera. It captures the image and sends it to the brain to be developed. The 
macula is the highly sensitive area of the retina. It is responsible for our critical focusing vision. It is the part of the retina most used. We use our macula when we read or stare intently. 

The area in the anterior chamber where the cornea and iris join is known as the ocular angle. This is comprised of several structures that make up the eye’s drainage system. This structure includes the outermost part of the iris, the front of the ciliary body, the trabecular meshwork, and the Canal of Schlemm. Aqueous is formed in the ciliary body behind the iris. It flows through the pupilary space into the anterior chamber. From there, the fluid travels into the angle structures and drains from the eye. As the aqueous fluid leaves the angle, it passes through a filter called the trabecular meshwork. After leaving the trabecular meshwork, the aqueous travels though a tiny channel in the sclera called the Canal of Schlemm. The aqueous then flows into other tiny channels and eventually into the eye’s blood vessels. The production and drainage of aqueous fluid determines the eye’s intraocular pressure.

The aqueous is the thin, watery fluid that fills between the cornea and the iris. It is continually produced by the ciliary body, the part of the eye that lies just behind the iris. The fluid nourishes the cornea and the lens and gives the front of the eye its form and shape. The choroid lies between the retina and the sclera. It is composed of layers of blood vessels that 
nourish the back of the eye. 

The choroid connects with the ciliary body toward the front of the eye and is attached to edges of the optic nerve at the back of the eye. The ciliary body lies just behind the iris. Attached to the ciliary body are tiny fiber “guy wires” called zonules. The crystalline lens is suspended inside the eye by the zonular fibers. Nourishment for the ciliary body and iris come from blood vessels. 

One function of the ciliary body is the production of aqueous humor, the clear fluid that fills the front of the eye. It also controls accommodation by changing the shape of the crystalline lens. When the ciliary body contracts, the zonules relax. This allows the lens to thicken, increasing the eye’s ability to focus up close. When looking at a distant object, the ciliary body relaxes, causing the zonules to contract. The lens becomes thinner, adjusting the eye’s focus for distance vision. With age, everyone develops a condition known as presbyopia. This occurs as the ciliary body muscle and lens gradually lose elasticity, causing difficulty in reading.

The conjunctiva is the thin, transparent tissue that covers the outer surface of the eye. It begins at the outer edge of the cornea, covering the visible part of the sclera, and lining the inside of the eyelids. It is nourished by tiny blood vessels that are nearly invisible to the naked eye. The conjunctiva also secretes oil and mucous which moisten and lubricate the eye. The cornea is the transparent, dome-shaped window covering the front of the eye. It is a powerful refracting surface, providing two-thirds of the eye’s focusing power. Like the crystal on a watch, it gives us a clear window to look through. Because there are no blood vessels in the cornea, it is normally clear, and has a shiny surface. The cornea is extremely sensitive. There are more nerve endings in the cornea than anywhere else in the body!

The adult cornea is about 1½mm thick. And is comprised of five layers: the epithelium, Bowman’s membrane, the stroma, Descemet’s membrane, and the endothelium (a layer of cells that covers the surface of the cornea). It is only about 5-6 cell layers thick and quickly regenerates when injured. If the injury penetrates more deeply into the cornea it may leave a scar. Scars leave opaque areas, causing the cornea to lose its clarity and luster. Bowman’s membrane lies just beneath the epithelium. Because this layer is very tough and difficult to penetrate, it protects the cornea from injury. The stroma is the thickest layer and lies just beneath Bowman’s membrane. It is composed of tiny collagen fibrils that run parallel to each other. The special formation of the collagen fibrils gives the cornea its clarity. Descemet’s membrane lies between the stroma and the endothelium. The endothelium is just underneath Descemet’s membrane and is only one cell layer thick. This layer pumps water from the cornea, keeping it clear. If damaged or diseased, these cells will NOT regenerate.

The six tiny muscles that surround the eye and control its movements are known as extra-ocular muscles. The primary function of the four rectus muscles is to control the eye’s movement from left to right and up and down. The two oblique muscles rotate the eyes inward & outward. All six muscles work in unison to move the eye. As one contracts, the opposing muscle relaxes, creating smooth movements. In addition to the muscles of one eye working together, the muscles of both eyes work in unison so that the eyes are always aligned.

