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Chapter 8 Lecture Notes
Special Senses: Smell, Sight, Taste and Hearing
Equilibrium (5th sense) is housed in the ear.
*Special sense receptors are either large complex sensory organs (eyes/ears)
or localized clusters of receptors (taste buds/olfactory epithelium).
The eye and vision:
 Of all sensory receptors in the body 70% are in the eyes.
 Optic tracts that carry information from the eyes to the brain are
massive bundles, containing over a million nerve fibers.
External and accessory structures:
 Meibomian glands: Modified sebaceous glands associated with the
eyelid edges that produce an oily secretion that lubricates the eye.
 Ciliary glands: Modified sweat glands that lie between the eyelashes.
 Conjunctiva: Delicate membrane that lines the eyelids and covers
part of the outer surface of the eyeball. It ends at the edge of the
cornea by fusing with the corneal epithelium. It secretes mucus, which
helps to lubricate the eyeball and keep it moist.
 Lacrimal Apparatus: Consists of the lacrimal gland and a number of
ducts that drain the lacrimal secretions into the nasal cavity.
 Lacrimal Glands: Located above the lateral end of each eye. They
continually release a dilute salt solution (tears) onto the anterior
surface of the eyeball through several small ducts.
 Lacrimal Canals: Tears flush across the eyeball into the lacrimal
canals medially, then into the lacrimal sac and finally into the
nasolacrimal duct which empties into the nasal cavity.
o Lacrimal secretion also contains antibodies and lysozyme, en
enzyme that destroys bacteria. When lacrimal secretion
increases substantially, tears spill over the eyelids and fill the
nasal cavities, causing congestion and the “sniffles”.
 Six extrinsic or external eye muscles are attached to the outer
surface of each eye. These muscles produce gross eye movements and
make it possible for the eyes to follow a moving object
Internal Structures: The eyeball
Sclera: The outermost tunic, thick white connective tissue whose
function is protection. (White of the eye)
Cornea: The transparent window of the sclera through which light
enters the eye. The cornea is well supplied with nerve endings which
cause blinking and tearing when the cornea is touched. Cornea can be
transplanted from one person to another without fear of rejection. (no
blood vessels and it is beyond the reach of the immune system).
Choroid: The middle coat of the eyeball. This blood-rich tunic
contains a dark pigment that prevents light from scattering inside the
eye. Anteriorly the choroid forms two smooth muscle structures, the
Ciliary body to which the lens is attached, and the iris.
Iris: the iris has a rounded opening the pupil, through which light
passes. Circularly and radially arranged smooth muscle fibers form
the iris which acts like the diaphragm of a camera, that is it regulates
the amt. of light entering the eye so one can see as clearly as possible.