Macro and Microscopic

 Protected by bony orbits of the skull  Send information to brain directly via optic nerve.

 From 2 outgrowths of the brain that form the optic nerves & the optic cup:  The posterior lining of the eye contains photoreceptors  In a mature eye called the retina

 Palpebrae: eyelids!  Thin flaps of skin  Controlled by orbicular muscles  Close when objects are placed near the eye: reflex arc  Eyelashes  Edges of eyelid  Protect from dirt

1. Iris 2. Lacrimal caruncle 5. Lower lid 7. Pupil 8. Sclera 9. Upper lid

 Sclera  Choroid  Retina

 Tough, outermost, white layer  Surrounds and protects the eyeball.  Its front surface, the cornea, is transparent to let light enter the eye.  Lacks blood vessels  Gets nutrition through diffusion  Ideal for transplants

 The choroid coat is the middle layer of the eyeball.  Consists of the colored portion of the eye known as the iris.  Iris has a hole in its center called the pupil.  Light enters through the pupil and the size of the pupil is regulated by the iris.  Constricting = parasympathetic muscles  Dilating = Sympathetic muscles

 Too much light “bleaches” the photopigment, rhodopsin to opsin  Reduces ability to see  Opsin must be “reconverted” to rhodopsin  This is related to “night vision” issues with sudden darkness…

 Transparent body  Lies directly behind the pupil  Held in place by ciliary muscles (run in circular, longitudinal, radial orientation; change lens shape)  Focuses light rays of images on the retina  Images inverted (both L to R and Up to Down)  Visual cortex reorients these

 Increases amount of energy reaching photoreceptive cells  What happens when source moves closer?  All light isn’t focused on retina  Makes image “fuzzy” because adjacent cells stimulated  An “accommodating” lens clears image up by refocusing light

 Close image = ROUND lens (decreases radius of curvature)  Far image = FLAT lens (increases radius of curvature)  To round the lens, contract muscles in ciliary body:  Contracting a circular muscle reduces the aperture  This decreases the tension on the suspensory ligaments, allowing lens to “round up”

 Innermost layer of the eyeball.  Contains microscopic structures:  Rods ▪ Low-light ▪ Non-color vision  Cones ▪ Bright-light ▪ Color vision

 At center of retina, have fovea centralis  Concentration of cone cells  1:1 cell/neuron ratio (gives good resolution)  Farther outward, mix of rods and cones, with just rods in peripheral vision  Mostly b & w, low-light, low-resolution peripheral vision  Eye directs fovea centralis at objects to maximize clarity  A “blind spot” occurs where the optic nerve/tract exits the eye

Optimal vision Blind spot

 Aqueous humor—in the anterior cavity in front of the lens  Provides nutrients to cornea, lens, other structures  Vitreous humor—in the posterior cavity behind the lens  Gelatinous  Holds retina to outer wall of choroid  May contain “floaters”, which must be removed surgically

 Mucous membrane covering the front surface of the sclera and lining the eyelid  Produces tears  Barrier to microbes  Susceptible to trauma, infections, chemical irritation, and allergic reactions

 Conjunctiva: thin, transparent epithelium covers the surface  Tiny blood vessels

 Innermost layer of retina contains rods and cones  Impulse travels from the rods and cones through the bipolar and ganglionic layers of retina  Nerve impulse leaves the eye through the optic nerve; the point of exit is free of receptors and is therefore called a blind spot  Visual interpretation occurs in the visual cortex of the cerebrum