The eyelids protect the eye from the environment, injury, and light. They also maintain a smooth corneal surface by spreading tears evenly over the eye. The lids are composed of an outer layer of skin, a middle layer of muscle and tissue, and an inner layer of moist, conjunctival tissue. Meibomian glands line the inner edge of the lids. The glands produce oil that lubricates the eye. Rows of lashes protect the eyes from the elements and debris. The colored part of the eye is called the iris. It controls light levels inside the eye similar to the aperture on a camera. The round opening in the center of the eye is called the pupil. The iris is embedded with tiny muscles that dilate and constrict the pupil size. The sphincter muscle lies around the very edge of the pupil. In bright light it contracts, causing the pupil to constrict. The dilator muscle runs radially through the iris like spokes of a heel. The muscle dilates the eye in dim lighting. 

The iris is flat and divides the anterior and posterior chambers. Its color comes from microscopic pigment cells called melanin. The color, texture, and patterns of each person’s iris are as unique as a fingerprint! The crystalline lens is located just behind the iris. Its purpose is to focus light on the retina. The nucleus is surrounded by a softer material called the cortex. The lens is encased in a capsular-like bag and suspended within the eye by the zonules. In young people, the lens changes shape to adjust for close or distance vision. This is called accommodation. With age, the lens gradually hardens, diminishing its ability to accommodate. The macula is 
located somewhere near the center of the retina, temporal to the optic nerve. It is a small and highly sensitive part of the retina responsible for detailed central vision. The fovea is the very center of the macula. 

The optic nerve transmits electrical impulses from the retina to the brain. It connects to the back of the eye near the macula. When examining the back of the eye, a portion of the optic nerve called the optic disc is visible. The retina’s sensory receptor cells are absent from the optic nerve. Because of this, everyone has a normal blind spot. This is not normally noticeable 
because the vision of both eyes overlaps. The pupil is the opening in the center of the iris. The size of the pupil determines the amount of light that enters the eye. This is controlled by the sphincter and dilator muscles of the iris. Doctors often evaluate the reaction of pupils to light to determine a person’s neurological function.

The retina is a multi-layered sensory tissue that lines the back of the eye. It contains millions of photoreceptors that capture light rays and convert them into electrical impulses. These travel on to the brain, where they are turned into images. There are two types of photoreceptors in the retina: rods and cones. The retina contains about six million cones, contained in the macula. Cones function best in bright light an allow us to distinguish and appreciate color. There are about 125 million rods. They are spread throughout the periphery of the retina, and function best in dim lightning. The rods are responsible for peripheral and night vision. 

The sclera is known as the “white” of the eye. It is tough, opaque tissue that serves as the eye’s protective outer coat. The optic nerve is attached to the sclera at the very back of the eye. In children the sclera is thinner and more translucent, allowing the underlying tissue to show through and give it a bit of a bluish cast. As we age, the sclera tends to become more yellow. 

Tears are formed by tiny glands that surround the eye. The tear film is comprised of three layers: oil, water, and mucous. The lower mucous layer serves as an anchor for the tear film and helps it adhere to the eye. The middle layer is comprised of water. The upper, oil layer seals the tear film and prevents evaporation. The tear film keeps the eye moist, creates a smooth surface for light to pass through the eye, nourishes the front of the eye, and provides protection from injury and infection. With each blink, the eyelids sweep across the eye, spreading the tear film evenly over the surface. The blinking motion of the eyelids forces the tears into tiny drains found at the inner corners of the upper and lower eyelids. These glands are called puncta. The tears travel from the puncta into the lacrimal sacs. The lacrimal sacs drain into the nasolacrimal duct which connects to the nasal passages. The connection between the tear production system and the nose is the reason your nose runs when you cry. Some people can actually “taste” eye drops as they drain from the nasal passage into the throat.

The vitreous is a thick, transparent substance that fills the center of the eye. It is composed mainly of water and comprises about two-thirds of the eye’s volume, giving it volume and shape. The viscous properties of the vitreous allow the eye to return to its normal shape if compressed. In children, the vitreous has a consistency similar to an egg white. With age it gradually thins and becomes more liquid. It is firmly attached to certain areas of the retina. As the vitreous thins, it separates from the retina, often causing floaters.