Special Senses The Eye

Overview of the Eye Eye acts like a camera Lens adjusts to bring object into focus Pupil constricts to allow less light to enter in bright setting or dilates to allow more light to enter in darker setting Through bending of light rays, images reaches retina Sensitive nerve cell layer of eye Image is transmitted to brain for interpretation

Front view of the eye Sclera = white portion of eye tough, maintains shape of eyeball serves as protective covering for the eye Iris = colored portion of the eye Pupil= opening in center of eye that controls amount of light entering eye

Lacrimal gland = produces tears Conjunctiva = thin mucous membrane that lines the eye and inner part of eyelid Lacrimal gland = produces tears located at upper outer edge of each eye Lacrimal Duct = Tears drain thru duct located at inner edge of eye

■ Vitreous humor = fluid that prevents eye from collapsing forward Cornea = transparent layer covering colored part of eye that holds in fluid Aqueous humor = maintains pressure to keep outer cornea inflated & helps fight infection Lens = colorless biconvex structure that focuses image clearly on retina Aqueous Humor Vitreous humor ■ Vitreous humor = fluid that prevents eye from collapsing forward

Retina Sensitive nerve cell layer Nerve cells of retina changes energy of light rays into electrical nerve impulses transmits nerve impulses via optic nerve to brain for interpretation Nerve cells of retina Rods are responsible for vision in dim light and for peripheral vision Cones responsible for colors and vision in bright light

Cones The retina contains approximately 6 million cones. contained in the macula, the portion of the retina responsible for central vision. They are most densely packed within the fovea, the very center portion of the macula. Cones function best in bright light and allow us to appreciate color. Fovea

Rods There are approximately 125 million rods. They are spread throughout the peripheral retina and function best in dim lighting. The rods are responsible for peripheral and night vision.

How the eye focuses Lenses and cornea focuses image precisely on the retina Cornea of eye is overly curved or eyeball is too long. This combination brings images of distant objects to a focus point in front of the retina. Normal Vision cornea of eye is not curved enough or eyeball is too short. This combination causes the focus point of the eye to be located behind the retina. Near-sighted Vision Astigmatism Far-Sighted Vision Cornea of eye has a football shape and does not bring light to focus at a single point. Instead, it focuses images over a range of points producing a blurred image

Figure 35-14 The Eye Section 35-4 Choroid Retina Blood vessels Optic nerve Fovea Vitreous humor Sclera Ligaments Iris Pupil Cornea Aqueous humor Lens Muscle

Pathway for Vision Reception Light Cornea Iris and pupil Lens Retina to photoreceptors Rods Cones (macula and fovea) Optic Nerve Brain