The Eye: Structure

Macro and Microscopic

Eyes

Protected by bony orbits of the skull Send information to brain directly via

optic nerve.

Development

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

External features

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

Eye Musculature

External Structures of the Eye

1. Iris2. Lacrimal

caruncle5. Lower lid7. Pupil8. Sclera9. Upper lid

3 primary layers…

Sclera Choroid Retina

Sclera

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

Cornea

Choroid

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

Why an iris?

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…

Iris and Choroid

Lens

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

Lens Function

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

To focus image… 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”

Pupil, Lens, Ciliary muscle (body)

Retina Innermost layer of the eyeball. Contains microscopic structures:

Rods▪ Low-light▪ Non-color vision

Cones▪ Bright-light▪ Color vision

Vertebrate Eye

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

Retina through Opthalmascope

Optimal vision

Blind spot

Eye Fluids

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

Anterior and Posterior Chambers

Conjunctiva

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

Visual Pathway

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