Anatomy and Physiology of the eye
Eye Anatomy & Physiology
The eyelashes are our first line of defence from foreign objects such as dust, dirt, pollen or insects entering the eye. The modified hairs of the eyelid known as cilia also protect the eyes against the elements such as wind, rain and sunlight.
The upper eyelid houses 100-150 upward-curving longer lashes than the lower eyelid, which usually contains 50-75 downward, shorter lashes. The root of each eyelash is anchored to the root hair plexus, which tells the brain to blink instantly to protect the delicate eye from injury.
Eyelashes are also connected to a network of capillaries and glands to aid in lubrication, nutrition and microbe defence. The Glands of Moll are small, modified sweat glands that open near the base of the eyelash. The Glands of Zies are small, modified oil glands that secrete from the eyelid margin directly into the tears. Both glands work together to keep lashes supple and oily and attract debris, and to keep themselves coated with a layer of protection.
1, The Eyelids
The eyelids are made up of a thin fold of skin, which can be opened and closed by special muscles. Eyelids are split into upper and lower portions that meet at the medial and lateral canthi of the eye. The opening between the eyelids is called the palpebral aperture or opening.
The eyelid consists of five main layers (superficial to deep):
- Skin and subcutaneous tissue
- Orbicularis oculi
- Tarsal plates
- Levator apparatus
- Conjunctiva
The skin and subcutaneous tissue make up the most superficial layer of the eyelid. The skin is amongst the thinnest in the human body. The subcutaneous layer comprises loose connective tissue but lacks subcutaneous fat.
The eyelashes are attached to the eyelid with their accompanying modified sweat glands, the ciliary glands of Moll. There are also sebaceous glands called glands of Zeis located in this layer.
The orbicularis oculi have three distinct sections, palpebral, lacrimal and orbital. The muscle is attached to the lacrimal bone and attaches to the skin of the orbit, superior and inferior tarsal plates.
- The Palpebral part is responsible for closing the eyelids.
- The Lacrimal part drains the tears.
- The Orbital part tightly closes the eyelid.
The tarsal plates are located deep to the palpebral region of the orbicularis oculi muscle. There are two plates, the superior tarsus of the upper eyelid and the inferior tarsus of the lower eyelid.
The tarsal plates form the scaffolding of the eyelid and are composed of dense connective tissue. In the tarsal plates lies the Meibomian glands or the tarsal glands, which are specialised types of sebaceous glands that secrete an oily substance over the surface of the eye, which slows down the evaporation of the eye’s tear film. This oily substance also prevents the eyelids from sticking together when closed.
The eyelid has a rich arterial supply from numerous vessels such as the ophthalmic artery, facial artery and superficial temporal artery. Venous drainage is provided by a rich network around the eyelid.
Sensory innervation to the eyelids is supplied by the trigeminal nerve.
2, Sclera
The Sclera is better known by the more common name, the ‘white of the eye’. The Sclera helps to protect and support the structure of the eye. It’s composed of tough tissue, which ensures the eyeball keeps its structure and shape. This tough tissue reduces injury to the eye and also provides a surface for the ocular muscles to attach to.
3, Tear Duct
The tear duct, or the nasolacrimal duct as it is more formally known, is found in the corner of the eyes, closest to the nose.
These ducts carry away excess tears from the surface of the eyes and carried through to an area inside your nose. This explains why when you cry or suffer from watery eyes or an allergy that you can sometimes taste the salt from your tears in your mouth and why your nose begins to run.
4, Lacrimal Gland
The lacrimal glands are almond-shaped structures found in the upper part of the eye socket above each eye. The basic function of this gland is to produce tears which are then collected on the conjunctiva of the upper eyelid. Tears help to nourish and hydrate the cornea and keep the eye clean and lubricated.
5, conjunctiva
The conjunctiva makes up the lining inside the eyelids and almost covers the Sclera, and is nourished by tiny blood vessels, almost invisible to the naked eye. The conjunctiva acts as a vessel for the tears to be spread over the eyes surface to properly lubricate them.
Conjunctivitis is a common condition associated with the conjunctiva when the area becomes inflamed, usually due to infection.
