Compound eyes are very effective at detecting movement.
(a) Fly eyes
Single-lens eyes are found in some jellies, polychaetes, spiders, and many molluscs.
They work on a camera-like principle: the iris changes the diameter of the pupil to control how much light enters.
In vertebrates the eye detects color and light, but the brain assembles the information and perceives the image.
Structure of the Eye
Main parts of the vertebrate eye:
The sclera: white outer layer, including cornea
The choroid: pigmented layer
The iris: regulates the size of the pupil
The retina: contains photoreceptors
The lens: focuses light on the retina
The optic disk: a blind spot in the retina where the optic nerve attaches to the eye.
The eye is divided into two cavities separated by the lens and ciliary body:
The anterior cavity is filled with watery aqueous humor
The posterior cavity is filled with jellylike vitreous humor
The ciliary body produces the aqueous humor.
Fovea = center of visual field
Optic disk (blind spot)
Central artery and vein of the retina
Humans and other mammals focus light by changing the shape of the lens.
Ciliary muscles relax.
(b) Distance vision
(a) Near vision (accommodation)
Suspensory ligaments pull against lens.
Lens becomes flatter.
Lens becomes thicker and rounder.
Ciliary muscles contract.
Suspensory ligaments relax.
The human retina contains two types of photoreceptors: rods and cones
Rods are light-sensitive but don’t distinguish colors.
Cones distinguish colors but are not as sensitive to light.
In humans, cones are concentrated in the fovea, the center of the visual field, and rods are more concentrated around the periphery of the retina.
Sensory Transduction in the Eye