CHAPTER 20
Eye
389
Tunica Vasculosa (Tunica Media)
Figure 20-9A
Overview of the Iris and Nearby Structures
Figure 20-9B
Iris
Figure 20-9C
Constrictor Pupillae Muscle and its Function
Figure 20-9D
Dilator Pupillae Muscle and its Function
Figure 20-10A
Anterior Surface of the Lens
Figure 20-10B
Posterior Surface of the Iris
Figure 20-11A
Overview of the Ciliary Body and Nearby Structures
Figure 20-11B
Ciliary Processes and the Ciliary Muscle
Figure 20-11C
Ciliary Body, Transverse and Posterior Views
Figure 20-12A
Posterior Portion of the Ciliary Body
Figure 20-12B
Ora Serrata
Figure 20-12C
Clinical Correlation: Outfl ow of Aqueous Humor and Glaucoma
Retina (Tunica Interna)
Figure 20-13A
Overview of the Retina; Fovea
Figure 20-13B
Macular Region of the Retina and the Retinal Layers
Figure 20-13C
Peripheral Region of the Retina and Distribution of the Rods and Cones
Figure 20-14A,B
A Representation of Rods and Codes
Table 20-1
Comparison of Rods and Cones
Figure 20-15A–C
Retinal Layers
Synopsis 20-1
Retinal Layers
Figure 20-16A
Clinical Correlation: Age-Related Macular Degeneration
Figure 20-16B
Clinical Correlation: Retinal Detachment
Optic Nerve
Figure 20-17A
Optic Nerve and Optic Disk
Figure 20-17B
Clinical Correlation: Papilledema
Synopsis 20-2
Clinical Terms for the Eye
Introduction and Key Concepts
for the Eye
The
eye
is the organ of vision, perhaps the most important of the
sensory modalities. The eye converts light into nerve impulses in
a way that allows the brain to be aware of the individual’s visual
surroundings. In our study, we divide the structure of the eye
into three general categories: those structures which
protect
the
eye (
eyelids
); those structures which help
form a visual image
of
what the individual is looking at (
cornea
,
lens
,
sclera
, and asso-
ciated structures); and those structures which
convert the visual
image into nerve impulses
and conduct the impulses to the brain
(
retina
and
optic nerve
), where they are analyzed to produce the
sensation of vision.
The basic structure of the eye is that of a hollow sphere
with optical elements on the anterior surface that focus an
inverted image of the surroundings onto the inside of the pos-
terior wall. The primary structural element of the sphere is
the
tunica ±
brosa
, or
tunica externa
, which consists of the
sclera
and the
cornea
. Lining the inside of the tunica externa
is the
tunica vasculosa
, consisting of the
choroid
, the
ciliary
body
, and the
iris
. The eye is ±
lled with a transparent liquid
(
aqueous humor
) and a transparent gel (
vitreous body
). In the
anterior portion of the eye is the
lens,
which is fl
exible and
can adjust the focus of the image depending on the distance
from the eye to the object being viewed. The image is focused
on the
retina
, a layer of neurons and neural receptors that line
the internal surface of the posterior two thirds of the eye. The
axons of the
ganglion cell neurons
in the retina leave the eye
and form the
optic nerve
.
The Eyelids
The
eyelids
protect the eyes from injury by foreign objects and
also maintain a thin ± lm of
moisture
on the surface of the cornea
that prevents the cornea from drying out and becoming opaque.
Each eyelid consists of an outer layer of
skin
; a middle layer of
muscle
,
glands
, and
connective
tissues
(
tarsal
plate
); and an inner
layer of
conjunctival
tissue
(
palpebral
conjunctiva
). There are
several types of glands in the eyelid that aid in keeping the cor-
nea moist, including
meibomian glands
,
glands of Zeis
,
glands
of Moll
, and
accessory lacrimal glands
(Figs. 20-3 and 20-4).
Several muscles are associated with the eyelids. These
include (1) the
orbicularis oculi muscle
, a circular sheet of stri-
ated muscle that is innervated by the facial nerve (cranial nerve
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