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Chapter 2

Eye Anatomy
The Eyes Have It
M’aam, your husband came out of surgery just fine …

by Tim Root
… thanks to the hard work of our staff and the unflappable tenacity of the doctors …

… we’ve managed to save your husband’s eyes!

Now … where would you like them sent?

Basic Eye Anatomy
by Tim Root, M.D. Before discussing conditions affecting the eye, we need to review somebasic eye anatomy. Anatomy can be a painful subject 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 tarsalplate 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 styeis 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 deeper chalazions are not.

You can’t depend on your eyes when your imagination is out of focus. Mark Twain

Eyelid Movement
Two muscles are responsible for eyelid movement. The orbicularis oculi closes the eyelids and is innervated bycranial nerve 7. Patients with a facial nerve paralyses, such as with Bell’s Palsy, can’t close their eye and their eyelids may need to be patched (or sutured closed) to protect the cornea from exposure. The levator palpebrae opens the eye and is innervated by CN3. Oculomotor nerve (CN3) palsy is a major cause of ptosis 22

(drooping of the eye). In fact, a common surgical treatment for ptosisinvolves shortening the levator tendon to open up the eye.

CN 3 opens the eye like a pillar CN 7 closes like a fish-hook

Conjunctiva
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 semitransparent conjunctiva to the white sclera of the eyeball underneath. The conjunctiva starts atthe 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 move freely. When people getconjunctivitis, 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 onlyresponsible 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 punctual holes are small, but can be seen with the naked eye. 23

After entering the puncta, tears flow down the lacrimal tubing and eventually drain into the nose at the inferior turbinate. This drainage pathway explains why you geta 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 its own, but sometimes we need to force open the nasolacrimal duct with a metal probe.

Most lacerations through the eyelid can be easily reaproximated and repaired. However, if a laceration occurs in the nasal quadrant of...
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