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Special Senses
Vision
Anatomy of the Eye
● 70% of all the sensory receptors in the body
are in the eyes
● Vision is the sense that requires the most
“learning”, and the eye is often “fooled”
● Most of the eye is enclosed and protected by
a cushion of fat and the walls of the bony
orbit
● the accessory structures include:
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extrinsic eye muscles
eyelids
conjunctiva
lacrimal apparatus
● Eyelids protect the anterior of the eye. The
eyelids meet at the medial and lateral
corners of the eye (medial and lateral
commissure (canthus))
● palpebral fissure - the space between the
eyelids in an open eye
● Tarsal glands - modified sebaceous glands
associated with the eyelid edges
○ produce an oily secretion that lubricates the eye
○ ciliary glands - modified sweat glands that lie
between the eyelashes
● Conjunctiva - a delicate membrane lining
the eyelids and covering part of the outer
surface of the eyeball
● The conjunctiva fuses with the corneal
epithelium
● The conjunctiva helps to lubricate the
eyeball and keep it moist
Lacrimal Apparatus
● The lacrimal Apparatus consists of the
lacrimal gland and a number of ducts that
drain the lacrimal secretions into the nasal
cavity.
● The lacrimal glands are located above the
lateral end of each eye and release a dilute
salt solution (tears) onto the anterior surface
of the eyeball through several small ducts.
● tear drainage from the eye follows the
following “tract”
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lacrimal canaliculi
lacrimal sac
nasolacrimal duct
nasal cavity
● Lacrimal secretion contains
○ antibodies
○ the enzyme lysozyme - enzyme that destroys
bacteria
● The lacrimal apparatus
○ cleanses the eye
○ protects the eye surface
○ moistens and lubricates
● The six extrinsic (external) eye muscles
produce gross eye movements
○ possible to follow a moving object
Name
Action
Cranial Nerve
Lateral rectus
Moves eye laterally
abducens (VI)
Medial rectus
moves eye medially
Oculomotor (III)
Superior rectus
elevates eye and turns it
medially
Oculomotor (III)
Inferior rectus
depresses eye and turns it
medially
Oculomotor (III)
Inferior oblique
elevates eye and turns it
laterally
Oculomotor (III)
superior oblique
depresses ey and turns it
laterally
Trochlear (IV)
Internal Structures: The Eyeball
● The eyeball is a hollow sphere whose wall is
composed of three layers and its interior is
filled with fluids (humors)
● The lens,the main focusing apparatus of the
eye, is supported upright within the eye
cavity
Layers of the Wall of the Eyeball
● Fibrous layer - outer layer
○ sclera - thick, glistening white connective tissue -”
white of the eye”
○ cornea - transparent “window” through which light
enters the eye
■ abundant nerve endings (pain)
■ great ability to repair itself when damaged
■ contains no vascular tissue, therefore, is beyond
immune system’s reach - may be transplanted
without fear of rejection
● Vascular layer - Middle layer has three
distinguishable regions
○ choroid - blood-rich nutritive tunic that contains a
dark pigment that prevents light from scattering
within the eye
○ Muscle structures - ciliary body - attaches to lens
and ciliary zonule - suspensory ligament of lens,
○ iris - pigmented, circularly and radially arranged
smooth muscle fibers that regulates the amount of
light entering the eye
■ pupil - opening in the center of the iris through
● Sensory layer - Innermost layer containing
the two-layered retina
○ pigmented layer ■ absorbs light and prevents light from scatering
inside the eye.
