Anatomy and physiology of the eye
The eye is the organ of sight (vision). A person has 2 eyes located below the forehead and eyebrows at the front of the face, within the eye socket area of the skull. One eye is located on either side of the bridge of the nose. Only about one-sixth of the eyeball can be seen – the rest is protected by bone and surrounding tissue of the eye socket (orbit).
The eye is made up of 3 main parts:
- orbit (eye socket)
- accessory (adnexal) structures
The main part of the eye is the eyeball (also called the globe). Each eye is sphere-shaped and is about 2.5 cm (1 inch) in diameter. The eyeball is rich in blood vessels. The inside of the eyeball is filled mostly with a clear, jelly-like fluid called vitreous humor. Vitreous humor fills the back (posterior) part of the eye. It helps support the internal structures and maintain the shape of the eye.
The outer part of the eyeball is called the wall of the eye. It can be divided into 3 layers (or tunics): an outer, middle and inner layer (from the outside to the inside of the eye).
The outermost layer or covering of the wall of the eye is made up of the sclera and cornea and is called the fibrous tunic.
- sclera – The sclera is the tough, white connective tissue that covers most of the outside of the eyeball. The sclera is seen as the white portion of the eye and serves as the protective covering. The optic nerve and blood vessels pass through the sclera in the back of the eye. Muscles that control the movement of the eye attach to the sclera.
- cornea – The cornea is the clear, dome-shaped covering at the front of the eye that lets in light. The cornea covers the pupil and the iris. It does not contain any blood vessels.
The middle layer of the wall of the eye is called the uvea or vascular tunic. The uvea has 3 main parts:
- iris – The iris is the thin, muscular, coloured part of the eye. It is located at the front (anterior) of the eye, between the cornea and the lens.
- The iris opens and closes the pupil (the small central opening) to change the amount of light entering the eye.
- choroid – The choroid is a thin layer of tissue that contains many tiny blood vessels that supply oxygen and nutrients to the retina.
- The choroid contains many pigment-producing cells called melanocytes. These cells help absorb any excess light and minimize reflections within the eye.
- ciliary body – The ciliary body lies just behind the iris and extends forward from the choroid.
- It is the muscular ring of tissue that helps the eye focus. It changes the shape of the lens so it can focus on near or far objects.
- The ciliary body contains cells that make aqueous humor, which is the clear fluid in the front of the eye between the cornea and lens.
The innermost layer of the wall of the eye is made up of the retina or neural tunic. The retina is the thin layer of cells at the back of the eyeball and works like the film of a camera. It is made up of nerve cells that are sensitive to light. These cells are connected to the brain by the optic nerve, which sends information from the eye to the brain and allows us to see.
The lens is a transparent structure in the inner part of the eye, which lies directly behind the cornea and iris. The lens changes shape to allow the eye to focus on objects. The lens focuses light rays on the retina.
The orbit (eye socket) is a bowl-shaped cavity made up of bone formed from the skull that contains the eyeball and the connective tissues surrounding the eyeball. The bone and connective tissues cushion and protect the eye. Muscles attached to the eyeball make it move in different directions. These small muscles attach to the sclera near the front of the eye and to the bones of the orbit at the back. The orbit also contains nerves, fat, blood vessels and a variety of connective tissues.
The accessory (adnexal) structures of the eye include the eyelids, conjunctiva, caruncle and lacrimal (tear) glands.
The eyelids (palpebrae) are folds of skin that cover and protect the eye. Muscles raise and close the eyelids.
- The eyelids contain glands, which produce an oily secretion that covers the tear layer and prevents tears from evaporating and the eyelids from sticking together.
- The eyelid is described as having an anterior (front) and a posterior (back) lamella.
- The anterior lamella consists of skin, a layer of fatty connective tissue and a layer of muscle fibres. It helps protect the eye and regulate the amount of light that reaches the eye.
- The posterior lamella consists of a layer of muscle, the palpebral conjunctiva and the tarsal plates. The tarsal plates are 2 thick plates of dense connective tissue found inside each eyelid (upper and lower) that help form and support the eyelid.
- Eyelashes grow from the edges of the eyelids. They help protect the eye from dust and debris.
The conjunctiva is a clear mucous membranemucous membraneThe thin, moist layer of tissue that lines some organs and body cavities, including the nose, mouth, lungs, airways, vagina and gastrointestinal (GI) tract. that lines the inner surface of the eyelids and the outer surface of the eye. The conjunctiva secretes mucus to lubricate the eyeball and keep it moist.
- Bulbar conjunctiva is the part of the conjunctiva that covers the front, outer surface of the eyeball.
- Forniceal conjunctiva is the loose fold that connects the conjunctival membrane that lines the inside of the eyelid with the conjunctival membrane that covers the eyeball.
- Palpebral (or tarsal) conjunctiva is part of the conjunctiva that covers the inner surface of the eyelids.
- The plica is a small fold of conjunctival tissue next to the caruncle in the inside corner of the eye.
The caruncle is the small, pinkish portion of the innermost corner of the eye (or inner canthus) that contains oil and sweat (sebaceous) glands and conjunctival tissue.
The lacrimal gland (tear gland) is the almond-shaped gland located at the upper, outer corner of each eye.
- The lacrimal gland secretes tears to help keep the surface of the eye and lining of the eyelids moist and lubricated. Tears help reduce friction, remove dust and debris from the eye and prevent infection.
- Small lacrimal ducts (lacrimal canaliculi) drain tears from the lacrimal gland through very tiny, openings (lacrimal punctum) inside the inner corner of each eyelid.
The eye is the organ that works with the brain to provide us with the sense of sight. It works much like a camera. The main function of the eye is to collect light and turn it into electric signals, which are sent to the brain. The brain then turns those signals into a visual image or picture for us to see. We have 2 eyes, so 2 pictures are usually created. If we lose the vision in one eye, we continue to see most of what we could see before.
When light enters the eye, it first passes through the cornea. The light then passes through the pupil, where the iris adjusts the amount of light entering the eye. The light then passes through the lens of the eye. The lens focuses light rays onto the retina, where it is changed into a signal that is transmitted to the brain by the optic nerve. The signal is received and interpreted by the brain as a visual image.