Eyelids and lacrimal apparatus
After completing this study unit you will be able to:
- Identify the various components of the eyelids and name their functions.
- Describe the anatomical location of the various structures belonging to the lacrimal apparatus and understand the movement of lacrimal fluid through them.
The eye is an important sensory organ, which allows us to perceive and visualize the world around us. The accessory structures of the eye are equally important, as they lubricate and protect this organ from our often harsh surroundings.
The eyelids are thin, elastic folds of skin that can cover the anterior aspect of the eyeball. With the help of the eyelashes (cilia) along their borders, they prevent the entry of foreign particles into the eye as well as exposure to excessive light. Several glands located in the eyelid produce important secretions for the lubrication and protection of the eye: the ciliary glands (of Moll) produce a lipid-based compound that prevents the evaporation of the tear film; the sebaceous glands (of Zeis) produce sebum for the hair follicles of the cilia; and the tarsal glands (a.k.a. Meibomian glands) secrete a sebaceous substance, also known as meibum, creating an oily layer across the surface of the eyeball that helps to prevent evaporation of the tear film.
Watch the following video to learn more about the different components of the eyelids.
The lacrimal apparatus is responsible for the production, movement and drainage of lacrimal fluid from the eyeball. The lacrimal gland produces this fluid (which forms the aqueous layer of the tear film) and has two parts: a superior orbital part and an inferior palpebral part. The aqueous secretions of the lacrimal gland reach the eye through excretory ducts, move across the eyeball, lubricating its anterior surface, and are drained by several structures in the medial corner of the eye. When we cry and produce an excess of this fluid, it can no longer be adequately drained and overflows out of our eyes to form tears. Follow the journey in this video!
Take a quiz
Do you have an eye for detail when it comes to this topic?! Test yourself with the quiz below:
Want to test yourself on the entire contents of the orbit? Try the following quiz, which you can further customize yourself, right down to specific topics and terms!
Have another look at each of the individual structures of the eyelids in the gallery below.
Revise all of the structures of the lacrimal apparatus in the following gallery.
|Function||Shield eyes from dust and other foreign particles; protect eyes from injury and excessive light; maintain a moist surface on the cornea|
|Layers||From superficial to deep: Skin and eyelashes (cilia), subcutaneous tissue, skeletal muscle (orbicularis oculi muscle/levator palpebrae superioris), tarsal plate, palpebral conjunctiva|
|Secretions of the eyelid||
Ciliary glands (of Moll): Secrete a lipid-based compound, prevents evaporation of tear film
Sebaceous glands (of Zeis): Secrete sebum for hair follicles of the eyelashes (cilia)
Tarsal glands (Meibomian glands): Secrete a sebaceous substance (meibum), for an oily layer across the anterior eyeball to prevent evaporation of tear film
|Function||Production, movement and drainage of lacrimal fluid|
Production of lacrimal fluid: Lacrimal gland
Excretion: 6-12 excretory ducts of lacrimal gland
Drainage: through lacrimal caruncle, lacrimal papilla, lacrimal canaliculus, lacrimal sac, nasolacrimal duct → inferior nasal meatus
Continue your learning
Now that you are an expert on the eyelids and lacrimal apparatus you can advance further and learn about the microscopic anatomy of these structures in the following histology study units.