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.