Key Insights into the Anatomy of the Eye

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  • Topic: Eye, Retina, Orbit
  • Pages : 8 (3206 words )
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  • Published : March 18, 2012
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Before discussing conditions affecting the eye, we need to review some basic eye anatomy. Anatomy can be painful for some (personally, I hated anatomy in medical school) so I’m going to keep this simple. Let us start from the outside and work our way toward the back of the eye.Eyelids The eyelids protect and help lubricate the eyes. The eyelid skin itself is very thin, containing no subcutaneous fat, and is supported by a tarsal plate. This tarsal plate is a fibrous layer that gives the lids shape, strength, and a place for muscles to attach.Underneath and within the tarsal plate lie meibomian glands. These glands secrete oil into the tear film that keeps the tears from evaporating too quickly. Meibomian glands may become inflamed and swell into a granulomatous chalazion that needs to be excised. Don’t confuse a chalazion with a stye. A stye is a pimple-like infection of a sebaceous gland or eyelash follicle, similar to a pimple, and is superficial to the tarsal plate. Styes are painful, while chalazions are not.Eyelid Movement

Two muscles are responsible for eyelid movement. The orbicularis oculi closes the eyelids and is innervated by cranial nerve 7. Patients with a facial nerve paralyses, such as after Bell’s Palsy, can’t close their eye and the eye may need to be patched (or sutured closed) to protect the cornea. The levator palpebrae opens the eye and is innervated by CN3. Oculomotor nerve palsy is the major cause of ptosis (drooping of the eye). In fact, a common surgical treatment for ptosis involves shortening the levator tendon to open up the eye. CN 3 opens the eye like a pillar

CN 7 closes like a fish-hookConjunctiva
The conjunctiva is a mucus membrane that covers the front of the eyeball. When you examine the “white part” of a patient’s eyes, you’re actually looking through the semi-transparent conjunctiva to the white sclera of the eyeball underneath. The conjunctiva starts at the edge of the cornea (this location is called the limbus). It then flows back behind the eye, loops forward, and forms the inside surface of the eyelids. The continuity of this conjunctiva is important, as it keeps objects like eyelashes and your contact lens from sliding back behind your eyeball. The conjunctiva is also lax enough to allow your eyes to freely move. When people get conjunctivitis, or “pink eye,” this is the tissue layer affected.There is a thickened fold of conjunctiva called the semilunar fold that is located at the medial canthus - it is a homolog of the nictitating membrane seen on sharks.Tear Production and Drainage The majority of tears are produced by accessory tear glands located within the eyelid and conjunctiva. The lacrimal gland itself is really only responsible for reflexive tearing. Tears flow down the front of the eye and drain out small pores, called lacrimal punctum, which arise on the medial lids. These puncta are small, but can be seen with the naked eye. After entering the puncta, tears flow down the lacrimal tubing and eventually drain into the nose at the inferior turbinate. This explains why you get a runny nose when you cry. In 2-5% of newborns, the drainage valve within the nose isn’t patent at birth, leading to excessive tearing. Fortunately, this often resolves on it’s own, but sometimes we need to force open the pathway with a metal probe.Lid Lacerations Most lacerations through the eyelid can be easily reaproximated and repaired. However, if a laceration occurs in the nasal quadrant of the lid you have to worry about compromising the canalicular tear-drainage pathway. Canalicular lacerations require cannulation with a silicone tube to maintain patency until the tissue has healed.Warning: Drug absorption through the nasal mucosa can be profound as this is a direct route to the circulatory system and entirely skips liver...
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