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No. 19
1. Introduction of the Sensory Organs
2. Visual Organ
PART Ⅴ The Sensory Organs
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Sensory organs include the
receptors and accessory organs.
The receptors receive the stimulation
from the external or internal
environment of the body, and
convert it into nerve impulses.
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Classification of the receptors:
On the bases of the location and the origin of the
senses, receptors may be divided into three kinds.
1. The exteroceptors They are located in the
skin, the mucous membrane of the nasal and oral
cavity, pain, light and sound from the external
environment.
2. The proprioceptors They are located in the
muscles, tendons, joints and ligatents, and
receive stimuli from there they located.
3. The interoceptors or visceroceptors They
are located in the wall of viscera, the heart and
blood vessels, and pick up information about the
internal environment.
Chapter 1 The Visual Organ
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The visual organ (eye) receives
stimulation of light and convert it
into nerve impulses. The nerve
impulse is conducted through the
visual pathways to the visual center
in the brain, and produces visual
sensation and visual reflex.
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The eye is the sense organ that contains
the receptors for vision. Each eye is
situated in a bony cavity in the skull called
the orbit.
The visual organ consists of eyeball and
accessory organs of eye. Most portions of
the eye are concerned with gathering and
focusing light rays from that are sensitive
to light rays.
Section 1 The Eyeball
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The eyeball is embedded in the fat
inside the orbit, but is separated
from it by the fascial sheath of
eyeball (capsule of Tenon).
Ⅰ. The Shape of Eyeball
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The eyeball is spherical in shape with
a diameter of approximately 2.5 cm.
Optic and visual axes:
The central points of the anterior and
posterior surfaces of the eyeball are
known as the anterior and
posterior poles, the line joining the
two poles is the axis oculi.
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An imaginary line encircling the
eyeball, midway between the
anterior and posterior poles, is the
equator.
Of more importance is the axis
optica, it joins the center of the
pupil to the fovea centralis of the
retina. The optic axis and the visual
axis decussate sharply.
Ⅱ. The Structure of Eyeball
The eyeball comprises the wall of the
eyeball and the contents enclosed by it.
Ⅰ) The Wall of Eyeball
 It consists of three tunics, from outside
inwards, which are the fibrous tunic, the
vascular tunic and the retina.
1. The fibrous tunic
 It consists of the posterior 5/6 of the
fibrous tunic, the sclera, and the anterior
1/6 , the cornea.
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1) The cornea
Characteristics:
It is the anterior one-sixth of the fibrous tunic. It
is transparent and more convex part of the
external tunic, through which the light enters the
eye.
The cornea is a non-vascular structure, but
numerous sensory nerve terminals supply the
cornea, this makes it very sensitive to pain and
touch stimulation.
Function:
It has the supporting, protecting and dioptric
functions.
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2) The sclera
Characteristics
It is the posterior 5/6 of the fibrous tunic. It
consists of strong fibrous connective tissue, is
white in colour and opaque.
Posteriorly, the sclera is pierced by the optic
nerve fibers and is continuous with dura mater
through the fibrous sheath of this nerve. Where
the optic nerve fibers pierce the sclera, the latter
has the appearance of a cribriform plate and is
named the cribribform plate of sclera, where
is a weakened point of the sclera.
A circular sinus venous sclerae is in the sclera,
nearer to the cornea.
Function:
It plays an important role for maintaining the
shape of the eyeball and protecting the contents
within it.
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2. The vascular tunic
Divisions:
It is the middle coat, and consists of the iris,
ciliary body and choroids.
Characteristics
This layer contains numerous blood vessels and
pigment cells and is brown in colour.
Function
Its function is to provide the nutrition for the
tissue inside the eyeball, and to regulate the
amount of light entering the eyeball.
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1) The iris
①Characteristics
It is the most anterior coloured part of the
vascular tunic.
In shape the iris is a circular, muscular perforated
diaphragm. Near its center is a round aperture,
named pupil which controls the amount of light
entering the eye. The iris uncompletely separates
the anterior chamber from the posterior chamber.
The anterior and posterior chambers of the eye
communicate through the pupil.
Colour:
The colour of iris varies from different races, e.
g. , it is:
light blue in White,
brown in Chinese,
very dark brown in Black people.
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② Structure
Iridocorneal angle (angle of anterior chamber):
Trabecular reticulum:
Smooth muscles:
The iris contains two sets of smooth muscles.
The circularly arranged muscle fibers form the
sphincter pupillae which constricts the pupil.