■ acts as phagocytes to remove dead or damaged
receptor cells
■ Stores vitamin A
○ neural layer - transparent inner layer containing
photoreceptors - rods and cones
● Electrical signals from the photoreceptors
pass through a two-neuron chain - bipolar
cells and the ganglion cells - before leaving
the retina via the optic nerve
● blind spot (optic disc) - area around the
optic nerve that does not contain
photoreceptors
● Rods are most dense at the edge of the
retina and decrease in number as the center
is approached
● Rods allows us to see in gray tones in dim
light, and they provide for peripheral vision
● cones allow us to see the details of our world
in color under bright light conditions
● Cones are more dense in the center of the
retina and decrease in density toward the
edge
● fovea centralis - tiny pit containing only
cones lateral to the blind spot. The area of
greatest visual acuity (sharpest vision)
● Varieties of cones respond to different
wavelengths of light (figure 8.6)
○ blue cones
○ green cones
○ green to red (red cones)
● Signals from more than one type of cone is
interpreted as intermediate colors.
● The lack of cones leads to colorblindness
Lens
● A flexible biconvex crystal-lke structure held
upright in the eye by the ciliary zonule
attached to the ciliary body.
● cataracts - result from the lens becoming
more opaque and hardened as one ages
● the lens divides the eye into two segments
● Anterior (aqueous) segment contains a clear
watery fluid, the aqueous humor
○ similar to blood plasma and is secreted continuously
by the choroid
○ maintains pressure inside the eye (intraocular
pressure)
○ provides nutrients for the lens and cornea which
lacks vascular tissue
● Posterior (vitreous) segment contains a gellike substance, the vitreous humor
○ reinforces the eyeball internally (avoiding collapsing
Refraction of light through the eye
● Refraction - the bending of light when the
speed of light changes as it passes from one
substance to another substance that has a
different density
● Light rays are bent in the eye as they
encounter the cornea, aqueous humor, lens,
and vitreous humor
● The refractive power of the cornea and
humors is constant
● The refractive power of lens can be changed
by changing its shape
○ the greater the lens convexity, or bulge, the more it
bends the light
○ the flatter the lens, the less it bends the light
● The lens does not need to change shape to
focus properly on the retina for light from a
distant source (over 20 ft)
● Light from a close object tends to diverge
(spread out) and the lens must bulge more
to make close vision possible
○ the ciliary body contracts, allowing the lens to
become more convex
● Accommodation - the ability of the eye to
focus specifically for close objects
● The image formed on the retina because of
accommodation is a real image
○ left-right reversed, inverted, and smaller than the
object
● Vision problems occur when:
○ the lens is too weak (underconverging)
○ the lens is too strong (overconverging)
○ structural problems of the eyeball
■ myopia (nearsightedness) - eyeball too long
■ hyperopic (farsightedness) - eyeball is too short
Visual Fields and Pathways
● Axons from the retina are bundled as the
optic nerve.
● Fiber tracts from the optic nerve split
○ optic tracts ■ fibers from medial side of each eye cross over to
the opposite side of the brain at the optic
chiasma
■ Fibers from the lateral side of the eye stay on the
same side of the brain
● The optic tract fibers synapse with neurons
in the thalamus
● Thalamus axons form the optic radiation
which runs to the occipital lobe of the brain
(visual center)
● Optic radiation axon synapse with the
cortical cells, and visual interpretation
(seeing) occurs
● Each side of the brain receives visual input
from both eyes - lateral field of vision of the
eye on it own side and from the medial field
of the other eye.
● the visual fields of both eyes overlap
● biocular vision - (two-eyed vision) provides
for depth perception (3-D vision)
Eye Reflexes
● Internal eye muscles (controlled by
autonomic nervous system)
○ Ciliary body muscles - alters lens curvature
○ radial and circular muscles of the iris - controls pupil
size
● External eye muscles (rectus and oblique
muscles)- control eye movement and make it
possible to follow moving objects
○ convergence - reflexive movement of the eyes
medially when we view close objects.
■ both eyes are aimed toward the near object
being viewed
○ Extrinsic muscles are controlled by somatic fibers of
cranial nerves III, IV, and VI
● Photopupillary reflex - the constriction of
the pupils immediately after sudden
exposure to bright light.
● accommodation pupillary reflex - the
reflexive constriction of the pupils when we
view close objects
○ provides for more acute vision