462
UNIT THREE
Sense of Sight
A number of accessory organs assist the visual receptors in
the eyes. These include the eyelids and lacrimal apparatus
that help protect the eyes and a set of extrinsic muscles that
move them.
Visual Accessory Organs
Each eye, lacrimal gland, and associated extrinsic muscles
are housed in the orbital cavity of the skull. The orbit, lined
with the periosteums of various bones, also contains fat,
blood vessels, nerves, and connective tissues.
Each
eyelid
(palpebra) is composed of four layers—skin,
muscle, connective tissue, and conjunctiva. The skin of the
eyelid, the thinnest of the body, covers the lid’s outer sur-
face and fuses with its inner lining near the margin of the lid
(f g. 12.22)
.
The muscles that move the eyelids include the
orbicu-
laris oculi
and the
levator palpebrae superioris.
Fibers of the
orbicularis oculi encircle the opening between the lids and
spread out onto the cheek and forehead. This muscle acts as
a sphincter that closes the lids when it contracts.
Crista
ampullaris
Hair cell
Supporting cells
Sensory nerve fibers
Hairs
Cupula
(a) Head in still position
(b) Head rotating
(c)
Crista ampullaris
Semicircular canal
Endolymph
Ampulla
FIGURE 12.21
±Equilibrium.±(
a
) When the head is stationary, the
cupula of the crista ampullaris remains upright. (
b
) When the head is
moving rapidly, (
c
) the cupula bends opposite the motion of the head,
stimulating sensory receptors.
FIGURE 12.20
A crista ampullaris is in the ampulla of each
semicircular canal.
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