The radially arranged muscle fibers form the
dilator pupillae which dilates the pupil.
The size of the pupil is altered by the contraction
of the sphincter pupillae or the dilator pupilae, so
altering the amount of light entering the eye.
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2) The ciliary body
① Characteristics in structure
It lies at the inner surface of the junction of the
cornea and the sclera. It is the anterior extension
of the choroid and connects in turn to the iris.
This body runs like an annulus and consists of the
ciliary ring, the ciliary processes and the ciliary
muscles.
The ciliary processes are situated in the anterior
portion of the cillary body.
The lens is attached to the inner side of the
ciliary processes by the ciliary zonule
(suspensory ligament) .
② Function:
 The ciliary muscle may contract and
relax, regulate the convexity of the
lens to change its dioptric effect.
 The ciliary body secretes the
aqueous humor to supply nutrition
for the cornea and to maintain the
normal intraocular pressure.
3) The choroids
 It covers the inner surface of the sclera
and extends as far forwards as the ora
serrata of the retina. The choroids
occupies posterior 2/3 of the vascular
tunic, and contains rich pigment cells and
vessels.
 Function
 It supplies the nutrition for outer layer of
retina, absorbs the scattered light in the
eye.
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3 The inner tunic (the retina)
It is the neural sensory stratum lining the internal
surface of the vascular tunic.
1) Arrangement and structure of retina
Two layers:
The outer becomes the pigment cell lamina.
The inner is nervous layer: From behind forwards
it is divided into:
The optic part (pars optica)
The ciliary part (pars ciliaris)
The iridial part (pars iridica)
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The iridial and the ciliary parts are known
as the blind part, because of lacking light
sensitive elements of the retina.
The optic part of retina lines the internal
surface of the choroids, and extends from
the optic disc to the ora serrata which is
the irregular edge of the optic parts at the
posterior border of the ciliary body.
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Structure of the optic part (pars optica)
The optic part is composed of three layers of cells:
Outer layer: rod and cone cells. They are
photoreceptors.
Middle layer: bipolar cells. They convey the
nerve impulses from the photoreceptors to the
ganglion cells. They are the first order neurons in
the visual pathway.
Inner layer: ganglion cells: They are the second
order neurons in the visual pathway.
2) Morphology
 Macula lutea and fovea centralis
 Near the center of the posterior part
of the retina, there is an oval
yellowish area, named the macula
lutea (yellow spot), it shows a
central depression termed the fovea
centralis, where visual acuity is
highest.
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Optic disc and “blind spot”
About 3.5 mm to the nasal side of the
macula lutea, the optic nerve pierces the
retina and forms the optic disc with a
diameter of about 1.5 mm. The
circumference of the disc is slightly raised,
while the central part presents a slight
depression. The center of the disc is
pierced by the central artery and vein of
retina. The optic disc is insensitive to light,
and termed the “blind spot”.
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On ophthalmoscopic examination,
the normal disc is seen to be pink
during life.
The connection between the inner
and the outer lamina may be
separated from the outer lamina and
results in the retinodialysis.
Ⅱ) The Contents of the Eyeball
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Inclusion:
The aqueous humor
The lens
The vitreous body
Function:
They are all transparent and avascular,
with the cornea altogether form the
refractive media.
Each plays a part in refracting the light
entering the eye.
1. Chambers of eyeball and aqueous
humor
(1) The chambers of eyeball
 Definition:
 They are the spaces between the cornea
and the lens, filled with aqueous humor
and incompletely divided into the anterior
and the posterior chambers by the iris.
The anterior and the posterior chambers
communicate through the pupil.
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Iridocorneal angle and space of iridocorneal angle
(Fontana’s space)
At the peripheral part of the anterior chamber,
the cornea and the iris meet to form the
iridocorneal angle.
The anterolateral wall of the angle is made up by
Trabecular reticulum. The spaces between
trabeculae are termed the space of iridocorneal
angle (Fontana’s space), through which the
aqueous humor in the anterior chamber of
eyeball may communicate with the sinus
venosus sclerae (Schlemm’s canal)
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(2) The aqueous humor
Characteristics:
It is colourless, transparent and watery fluid. It
fills the chambers of the eye.
Production and circulation:
It is formed by active transport and diffusion
from the capillaries of the ciliary body, from
which it passes into the posterior chamber. Then
it passes into the anterior chamber through the
pupil and escapes from the iridocorneal angle into
the sinus venosus sclerae through the space of
iridocorneal angle, and finally, is drained into the
scleral veins.