6, Pupil
The Pupil is the small black hole found in the centre of the eye and surrounded by the colourful iris, and leads directly to the inside of the eye. The Pupil acts as an entry point for light to enter the eye and surrounding muscles within the iris called the pupillae help to adjust the Pupil’s diameter depending on the light conditions. The Pupil’s contract and dilate to allow less or more-light to enter the eye, allowing you to see better in darker conditions and protecting the eye when the light is stronger.
7, Iris
The iris is the colourful ring that surrounds the Pupil, and it consists of tiny pigmented cells called melanin, which determines the colour of the eyes.
During light conditions, the iris causes the Pupil to decrease in size, and when it is darker, the iris causes the Pupil to enlarge to 6mm in diameter to allow more light to enter the eye. The iris acts as a wall that separates the anterior chamber between the cornea and iris.
Inside the eye
1, cornea
The cornea is a thin layer that covers the iris, pupil and anterior chamber. A healthy cornea is completely transparent. It has a vital number of roles, including its ability to focus incoming light and making up the eye’s total optical power. It also protects the iris and pupil from foreign objects entering inside the eye. It works as a light filter to screen out the sun’s harmful rays and prevents them from reaching the retina, where they may cause damage.
2, Crystalline Lens
This is a transparent structure that is convex on both sides and suspended by suspensory ligaments. Located just behind the iris, the crystalline lens is not visible from the outside unless using specialist equipment. The cillary muscles can adjust the shape of the lens, which allows it to focus incoming light, ensuring that images are Sharpley focused on the retina at the back of the eye.
As the cillary muscles contract, the supersensory ligaments which hold the lens in place will relax. This causes the lens to become more spherical in shape, allowing the eye to focus on close up objects and the muscles relax again, making the lens flatter to allow the eye to focus on objects in the distance.
3, Cillary Body
The cillary body surrounds and attaches to the crystalline lens. It is circular in shape and separates the anterior chamber from the vitreous. There are three components that make up the cillary body: the suspensory ligaments or zonules, cillary muscle and cillary processes. The ligaments and muscles work in tandem to alter the shape of the crystalline lens and adjust the focus of the light entering the eye.
4, retina
The retina is light-sensitive and coats the inner part of the eye. It connects directly to the brain via the optic nerve. The retina processes the incoming light and converts the energy into electrical signals. These signals are then sent to the brain through the optic nerve, where they are translated into images.
5, macula
The macula is a sensitive, circular area on the surface of the retina that appears darker than the space around it. It contains a large number of photoreceptors in comparison to the rest of the retina. This allows it to interpret higher levels of colour, providing the majority of the central vision.
6, Optic Disc
The optic disc signifies the beginning of the optic nerve and has first contact with the retina. This is the start of the visual information received by the eye, beginning its journey to the brain. As there are no photoreceptors on the surface of the optic disc, it is unable to process any visual information and is the reason behind the blind spot’ in your vision.
7, Centre Retinal Vein and Artery
The central retinal vein and artery line the back of the retinal wall. The artery supplies blood through to the sclera and across to the retina. The vein carried away blood or other waste products away from the retina.
8, Optic Nerve
The optic nerve travels from the brain to the back of the eye, where it becomes the optic disc once it meets the retinal wall. It comprises 1.2 million nerve fibres that work together to send the impulses that will be converted into a visual image. The right side of the brain receives information from the left visual field of both eyes, whereas the left side of the brain receives information from the right visual field of both eyes.
9, Vitreous
Around 80% of the eye’s total mass comprises vitreous, a clear gel that fills the space between the crystalline lens and the retina. Its main function is to keep the layers of the retina tightly pressed together as well as maintaining the general structure of the eyeball.
The Vitreous is completely stagnant and contains no blood vessels to carry away waste. This means that anything that enters the eye will remain there unless surgically removed.
10, Muscles of the eye
There are four muscles responsible for facial expression and control of eyebrow position.
These are:
| Frontalis | Elevates the brows |
| Orbicularis oculi | Closes the eyelids and draws the brows towards the eyes |
| Corrugator supercilia | Pulls the nasal eyebrow inferiorly and medially, producing vertical glabellar wrinkles |
| Procerus | Pulls the eyebrow inferiorly |
The eyebrow growth cycle can last 12-15 weeks. The anagen phase lasts around 30-45 days. The telogen phase lasts for around 2-4 months.