Anatomy: External structures
- Tear and Fat Glands
- Extrinsic / Extraocular muscles
- upper and lower eyelids are part of our skin "folded up"
- the eyebrow marked the extent of the upper eyelid
- the junction of the upper and lower eyelids are called lateral & medical canthus
- there are small muscles controlling the opening and closure
- prevent entry of excessive light and foreign particles
- Blinking (the rapid opening and closure of eyelids at intervals of approx. 6 sec, maybe voluntary or reflex) lubricate the eye surface by spreading tear evenly
Lacrimal (tear) glands are located outside the cornea of each eye. Tears are a watery solution of sodium chloride(NaCl) and hydrocarbonate(NaHCO3) salts. The excess tears are drained away by the lachrymal duct to the nasal cavity.
Uses of tears:
- keep the cornea moistened for clarity
- wash foreign bodies out
- stimulated on having strong emotions e.g. anger, grief (though the exact mechanism is not known)
- contains an enzyme called lysozyme to kill bacteria
Meibomian (fat) glands are located at the edge of the eyelids. The fatty secretions lubricates the eyelids and also keep tears from flowing out of our eyes.
Eyelashes, together with eyebrows, stop dust and sweat from running into our eyes.
We call these 3 antagonistic pairs of muscles extrinsic because they are external to our eye, in contrast to our ciliary muscles inside our eye.
The 3 pairs are:
- medial & lateral rectus (horizontal)
- superior & inferior rectus (vertical)
- superior & inferior oblique (torsional / twisting movements).
They can be under voluntary control, but more often, they perform automatic movements. The 3 nerves responsible are the oculomotor, trochlear and abducens nerves.
The muscles anchor our eye to the bony socket, change shape of eyeball for change of focus, move the eyeball independently of head and keep movements of both eyes in sychronization.
- covers the whole cornea (bulbar conjunctiva) and lines the inner side of the eyelids (palpebral conjunctiva)
- conjunctiva does not belong to fibrous coat.
- a transparent membrane for light to pass though
- the small buldge of pink tissue we see at the medial cornea of the eye, called caruncle, is also part of the conjunctiva.
- the infection in conjunctiva is trachoma.
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