The EYE
Vision
 Dominant
sense in humans
 Performed by eyes, in orbits of
skull
 Surrounded by accessory
structures
 Positioned by six extrinsic
muscles
Accessory Eye Structures
Eyebrows
 Eyelids (Palpebrae)
 Eyelashes
 Conjunctiva
 Lacrimal glands/ducts
 Extrinsic eye muscles

Functions of Accessory
Structures:

Eyebrows –
– physical protection

Eyelashes –
– blinking reflex initiation

Eyelids –
– lubrication
– contain sebaceous glands

CONJUNCTIVA
–
–
–
–
transparent mucous membrane
lines inner surface of eyelids
Folds over / covers eyeball surface
MAJOR LUBRICATION FUNCTION!
Conjunctivitis

The Lacrimal Apparatus (gland + ducts)!!
– Location: superior / lateral to eyeball
– Continuous lacrimal secretion ! (aka tears)
– Blinking: spreads lacrimal secretions over
eyeball surface
– Lacrimal canals (medial to eye) drain
secretion to lacrimal sac…
– Lacrimal sac to nasolacrimal duct to
nasal cavity!

Extrinsic Eye Muscles
– Move eyeball
– Six (6) per eye!
» LATERAL RECTUS
» MEDIAL RECTUS
» SUPERIOR RECTUS
» INFERIOR RECTUS
» INFERIOR OBLIQUE
» SUPERIOR OBLIQUE

Reminder:
What Cranial Nerves control these muscles?
III, IV, VI !!
Strabismus and Diplopia
The Eyeball

Sphere of three layers (TUNICS)
FIBROUS TUNIC
(Outermost)
VASCULAR TUNIC
(Middle layer)
SENSORY TUNIC
(Innermost)
Fibrous Tunic
CT
 Two major Regions:

1. SCLERA :
» The “white” of the eye
» Durable, provides shape to the eyeball
» Point of attachment for extrinsic eye muscles
» Continuous with Dura Mater
» Surrounds the Optic Nerve (II)
post.
ant.
2. CORNEA
» Most anterior portion of fibrous tunic
» Continuous with sclera
» Transparent
» “window” of the eye
Corneal Transplant
Vascular Tunic
blood vessels
 Three Major Regions:
– Choroid
– Ciliary Body
– Iris

1.
CHOROID
–
Structure:
» Located posteriorly
» Dark brown pigmentation (melanin)
–
Functions:
»
»
Provides nutrition to all tunics!
Melanin absorbs light (prevents reflection)
2.
CILIARY BODY
–
–
Structure:
Thick ring of :
» ciliary muscles and suspensory ligaments
» Ciliary processes (^ capillaries)
– Functions:
» Surround/control shape of lens
» Secrete fluid found in anterior chamber of eyeball
3.
IRIS
–
Structure:
» Most anterior portion of Vascular Tunic
» Located between cornea and lens
» Flattened appearance
» Contain smooth muscle fibers
» Surrounds the PUPIL (central opening that
allows light to enter the eye!)
» Contains MELANIN


^ melanin = brown eyes
melanin = green / blue /non brown eyes
– Functions:
»Controls diameter of the PUPIL!
»Reflexive to:
Amount
of Light present!
Distance of object being seen!
Emotional responses!
Pupil Constriction (PC) or Pupil Dilation (PD)???

Reading a book 6 inches from eyes?
– PC

Looking at a traffic sign while driving?
– PD

Solving a mathematical word problem?
– PD

Seeing something attractive approaching?
– PD

Seeing something repulsive or boring approaching?
– PC

Having a flashlight shown directly at face?
– PC

Entering a dark room?
– PD
Sensory Tunic
Aka… RETINA
 Structure:

– 2 Layers:
» Outer Pigmented Layer
 Absorbs
light/ prevents scattering & reflection
 Stores Vitamin A!!
» Inner Nervous Layer
 Contains
Photoreceptor Cells!
 Contains Optic Nerve
Neurons +
neuroglia!
The Eye and Vision
choroid
retina
sclera
Retina cont’d.
 Function
of the Retina
– Complex explanation:
» Processes light stimuli and sends impulses to
brain via the optic nerve!
– Simple explanation:
» ALLOWS YOU TO SEE!
Retina continued…
 So
What about these Photoreceptor Cells?
– 2 TYPES:
» RODS receptors for dim light and peripheral vision!
» CONES  receptors for bright light and color!
Rods
vs.
Cones

Absorb all wavelengths of
visible light

Absorb Red, Green, Blue
wavelengths

~200 rods relay impulse to
single neuron of OPTIC
nerve

1 cone to 1 OPTIC nerve
neuron impulse ratio!

Widely dispersed in retina;
allows for greater field of
vision

Allows for better visual acuity!
Vision in a Nutshell…
Rods and Cones receive light stimuli and
generate impulse!!
 Impulse passed to Bipolar Neurons!
 Impulse passed to Ganglion Neurons!
 Action Potential generated along
OPTIC NERVE!!

Structures Associated with the
Retina

OPTIC DISC
–
–
–
–
Area where optic nerve exits the eye!
Very weak… no support from sclera!
Lacks photoreceptors! (uh-oh…)
Aka Blind Spot
Central Artery
 Central Vein

– Transport blood to / from retina
– Enter/leave eye through center of Optic Nerve
– Form highly branched network of vessels!
The LENS
Structural Characteristics:
Biconvex!
Transparent!
Avascular! (Why??)
Flexible!
Held in place and changes position due to…
Suspensory ligaments !!

Function of Lens:
– Adjusts shape to focus light on the retina!!
– Divides Eye into
Anterior and Posterior Chambers!
Lens Problems …

As body ages lens becomes…
– Thicker
– More convex
– Less flexible

THE RESULT OF LENS PROBLEMS…??
Internal Chambers of the Eye
Lens is the DIVIDING LANDMARK!!
 Anterior Chamber:

– Filled with Aqueous Humor
» Watery, thin, clear fluid
» Constantly produced and drained

Posterior Chamber:
– Filled with Vitreous Humor
» Thick, clear gel
» Formed embryonically… lasts for lifetime
lens
aqueous
humor
vitreous
humor
CATARACTS
Glaucoma