463
CHAPTER TWELVE
Nervous System III
The
lacrimal apparatus
consists of the
lacrimal gland,
which secretes tears, and a series of
ducts,
which carry the
tears into the nasal cavity
(f
g. 12.23)
. The gland is in the
orbit, superior and lateral to the eye. It secretes tears contin-
uously, which pass through tiny tubules and fl
ow downward
and medially across the eye.
Two small ducts (superior and inferior canaliculi) collect
tears, and their openings (puncta) can be seen on the medial
borders of the eyelids. From these ducts, the fl
uid moves into
the
lacrimal sac,
which lies in a deep groove of the lacrimal
Fibers of the levator palpebrae superioris muscle arise
from the roof of the orbit and are inserted in the connective
tissue of the upper lid. When these ± bers contract, the upper
lids are raised, and the eye opens.
The connective tissue layer of the eyelid, which helps
give it form, contains many modi± ed sebaceous glands (tar-
sal glands). Ducts carry the oily secretions of these glands to
openings along the borders of the lids. This secretion helps
keep the lids from sticking together.
The
conjunctiva
is a mucous membrane that lines the
inner surfaces of the eyelids and folds back to cover the ante-
rior surface of the eyeball, except for its central portion (cor-
nea). Although the tissue that lines the eyelids is relatively
thick, the conjunctiva that covers the eyeball is very thin. It
is also freely movable and transparent, so that blood vessels
are clearly visible beneath it.
Orbicularis
oculi
Eyelash
Tarsal glands
Cornea
Conjunctiva
Eyelid
Superior
rectus
Inferior
rectus
Tendon of levator
palpebrae superioris
FIGURE 12.22
Sagittal section of the closed eyelids and the anterior portion of the eye.
A child in school with “pinkeye” is usually sent straight home. Bacteria
cause this highly contagious form of inF
ammation of the conjunctiva,
or
conjunctivitis
.
Viral conjunctivitis is not usually contagious. Allergy
or exposure to an irritating chemical may also cause conjunctivitis.
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