Chapter 15
The Special Senses
545
15
In this chapter we consider each of the five special senses in
turn. But keep in mind that our perceptions of sensory inputs
overlap. What we finally experience—our “feel” of the world—is
a blending of stimulus effects.
The Eye and Vision
Describe the structure and function of accessory eye
structures, eye layers, the lens, and humors of the eye.
Outline the causes and consequences of cataracts and
glaucoma.
Vision is our dominant sense: Some 70% of all the sensory re-
ceptors in the body are in the eyes, and nearly half of the cer-
ebral cortex is involved in some aspect of visual processing.
Te adult
eye
is a sphere with a diameter of about 2.5 cm
(1 inch). Only the anterior one-sixth of the eye’s surface is visible
(Figure 15.1a)
. Te rest is enclosed and protected by a cushion
of fat and the walls of the bony orbit. Te fat pad occupies nearly
all of the orbit not occupied by the eye itself. Te eye is a com-
plex structure and only a small portion of its tissues are actually
involved in photoreception. Before turning our attention to the
eye itself, let us consider the accessory structures that protect it
or aid its function.
Accessory Structures of the Eye
Te
accessory structures
of the eye include the eyebrows,
eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye
muscles.
Eyebrows
Te
eyebrows
are short, coarse hairs that overlie the supraor-
bital margins of the skull (Figure 15.1). Tey help shade the
eyes from sunlight and prevent perspiration trickling down the
forehead from reaching the eyes.
Eyelids
Anteriorly, the eyes are protected by the mobile
eyelids
or
palpebrae
(pal
9
pĕ-bre). Te eyelids are separated by the
palpe-
bral fissure
(“eyelid slit”) and meet at the medial and lateral an-
gles of the eye—the
medial
and
lateral commissures
(
canthi
),
respectively (Figure 15.1a).
Te medial commissure sports a fleshy elevation called the
lacrimal caruncle
(kar
9
ung-kl; “a bit of flesh”). Te caruncle
contains sebaceous and sweat glands and produces the whitish,
oily secretion (fancifully called the Sandman’s eye-sand) that
sometimes collects at the medial commissure, especially during
sleep. In most Asian peoples, a vertical fold of skin called the
epicanthic fold
commonly appears on both sides of the nose and
sometimes covers the medial commissure.
Te eyelids are thin, skin-covered folds supported internally
by connective tissue sheets called
tarsal plates
(Figure 15.1b).
Te tarsal plates also anchor the
orbicularis oculi
and
levator
palpebrae superioris
muscles that run within the eyelid. Te
orbicularis muscle encircles the eye, and the eye closes when
it contracts. Of the two eyelids, the larger, upper one is much
more mobile, mainly because of the levator palpebrae superioris
muscle, which raises that eyelid to open the eye.
(b) Lateral view;
some structures shown in sagittal section
Levator palpebrae
superioris muscle
Orbicularis
oculi muscle
Eyebrow
Tarsal plate
Palpebral
conjunctiva
Tarsal glands
Cornea
Palpebral
fissure
Eyelashes
Bulbar
conjunctiva
Conjunctival
sac
Orbicularis
oculi muscle
Eyelashes
Sclera
(covered by
conjunctiva)
Site where
conjunctiva
merges with
cornea
Lateral
commissure
Iris
Medial
commissure
Lacrimal
caruncle
Eyelid
Eyelid
Eyebrow
Pupil
Palpebral
fissure
(a) Surface anatomy of the right eye
Figure 15.1
The eye and accessory structures.
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