Transcript Accessory Structures of the Eye Lacrimal apparatus

Chapter 8
Special Senses
The Senses
 Special senses
 Smell
 Taste
 SIGHT
 Hearing
 Equilibrium
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The Eye and Vision
 Of all the sensory receptors in the body
 70% are found in the eyes
 Optic tracts
 Carry information from the eyes to the brain
 Consists of over a million nerve fibers per
eye
The Eye and Vision
 Protection for the eye
 Only 1/6th of the eye’s surface can be seen
 Remaining portion of the eye
 Enclosed in a bony orbit
 Cushioned by a layer of fat
 Accessory Structures
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Eyelids
Conjuctiva
Extrinsic eye muscles
Lacrimal apparatus
Accessory Structures of the Eye
• Eyelids
• Meet at the medial &
lateral corners of the
eye = (Medial &
lateral canthus)
 Nictitating
membrane
 Eyelashes
Accessory Structures of the Eye
Accessory Structures of the Eye
 Conjunctiva
 Delicate membrane
 Lines the eyelids & covers part of the
outer surface of the eyeball
 Connects to the surface of the eye
 Secretes mucus to lubricate the eye
 Conjuctivitis?
Accessory Structures of the Eye
Conjunctivitis
 PINKEYE
 Common condition
 Inflammation of the conjunctiva
 Clear membrane that covers the white part of
the eye & lines the inner surface of the eyelids
 No danger to the eye or your vision
 Causes
 Bacteria, viruses, allergies (hay fever),
irritants
Accessory Structures of the Eye
 Lacrimal apparatus
 Consists of the lacrimal glands and ducts  drain lacrimal
secretions into the nasal cavity
 Lacrimal glands
 Continually release tears
 Tears
 Antibodies
 Lysozyme
 Enzyme that destroys bacteria
 Clean, protect and moisten the eye surface
Accessory Structures of the Eye
 Extrinsic (external) eye muscles
 6 muscles attached to the outer surface of
each eye
 Produce eye movements
Internal Structures: The Eyeball
 Eyeball
 Hollow sphere
 Its wall is composed of three tunics (layers)
 Sclera or fibrous tunic
 “White of the eye”
 Outermost layer
 Choroid or vascular tunic
 Middle layer
 Sensory tunic
 Innermost layer
 Retina
 Interior is filled with fluids called humors
Internal Structures: The Eyeball
The Fibrous Tunic
 Sclera
 White connective tissue layer
 “White of the eye”
 Cornea
 Transparent “window”- central anterior portion
 Allows for light to pass through
 Repairs itself easily
 Only human tissue that can be transplanted from
one person to another without the fear of rejection
 No blood vessels  beyond the reach of the
immune system
Choroid Layer
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Middle layer
Blood-rich nutritive tunic
Pigment prevents light from scattering
Modified anteriorly into two structures
 Cilliary body – smooth muscle to which the
lens is attached
 Iris
 Pigmented layer that gives eye color
 Pupil – rounded opening in the iris through
which light passes
Sensory Tunic (Retina)
 Contains receptor cells (photoreceptors)
 Rods
 Cones
 Signals pass from photoreceptors and leave
the retina toward the brain through the optic
nerve
 Photoreceptors
 Distributed over the entire retina, except
where the optic nerve leaves the eyeball
 Optic disc or blind spot*
Neurons of the Retina
Neurons of the Retina & Vision
 Rods
 Most are found towards the edges of the
retina
 Allow dim light vision and peripheral vision
 Test fact of rod location by looking “offcenter” at objects at night…stars/glow-inthe-dark objects on ceiling
 Perception is in all gray tones
Neurons of the Retina & Vision
 Cones
 Densest in the center of the retina
 Fovea centralis
 Area of the retina with only cones
 Greatest visual acuity
 3 types of cones  respond to a particular
wavelength of light
 Blue light
 Green light
 Both green & red light
 ** Color blindness  lack of one to all
cone types
Color-Sensitive Cones
Lens
 Focuses light entering the eye on the
retina
 Flexible biconvex crystal-like structure
 Held in place by a suspensory ligament
 attached to the ciliary body
 Divides the eye into 2 segments
 Anterior (aqueous) segment
 Posterior (vitreous) segment
Lens
Internal Eye Chamber Fluids
 Anterior (aqueous) Segment
 Contains a watery fluid  Aqueous humor
 Found in chamber between the lens and
cornea
 Similar to blood plasma
 Helps maintain intraocular pressure
 Provides nutrients for the lens and cornea
 Reabsorbed into venous blood
 Blocked drainage = Glaucoma
Internal Eye Chamber Fluids
 Posterior (Vitreous) Segment
 Vitreous Fluid
 Gel-like substance behind the lens
 Keeps the eye from collapsing
 Lasts a lifetime and is not replaced
Lens Accommodation
 Light must be
focused to a point
on the retina for
optimal vision
 Resting eye is set
for distance vision
(over 20 ft away)
 The lens must
change shape to
focus for closer
objects
Vision
 Emmetropia
 Correct focus
 Myopia  nearsightedness
 Focus of light in front of retina
 Eyeball too long or lens too strong
 Distant objects are blurry
 Hyperopia  farsightedness
 Focus of light beyond the retina
 Short eyeball or lazy lens
 Near objects are blurry.
Emmetropia
Hyperopia