Eye and Vision
Exercise 26
BI 232
External Features
• Notice the pupil which is
surrounded by the
colored iris.
• The sclera is the white of
the eye which is covered
by a membrane called the
conjunctiva
• Eyelids come together at
lateral and medial
commissures.
• The lacrimal caruncle is
found in the medial
commissure.
Lacrimal Apparatus
• Consists of the lacrimal
gland and its accessory
structures.
• Produces water, alkaline
tears.
• Contain antibacterial
enzyme called lysozyme
for protection from
bacterial infections
3
Extrinsic Eye Muscles
• Superior oblique: primarily rotates the top of the
eye toward the nose and secondarily moves the
eye downward
• Innervated by CNIV (trochlear)
– Trochlea: Ligament sling
• Superior rectus: primarily moves the eye upward
and secondarily rotates the top of the eye toward
the nose
• Innervated by CNIII (oculomotor)
• Lateral rectus: moves the eye away from the nose
• Innervated by CNVI (abducens)
4
Extrinsic Eye Muscles
• Medial rectus: moves the eye toward the nose
• Innervated by CNIII (occulomotor)
• Inferior oblique: primarily rotates the top of
the eye away from the nose and secondarily
moves the eye upward (also CNIII)
• Inferior rectus: primarily moves the eye
downward and secondarily rotates the top of
the eye away from the nose (also CNIII)
5
Ear
Nose
6
Extrinsic Eye Muscles
7
8
Ciliary body
A
1. Ciliary processes
– Ciliary epithelium
– Secretes aqueous humor
2. Ciliary muscle
3
P
- (intrinsic eye muscle)
3. Suspensory ligament of the lens
A= anterior chamber
P= posterior chamber
1
2
9
Intrinsic Eye Muscles of the Iris

Pupils constrict (Parasympathetic)


Close vision and bright light
Pupils dilate (Sympathetic)

Distant vision and dim light
10
11
Neural pathway for vision
• After optic nerves exit the
eyeballs they meet at the
optic chiasm.
• Fibers from medial half of
retina cross over to the
opposite side.
• Optic tracts project to the
lateral geniculate bodies in
thalamus
• Some fibers are relayed to
superior colliculi
12
Histology of the Retina
13
Eye Dissection
• Do eye dissection
• Be able to ID structures
for quizzes and practical
Determination of the Near Point
• The minimum distance an
object can comfortably be
held in focus
• Changes as we age 
• Hold a paper with fine print
vertically at arm’s length in
front of you.
• Close one eye and slowly
move the paper closer until
either you see two objects
or it becomes blurry.
• Have partner measure
distance
Distribution of Rods and Cones
• Slowly move a small colored
object, without letting your
lab partner see it, from the
back of your lab partner’s
head, around the side
toward the front.
• With lab partner still
looking straight ahead, not
the angle when they are
able to see the object.(rods)
• Continue to move object
forward slowly until color
recognition occurs
Binocular Visual Field
• Close right eye and look
straight ahead with the left
• Lab partner moves an object
from behind your head from
the left until you can see
object.
• Measure angle with protractor
from tip of nose
• Continue moving object until it
disappears.
• Repeat the exercise with the
other eye
• Determine the total visual field
by adding the sums of above.
Visual Acuity
• Snellen eye chart used to
test visual acuity
• 20/20 normal
• 20/15 you can see at 20
feet what people with
normal vision see at 15
feet.
• 20/30 you see at 20 feet
what people with normal
vision see at 30 feet.
Astigmatism
• The degree of curvature
in the cornea or lens
varies from one axis to
another (is uneven or wavy)
• This causes light to focus on more than one
area of the retina creating a blurry image.
19
Ophthalmoscope
• Used for diagnosis of
variances in the eyeball
but also as a potential
indicator of diseases, such
as diabetes mellitus.
• Sit facing your lab partner
• Have scope set at zero
• Move in close to examine
the eye of your partner
• Look into the pupil
21
Retinal scan- Ultra-widefield
22
Refraction
• Light is bent when it passes from one
medium to another medium with a
different density
• Light passes through these before it hits the
retina:
– Cornea
– Aqueous humor
– Lens
– Vitreous humor
23
Focal Point & Focal Distance
•
•
Focal Point: The specific point of
intersection on the retina.
Focal distance: The distance between the
center of the lens and its focal point.
Determined by two factors:
1. Distance from the object to the lens
2. Shape of the lens
24
Focal Distance
•
•
Distance from the object to the lens: the closer an
object is, the greater the focal distance
Shape of the lens: the rounder the lens, the more
refraction occurs, so it has a shorter focal distance
25
Accommodation
• Accommodation is an alteration in the curvature of
the lens of the eye to focus an image on the retina
– Near objects: Lens becomes rounder
– Distance objects: Lens becomes flatter
26
Accommodation
• Emmetropia is
normal vision.
• The image will be
focused on the
retina’s surface
27
Accommodation Problems
• Myopia: Nearsighted
• The eyeball is too deep or
the curvature of the lens is
too great
• The focal point is in front of
the retina, so distance
objects are blurry
• Corrected with a diverging
lens
28
Accommodation Problems
• Hyperopia: Farsighted
• The eyeball is too shallow or
the curvature of the lens is too
flat
• The focal point is behind of the
retina, so near objects are
blurry
• Corrected with a converging
lens
29
Color Blindness
• Cones are responsible
for color vision.
• 3 types of cones each
able to absorb light at
specific wavelengths.
• Lack of one or more
types of cone can cause
color blindness
• Use Ishihara color
plates to test
30
Afterimages
• Photosensitive pigment of the
rods is rhodopsin, a lightsensitive retinal and the
protein opsin.
• When light strikes the retina,
rhodopsin splits into its two
component parts and
becomes pale (bleaching)
• You can test the time for
separation and reassembly of
the photopigments by staring
at a contrasting image on a
card
• Stare about 10-20 seconds and
then shuts your eyes.
Have your partner record time.
You should see colored image
against dark background (positive
afterimage)
After a few moments you should
see the reverse of the original
image (negative afterimage)
Blind Spot
• The optic disc is a region
where the nerve fibers
exit the back of the eye
and form the optic nerve.
• This region is devoid of
photoreceptors and is
called the blind spot.
• Follow the instructions
from your instructor or
book to find your blind
spot
The End
• Spend the rest of the lab locating structures
on the eye models
• Make sure that you understand the tests we
did in class because you will be asked about
them on quizzes and practicals