Functions:
In addition to the dioptric role, the aqueous
humor also maintains the normal interocular
pressure.
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3. The lens
It lies between the iris and the vitreous body.
(1) Characteristics of construction:
Characteristics
It is a transparent and elastic, biconvex body
lacking vessels and nerves, and the convexity of
its anterior surface is less than that of its
posterior surface.
Suspensory ligament (ciliary zonule):
A transparent, elastic membrane closely
surrounded the lens is known as the lens capsule
which is connected by the suspensory ligament
(the ciliary zonule) to the ciliary process.
The substance of lens consists of two parts:
Cortex of lens.
Nucleus of lens.
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(2) Function
The shape of the lens is maintained by the
tension of the suspensory ligament.
During near vision:
The ciliary muscle contracts, pulling the ciliary
body anterointernally. This relaxes the
suspensory ligament, and allows the elasticity of
the lens to increase its convexity, thus shortening
the focal length and bringing near objects into
sharp focus in the retina.
During far vision:
The condition is reversed.
(3) Clinical points
 Presbyopia:
 The lens becomes increasingly harder, and
decreases its elasticity with age, and
results in decrease of dioptric range, thus
focusing on near objects becomes
progressively more difficult. This condition
is known as presbyopia.
 Senile cataract:
 The lens may also becomes opaque in the
aged, this condition is termed senile
cataract.
4. The vitreous body
(1) Characteristics:
 It is a colourless, transparent and jelly-like
substance. It fills the space between the lens and
the retina (the vitreous chamber).
(2) Function:
 The vitreous body, in addition to the refracting
role, plays the supporting role for the retina.
(3) Clinical points:
 Retinodialysis:
 When the supporting role of the vitreous body is
weakened, retinodialysis may occur.
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Myopia and hyperopia:
Myopia may occur, when the diopter
of the refracting apparatus is too
high, or the axis bulbi is too long; on
the contrary, the hyperopia may
occur, if the axis is too short, or the
diopter is too low.
Section 2 The Accessory Organs
of Eye
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The accessory organs of the eye
includes the eyebrow, the eyelids,
the cojunctiva, the lacrimal
apparatus, the extraocular muscles,
the adipose body of orbit and the
orbital fasciae fasciae.
Ⅰ. The Eyelids (or palpebrae)
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Definition:
They are thin, movable folds of skin, and consist
of the upper and lower eyelids. They meet at the
medial and lateral angles of the eye.
palpebral fissure:
The gap space between the two eyelids is termed
the palpebral fissure.
The ciliary glands (of Zeis):
They are arranged in rows inside the free margin
of each eyelid and open near the attachments of
the eyelashes. Inflammation of these glands is
known as the sty.
Structure:
 From outside inwards, each eyelid includes:
①Skin
②Subcutaneous areolar tissue
③Muscular layer (Orbicularis oculi, Levator
palpebrae superioris, Müller muscle)
④Tarsus
⑤Conjunctiva.
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Müller muscle:
The deep lamella of the levator palpebae
superioris contains the nostriated muscle
fibers, termed the superior tarsalis (socalled Müller muscle). It is attached to
the upper margin of the superior tarsus.
The skin or eyelid is extremely thin, and
its subcutaneous areolar tissue is loose
and delicate, thus obvious edema of the
eyelid can occur under some pathological
conditions.
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Tarsus and tarsal glands:
The tarsus is formed by dense connective
tissue and is a framework of the eyelid.
The tarsal glands (of Meibom) are
embedded in deep surface of tarsus, and
open at the margin of eyelids, their
secretion ensures the airtight closure of
the eyelids. A swelling of the eyelid
because of the retained secretion of these
glands is known as the chalazion.
Ⅱ. The Conjunctiva
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It is a thin and transparent mucous
membrane which covers the eyeball
(bulbar conjunctiva) and is
reflected to the posterior surface of
both eyelids (palpebral
conjunctiva). Thus, it joins the
eyeball to the eyelids.
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Conjunctival fornices and
conjunctival sac:
The reflected parts of the conjunctiva
from the superior and inferior eyelids
onto the eyeball is respectively
named the superior and inferior
conjunctival fornices. The saccular
space formed by the conjunctiva is
named the conjunctival sac.
Ⅲ. The Lacrimal Apparatus
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It consists of the lacrimal gland and the lacrimal
passages.
Ⅰ) Lacrimal Gland:
It lies in the lacrimal fossa of the orbit.
Its excretory ducts (lacrimal ducts) open into the
lateral part of superior fornix of the conjunctival
sac.
