Accessory parts of the Eye

Accessory parts of the Eye
Photo: https://www.sciencealert.com/

Accessory (adnexal) parts of the eye are anatomical structures that surround and protect the eyeball and support the function of the eye. Those structures include the orbit, eyebrows, eyelids, lacrimal apparatus and extraocular muscles. Since the (dis)function of extraocular muscles is very important in the pathophysiology of strabismus and amblyopia and we plan to devote it a whole blog soon, we will not be dealing with the extraocular muscles today.

The eye is confined into the bony anatomical structure called the orbit

The orbit is an anatomical space in our skulls in which the eyeball, extraocular muscles, orbital fat, and nerves and vessels are situated. Seven different bones that are fused together, form the margins of the orbit – frontal, lacrimal, ethmoid, zygomatic, maxillary, palatine and sphenoid bone. Although the orbital bones are very hard structures, blunt trauma to the eye can damage them, resulting in the blowout fracture of the orbit. It is most commonly caused by sports injuries or motor vehicle accidents.

The exact function of the eyebrows remains debatable

Eyebrows are specialized hair located above the eyes. Their main evolutionary function is supposed to prevent sweat, water, and moisture from flowing into the eye. Nonetheless, some evolutionary biologists disagree and, as newer research shows, believe that the eyebrows developed as a mean of nonverbal communication by facial expressions. Indeed, a smooth forehead combined with dense eyebrows let us express a series of subtle emotions.

A healthy vision starts with healthy eyelids…

Eyelids are important organs that help to maintain a favourable environment for the proper function of the eye underneath. They are made up of several layers; skin on the outside, conjunctiva with meibomian glands on the inside and muscles and connective tissue in between. Along their margins, they feature specialized hair that is the eyelashes.

The meibomian glands that are on the inside of the eyelids, adjacent to the ocular surface, secrete the thin oily layer of the tears that prevents the evaporation of the tear film. The eyelids, especially the upper eyelid, are also efferent initiators of the blink reflex which shields our eyes from foreign bodies. By constant blinking, the eyelids spread the tears evenly along the ocular surface keeping it moist all the time. People normally blink spontaneously around 10 times per minute which is enough to keep our eyes moist and the ocular surface stable.

However, during prolonged computer use our blinking rate decreases, leading to a condition referred to as computer vision syndrome. It is a common disorder in the modern world and is characterized by discomfort, eye strain, and fatigue. I bet quite a few of you have already experienced similar symptoms at least once in your lives. Remember then, do not forget to blink during prolonged computer, tablet or cell phone use!

…continues through healthy lacrimation system…

Lacrimal apparatus is a system responsible for tear production and drainage and is under autonomic nervous system control. The tears are produced by the lacrimal gland that is located in the upper lateral region of the orbit. The tears then flow across the eye and are drained into the nasal cavity through the lacrimal duct at the eyes’ medial region next to the nose. I am sure most of you have already experience runny nose following extensive tearing – this is simply because the tears drain into the nasal cavity!

…and the quality of the tear film.

It is important to note, though, that the quality of tear film does not directly correlate to the number of tears that are produced, and that conditions such as dry eye disease can either be characterized by (I) the lack of tears or (II) the altered quality of tears. For easier comprehension let us first tell you a few words about the tear film itself. The tear film is made up of 3 layers: (a) an innermost mucin layer, that is produced by ocular surface epithelial cells, lubricates the ocular surface and prevents pathogens adherence and entrance; (b) a middle thick watery layer, that is secreted by the lacrimal gland, nourishes the cells of the ocular surface and has antibacterial as well as antioxidant functions; (c) a thin superficial oily layer, that is produced by meibomian glands under the eyelids, hinders the evaporation rate of the watery layer below.

If any of those layers get compromised it can lead to disorders and disease of the ocular surface. This is most commonly seen in the dysfunction of the lacrimal gland and the decreased production of the watery layer, or the dysfunction of meibomian glands and the lack of thin superficial oily layer. Whilst these conditions are not among the easiest to treat, various pharmacological and behavioral interventions exist to alleviate the discomfort and pain produced by ocular surface disorders.

I hope you enjoyed this blog post that talks about everything going on around the eye – but you can learn much more about vision and eyes inside our “Following the Light” blog series!

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