Eyes

Anatomy of the eye

The tissues of the eye can be split into three types:

refracting tissues that focus light

light-sensitive tissues

support tissues

We will look at each of these in turn.

Refracting tissues Refracting tissues focus incoming light onto the light-sensitive tissues, to give us a clear, sharp image. If they are the wrong shape, misaligned, or damaged, vision can be blurry.

The refracting tissues include:

The pupil: This is the dark spot in the center of the colored part of your eye, which, in turn, is called the iris. The pupil expands and shrinks in response to light, acting similarly to the aperture on a camera.

In very bright conditions, the pupil constricts or shrinks to around 1 millimeter (mm) in diameter to protect the sensitive retina from damage. When it is dark, the pupil can dilate or widen up to 10 mm in diameter. This dilation allows the eye to take in as much light as possible.

Iris: This is tLens: Once light has traveled through the pupil, it reaches the lens, which is a transparent convex structure. The lens can change shape, helping the eye to focus light accurately onto the retina. With age, the lens becomes stiffer and less flexible, making focusing more difficult.

Ciliary muscle: This muscular ring is attached to the lens and, as it contracts or relaxes, it changes the shape of the lens. This process is called accommodation.

Cornea: This is a clear, dome-like layer that covers the pupil, iris, and anterior chamber or fluid-filled area between the cornea and the iris. It is responsible for the majority of the eye’s focusing power. However, it has a fixed focus so cannot adjust to different distances.he colored portion of the eye. The iris is a muscle that controls the size of The cornea is densely populated with nerve endings and incredibly sensitive. It is the eye’s first defense against foreign objects and injury. Because the cornea must remain clear to refract light, it has no blood vessels.

Two fluids circulate throughout the eyes to provide structure and nutrients. These fluids are:

Vitreous fluid: Found in the back section of the eye, vitreous fluid is thick and gel-like. It makes up the majority of the eye’s mass.

Aqueous fluid: This is more watery than vitreous fluid and circulates through the front of the eye.The retina is the innermost layer of the eye. It houses more than 120 millionTrusted Source light-sensitive photoreceptor cells that detect light and convert it into electrical signals.

These signals are sent on to the brain for processing.

Photoreceptor cells in the retina contain protein molecules called opsins that are sensitive to light.

The two primary photoreceptor cells are called rods and cones. In response to particles of light, the rods and cones send out electrical signals to the brain.Cones: These are found in the central region of the retina called the macula, and they are particularly dense in a small pit at the center of the macula known as the fovea. Cones are essential for detailed, color vision. There are three types of cones, normally called:

• short or blue

• middle or green

• long or red

Cones are used to see in normal light conditions and allow us to distinguish colors.Optic nerve: This thick bundle of nerve fibers transmits signals from the retina to the brain. In all, there are around 1 million thin, retinal fibers called ganglion cells that carry light information from the retina to the brain.

The ganglion cells leave the eye at a point called the optic disc. Because there are no rods and cones, it is also referred to as the blind spot.