Anatomy clinical correlates: Eye

The eyes enable us to see trees of green, red roses too… and basically everything in the wonderful world around us. From an anatomical perspective, the eyes are sensory organs, and they’re protected by a hard bony casing called the orbit, and shielded from the outside environment by softer tissues like the eyelids. Unfortunately, all of these structures are prone to various diseases - but luckily, understanding eye anatomy can help us recognize and treat these conditions!

Let’s start with the eyelids. Remember that they’re controlled by a muscle called the orbicularis oculi, which is a ring of muscles with two different parts that are arranged in circumferential bands around the orbit. The outer and thicker ring is the orbital part, while the thinner part that lies nearer to the eyelids is known as the palpebral part. The orbicularis oculi muscles are innervated by the facial nerve, and when they contract, they bring the eyelids together to close the eye for protection. So with a facial nerve lesion, the function of the orbicularis oculi muscle is affected, which impairs the muscle’s ability to close the eyelids. First, this means that blinking and moisturizing the front of the eye with lacrimal secretions is impaired, so the cornea can dry out. Then, there is also the added risk of foreign bodies entering the eye due to impaired blinking, for example sand blowing into our face during a windstorm. Abrasions and infections can then result which can ultimately lead to corneal ulceration.

Now, the eye can also be subject to infection even if the eyelids are working properly. One of the most common ones is hordeolum - usually referred to as a “stye”. This is an abscess of the eyelid, typically presenting as localized erythematous and painful swelling on the eyelid. A hordeolum can be external, which is when it arises from either the gland of Zeis or the gland of Moll which both secrete sebum in the eyelash follicle on the margin of the eyelid.

A stye can also be internal, in which case it arises from the meibomian gland, causing a swelling under the conjunctival side of the eyelid. The meibomian glands are the tiny oil glands at the roots of our eyelashes.

Another type of eyelid lesion known as a chalazion has an origin similar to that of a stye, a blocked duct of a gland, and it can also even develop from an old stye. Chalazions, however, differ from the hordeolums as they generally present as a painless swelling on the inner part of the eyelid, and more often than not, they involve the meibomian gland.