1. The Eye and Vision 70 percent of all sensory receptors are in the eyes
Each eye has over a million nerve fibers
Protection for the eye
Most of the eye is enclosed in a bony orbit
A cushion of fat surrounds most of the eye

2. Accessory Structures of the Eye
Figure 8.1b

3. Accessory Structures of the Eye
Eyelids – composed of 4 layers
Outer layer – Skin (thinnest in the body)
Middle layer – Muscle – Levator palpebrae superioris. Raises the eyelid
Connective tissue layer
Inner layer – Conjunctiva - Membrane that lines the eyelids. Secretes mucus to lubricate the eye
Eyelashes
Meibomian glands – modified sebacious glands produce an oily secretion to lubricate the eye

4. Accessory Structures of the Eye
Lacrimal Apparatus
Lacrimal gland – produces lacrimal fluid
Lacrimal canals – drains lacrimal fluid from eyes
Lacrimal sac – provides passage of lacrimal fluid towards nasal cavity
Nasolacrimal duct – empties lacrimal fluid into the nasal cavity

6. Function of the Lacrimal Apparatus
Properties of lacrimal fluid
Dilute salt solution (tears)
Contains antibodies and lysozyme. Both are antibacterial
Protects, moistens, and lubricates the eye
Empties into the nasal cavity

8. Extrinsic Eye Muscles Muscles attach to the outer surface of the eye
Produce eye movements

10. Structure of the Eye The wall is composed of three tunics
1) Fibrous tunic – outside layer
2) Choroid – middle layer
3) Nervous tunic – inside layer

11. The Fibrous Tunic Sclera Cornea
White connective tissue layer (many collagen fibers)
Seen anteriorly as the “white of the eye”
Cornea
Transparent, central anterior portion
Allows for light to pass through.
Curved surface helps focus light
Repairs itself easily
The only human tissue that can be transplanted without fear of rejection. (has no blood vessels)

12. Choroid Layer Very vascular, blood-rich nutritive tunic
Pigment prevents light from scattering
Modified anteriorly into two structures
Cilliary body – smooth muscle. Contraction changes the shape of the lens to focus light
Iris
Pigmented layer that gives eye color
Pupil – rounded opening in the iris

13. Lens Biconvex, transparent,crystal-like structure
Held in place by a suspensory ligament attached to the ciliary body
The lens focuses light on the retina of the eye
It is elastic and can change shape to focus images at different distances
Contraction of the smooth muscles of the cilliary body makes the lens thicker or thinner
With age the lens can become harder and less transparent resulting in a Cataract.

15. Nervous Tunic (Retina)
Contains receptor cells (photoreceptors)
Rods
Cones
Most are found towards the edges of the retina
More numerous than cones
Allow dim light vision and peripheral vision
Perception is all in gray tones
Contain a light sensitive pigment called Rhodopsin which decomposes when it absorbs light energy

16. Cones Fovea centralis – area of the retina with only cones
Responsible for detailed color vision
Produce sharp images
Densest in the center of the retina
Fovea centralis – area of the retina with only cones
3 different types of cones exist
Each contains a different pigment that is most sensitive to either red, green or blue light
Lack of a particular cone type results in color blindness
No photoreceptor cells are at the optic disk, or blind spot

17. Figure 8.4

18. Internal Eye Chamber Fluids
Aqueous humor
Watery fluid 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 through the canal of Schlemm
Blockage of the drainage of aqueous humor results in an increase of intraocular pressure. The resulting condition, Glaucoma, can cause pain and blindness

19. Internal Eye Chamber Fluids
Vitreous humor
Gel-like substance behind the lens
Keeps the eye from collapsing
Lasts a lifetime and is not replaced

21. The Ear Houses two senses Receptors are mechanoreceptors
Hearing
Equilibrium (balance)
Receptors are mechanoreceptors
Different organs house receptors for each sense

23. The External Ear Involved in hearing only
Structures of the external ear
Pinna (auricle)
External auditory canal

24. The External Auditory Canal
Narrow chamber in the temporal bone
Lined with skin
Ceruminous (wax) glands are present
Ends at the tympanic membrane

26. The Middle Ear (Tympanic Cavity)
Air-filled cavity within the temporal bone
Only involved in the sense of hearing
Two tubes are associated with the inner ear
The opening from the auditory canal is covered by the tympanic membrane
The auditory tube connecting the middle ear with the throat
Allows for equalizing pressure during yawning or swallowing
This tube is otherwise collapsed

27. Bones of the Tympanic Cavity
Three bones span the cavity
Malleus (hammer)
Incus (anvil)
Stapes (stirrip)
Vibrations from eardrum move the malleus
These bones transfer and amplify sound to the inner ear

29. Organs of Hearing Organ of Corti Located within the cochlea
Receptors = hair cells on the basilar membrane
Gel-like tectorial membrane is capable of bending hair cells
Cochlear nerve attached to hair cells transmits nerve impulses to auditory cortex on temporal lobe

30. Mechanisms of Hearing
Vibrations from sound waves move tectorial membrane
Hair cells are bent by the membrane
An action potential starts in the cochlear nerve
Continued stimulation can lead to adaptation
Humans interpret as audible sound vibrations between 20 Hz to 20,000 Hz

33. Organs of Equilibrium Receptor cells are in two structures
Vestibule
Semicircular canals
Equilibrium has two functional parts
Static equilibrium
Dynamic equilibrium

34. Static Equilibrium Maculae – receptors in the vestibule
Report on the position of the head
Send information via the vestibular nerve
Anatomy of the maculae
Hair cells are embedded in the otolithic membrane
Otoliths (tiny stones) float in a gel around the hair cells
Movements cause otoliths to bend the hair cells

35. Function of Maculae

36. Dynamic Equilibrium
Crista ampullaris – receptors in the semicircular canals
Tuft of hair cells
Cupula (gelatinous cap) covers the hair cells
Action of angular head movements
The cupula stimulates the hair cells
An impulse is sent via the vestibular nerve to the cerebellum