546
UNIT 3
Regulation and Integration of the Body
15
Te eyelid muscles are activated reflexively to cause blinking
every 3–7 seconds and to protect the eye when it is threatened
by foreign objects. Each time we blink, accessory structure se-
cretions (oil, mucus, and saline solution) spread across the eye-
ball surface, keeping the eyes moist.
Projecting from the free margin of each eyelid are the
eye-
lashes
. Te follicles of the eyelash hairs are richly innervated
by nerve endings (hair follicle receptors), and anything that
touches the eyelashes (even a puff of air) triggers reflex blinking.
Several types of glands are associated with the eyelids. Te
tarsal glands
are embedded in the tarsal plates (Figure 15.1b),
and their ducts open at the eyelid edge just posterior to the eye-
lashes. Tese modified sebaceous glands produce an oily se-
cretion that lubricates the eyelid and the eye and prevents the
eyelids from sticking together. Associated with the eyelash fol-
licles are a number of smaller, more typical sebaceous glands,
and modified sweat glands called ciliary glands lie between the
hair follicles (
cilium
5
eyelash).
Homeostatic Imbalance
15.1
An infected tarsal gland results in an unsightly cyst called a
chalazion
(kah-la
9
ze-on; “swelling”). Inflammation of any of the
smaller glands is called a sty.
Conjunctiva
Te
conjunctiva
(kon
0
junk-ti
9
vah; “joined together”) is a trans-
parent mucous membrane. It lines the eyelids as the
palpebral
conjunctiva
and reflects (folds back) over the anterior surface of
the eyeball as the
bulbar conjunctiva
(Figure 15.1b). Te bulbar
conjunctiva covers only the white of the eye, not the cornea (the
clear “window” over the iris and pupil). Te bulbar conjunc-
tiva is very thin, and blood vessels are clearly visible beneath it.
(Tey are even more visible in irritated “bloodshot” eyes.)
When the eye is closed, a slitlike space occurs between the
conjunctiva-covered eyeball and eyelids. Tis so-called
con-
junctival sac
is where a contact lens lies, and eye medications
are o±en administered into its inferior recess. Te major func-
tion of the conjunctiva is to produce a lubricating mucus that
prevents the eyes from drying out.
Homeostatic Imbalance
15.2
Inflammation of the conjunctiva, called
conjunctivitis
, results
in reddened, irritated eyes.
Pinkeye
, a conjunctival infection
caused by bacteria or viruses, is highly contagious.
Lacrimal Apparatus
Te
lacrimal apparatus
(lak
9
rĭ-mal; “tear”) consists of the lac-
rimal gland and the ducts that drain lacrimal secretions into the
nasal cavity
(Figure 15.2)
. Te
lacrimal gland
lies in the orbit
above the lateral end of the eye and is visible through the con-
junctiva when the lid is everted. It continually releases a dilute
saline solution called
lacrimal secretion
—or, more commonly,
tears
—into the superior part of the conjunctival sac through
several small excretory ducts.
Blinking spreads the tears downward and across the eyeball
to the medial commissure, where they enter the paired
lacrimal
canaliculi
via two tiny openings called
lacrimal puncta
(liter-
ally, “prick points”), visible as tiny red dots on the medial margin
of each eyelid. From the lacrimal canaliculi, the tears drain into
the
lacrimal sac
and then into the
nasolacrimal duct
, which
empties into the nasal cavity at the inferior nasal meatus.
Lacrimal fluid contains mucus, antibodies, and
lysozyme
,
an enzyme that destroys bacteria. Tus, it cleanses and protects
the eye surface as it moistens and lubricates it. When lacrimal
secretion increases substantially, tears spill over the eyelids and
fill the nasal cavities, causing congestion and the “sniffles.” Tis
spillover (tearing) happens when the eyes are irritated or when
we are emotionally upset. In the case of eye irritation, enhanced
tearing washes away or dilutes the irritating substance. Te im-
portance of emotionally induced tears is poorly understood.
Homeostatic Imbalance
15.3
Because the nasal cavity mucosa is continuous with that of the
lacrimal duct system, a cold or nasal inflammation o±en causes
the lacrimal mucosa to swell. Tis swelling constricts the ducts
and prevents tears from draining, causing “watery” eyes.
Extrinsic Eye Muscles
How do our eyes move? Six straplike
extrinsic eye muscles
control the movement of each eyeball. Tese muscles originate
from the walls of the orbit and insert into the outer surface of
Lacrimal gland
Excretory ducts
of lacrimal glands
Lacrimal punctum
Lacrimal canaliculus
Nasolacrimal duct
Inferior meatus
of nasal cavity
Nostril
Lacrimal sac
Figure 15.2
The lacrimal apparatus.
Arrows indicate the flow of
lacrimal fluid (tears) from the lacrimal gland to the nasal cavity.
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