The External Eye Anatomy

• The eyelids: The upper and lower eyelids create moisture around the eye and protect the eye surface from any injury, infection, or disease. The eyelids contain muscles that enable them to open and close around the eye, and these muscles are covered with skin. The inner part of the eyelid is lined with mucous membrane, while the outer part is lined with eyelashes. As the eyelids open and close, this motion spreads the tear film over the eye’s surface, which in turn lubricates the eye. The eyelids also act to remove the tears from the eye’s surface by pushing the tears through the lacrimal puncta into the tear duct, which is also known as the “watermelon seed” effect.
• Eyelashes: The eyelashes are the hairs that grow along the edges of the upper and lower eyelids. The eyelashes act to protect the eye from foreign particles such as dust, pollen, and debris. The eyelashes are touch-sensitive and send signals to the eyelids to close when a foreign object comes in contact with the eyes. The eyelashes fall out on their own and take around six to eight weeks to grow back. Pulling out the eyelashes can cause damage to the eyelash follicles under the skin, from which the hairs grow. Eyelashes only grow to a certain length and never need to be trimmed. Eyelash color may differ from hair color, though people with dark hair will typically have darker lashes while people with light hair will usually have lighter lashes.
• Glands and ducts: The oil glands located inside the eyelids are known as meibomian glands. Their opening pores line the edges of the eyelids near the eyelashes. Around 25 to 40 meibomian glands line the upper eyelid and 20 to 30 line the lower eyelid. These glands secrete the oil that coats the eye and tear film to prevent the tears from evaporating too quickly. The lacrimal gland creates the water layer of the tears that lubricate, cleanse, nourish, and protect the surface of the eye. This gland is located on the upper lateral wall of the eye, under the eyebrow. The tear duct, also called the nasolacrimal duct, is located in the inner corner of the eye and is part of the tear drainage system that goes from the eye, through the back of the nose, and down the throat.

The tears are spread across the eye’s surface to moisten, clean, nourish, and protect the ocular tissues. When the eyelids open and close, they push the tears out of the eye through the tear duct, allowing them to be drained down the back of the nose. When the tear duct becomes clogged or blocked, the tears are unable to drain out of the eye properly, usually resulting in an irritated, watery eye.

The Internal Eye Anatomy

• Conjunctiva: It is a clear membrane that covers the white portion of the eye or the sclera. The conjunctiva also covers the inside of the eyelids. It produces mucus and tears to lubricate the eyes and keep microbes out of them.
  Ciliary body: The ciliary body sits between the choroid and the iris, and it works to produce the aqueous humor and hold the lens in place.
• Pupil: The pupil is the opening at the center of the iris through which light passes.
• Lens: The lens is the clear part of the eye behind the iris that helps to focus light and images on the retina.
• Iris: The iris is the colored part of the eye that surrounds the pupil. It regulates the amount of light that enters the eye.
• Cornea: The cornea is the clear, outer part of the eye’s focusing system located at the front of the eye. It functions as a window and allows light to enter the eye. It also begins the process of focusing light rays that allow us to see words and images clearly. The cornea provides 65-75% of the eye’s focusing power. The cornea does not contain any blood vessels but instead contains many nerve endings that make it extremely sensitive. That is why a scratch or a loose eyelash is so painful.
• Aqueous humor: This thick fluid rests in the anterior chamber and provides nutrients to the anterior and posterior parts of the eye. The liquid must drain regularly, and the body replaces it.
• Vitreous humor: The vitreous gel is a transparent, colorless mass that fills the rear two-thirds of the eyeball, between the lens and the retina.
• Sclera: The sclera is more commonly known as the whites of the eyes. This fibrous layer contains collagen and protects the inner components of the eye from damage.
• Choroid: This small, vascular layer sits between the eye’s sclera and retina. It provides the outer layers of the retina with nourishment (through blood vessels) and oxygen. While one won’t develop health issues in the choroid, it is the component that refracts light and is responsible for the red-eye effect while taking photos.
• Retina: The retina is the light-sensitive tissue at the back of the eye. The retina converts light into electrical impulses that are sent to the brain through the optic nerve. The retina has special cells called photoreceptors. These cells change light into energy that is transmitted to the brain. There are two types of photoreceptors: rods and cones. Rods perceive black and white and enable night vision. Cones perceive color and provide central (detail) vision.
• Fovea centralis: The fovea is a small depression in the retina that contains cones to aid in proper eyesight.
• Macula densa: The macula is the small, sensitive area of the retina that gives central vision. It is located at the center of the retina.
• Optic nerve: The optic nerve is the largest sensory nerve in the eye. The retina sends light as electrical impulses through the optic nerve to the brain. The optic nerve is made up of millions of nerve fibers that transmit these impulses to the visual cortex, the part of the brain responsible for our sight.
• Trabecular meshwork: This component is located at the base of the cornea. It drains the aqueous humor from the eye through the anterior chamber. Using tubes known as Schlemm’s canal, the trabecular meshwork lets fluid drain into the blood system.
• Blood vessels: The ophthalmic artery and its branch, the central retinal artery, provide oxygenated blood to each eye. Similarly, ophthalmic veins (vortex veins) and their branch, the central retinal vein, drain the deoxygenated blood from the eye. The blood vessels enter and leave the eye through the back of the eye.