Excretory channel of tears:
The secretion of lacrimal gland (tears) flows over
the anterior surface of the eyeball to the
lacrimal punctum and then passes to the
lacrimal ductules, the lacrimal sac and the
nasolacrimal duct.
The tears contains a special enzyme dissolving
bacteria, and help to remove the foreign material
and moist the front of the eyeball.
Ⅱ) The Lacrimal Passages
1. The lacrimal punctum. On the medial end of
the margin of each eyelid, there is a small conical
elevation termed the lacrimal papilla. A minute
opening on the apex of the papilla is known as
the lacrimal punctum.
2. The lacrimal ductule (canaliculus). The
upper and lower lacrimal ductule ascends or
descends at first, and then runs almost at an
acute angle to the medial side, with uniting to the
lacrimal sac.
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3. The lacrimal sac: It is lodged in the fossa of
lacrimal sac situated in the anteroinferior part of
the medial wall of the orbit.
The medial palpebral ligament and palpebral part
of the orbicularis oculi lie anterior to its upper
part. The contraction of orbicularis oculi may
dilate this sac, thus, the tears are sucked into the
lacrimal sac via the lacrimal ductules.
4. The nasolacrimal duct
It opens into the anterior part of the inferior
nasal meatu.
Ⅳ. The Extraocular Muscles
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Which include the levator palpebral
superioris, the four rectus and two
oblique muscles.
Ⅰ) The Four Rectus Muscles
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They arise from a common tendinous ring
which encircles the optic canal. These
muscles are inserted into the sclera in
front of the equator of the eyeball.
The medial rectus turns the anterior pole
of the eyeball medially.
The lateral rectus turns the anterior pole
laterally.
The superior rectus turns the eyeball
superomedially.
The inferior rectus turns the eyeball
inferomedially.
Ⅱ) The Two Oblique Muscles
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The superior obliquus (superior
oblique m.)
Contraction of this muscle turns the
anterior pole of the eyeball
inferolaterally.
The inferior obliquus (inferior
oblique m.)
Its action is to turn the eyeball
superolaterally.
Ⅲ) The Levator Palpebrae
Superioris
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It cannot move the eyeball, but rises
the upper eyelid upward.
Ⅳ. The Connective Tissue in the
Orbit
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The orbital cavity contains a lot of
adipose tissues, named the adipose
body of orbit. It fills the space
among the organs in the orbital
cavity, and plays the supporting and
protective roles for these organs
inside the orbital cavity.
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The eyeball is embedded in this adipose
tissue but is separated from it by a thin
membranous sac, named the sheath of
eyeball (the capsule of Tenon). This
sheath envelops the eyeball from the optic
nerve to the sclerocorneal junction. Its
inner surface is smooth, and is separated
from the outer surface of the sclera by the
episcleral space; the space is traversed
by delicate bands of connective tissue
extended between the fascia and the
sclera. The eyeball can freely rotate inside
the episcleral space.
Section 3 The Blood Vessels and
Nerves of Eye
Ⅰ. The Arteries
 Ophthalmic artery.
 The Central Artery of Retina
 It is the first and the smallest branch of
the ophthalmic artery.
 It pierces the inferior surface of the optic
nerve about 1.0～1.5 cm behind the
eyeball, and runs forwards to the optic
disc of the retina along the center of the
nerve.
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Branches:
In the optic disc, it divides into superior
and inferior branches, and these two
branches divide again into superior and
inferior nasal retinal arteries,
superior and inferior temporal retinal
arteries, which supply the internal lamina
of the retina.
The branches of the central artery of
retina can be seen through the pupil by
the ophthalmoscopy.
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Ⅱ. The Veins
Ⅰ) The ophthalmic veins
The superior ophthalmic vein
The inferior ophthalmic vein
Ⅱ) The central vein of retina
Ⅲ. The Nerves
Ⅰ) The optic nerve
Ⅱ) Other nerves
1. Oculomotor nerve: Controlling levator
palpebrae superioris, rectus superior, rectus
inferior, rectus medialis, obliquus inferior.
2. Trochlear nerve: Controlling obliquus superior.
3. Abducent nerve: Controlling rectus lateralis.
4. Parasympathetic nerve fibers in oculomotor
nerve: Controlling ciliary muscle and sphincter
pupilae.
5. Sympathetic nerve: Controlling dilator pupillae.
6. Ophthalmic nerve of trigeminal nerve:
Controlling the sense of eyeball.
7. Facial nerve: Controlling the secretion of
lacrimal gland.