Biology: The Eye.

Choroid

A dark layer underneath the sclera, is dark because it contains many PIGMENT CELLS BLOOD VESSELS The pigment stops light being reflected around inside the eye, so the image doesn't fog up. ( like a camera.)

Ciliary muscles

A ring of muscle.

Suspensory ligaments

A series of fibres that hold the lens in place.

Rod cells.

Are more sensitive in dim/dark light, but can't sense colour ( hence why we can't distinguish coloured objects in the dark.)

Cone cells.

Are sensitive to colours but aren't so good in dim/dark light. ( think c for colours and cones.) Cones are found all over the RETINA, but there are loads of them at the FOVEA.

Fovea

At the centre of the retina, lots of cone cells can be found here.

Optic nerve

Carries impulses from the receptors to the brain. And is, essentially, a bundle of sensory neurones st back of eye.

Accommodation (nearby object.)

Ciliary muscles contract. Suspensions ligaments slack. Lens more rounded.

Accommodation ( distant object.)

Ciliary muscles relax. Suspenders ligaments pulled tight. Lens become 'flat.'

The eye in bright light

Circular muscles contract Radial muscles relax Pupil constricts (gets smaller.)

The eye in dim light.

Circular muscles relax. Radial muscles contract. Pupil dilates (gets bigger.)

Retina (function.)

Contains the light receptors. ( which will send impulses to the brain.)

Pupil

Lets the light through. (Is black because there is no light escaping from the inside of the eye.)

Forming an image on the retina.

Light is refracted at the air/cornea boundary and again at the lens. As a result of refraction at the cornea and lens, the image on the retina is upside down/inverted. The brain interprets the image the right way up.

Iris

Pigmented, and controls how much light enters the eye through changing the size of the pupil ( in different conditions, light and dark, IRIS REFLEX.)

Iris reflex (diagram)

STIMULUS (light intensity.) -> RETINA (receptor)-> SENSORY NEURONES IN OPTIC NERVE-> UNCONSCIOUS PART OF BRAIN-> MOTOR NEURONES IN NERVE TO IRIS-> IRIS MUSCLES (effector)-> RESPONSE (change in size of pupil.)

Accommodation

The changes that take place in the eye which allow us to see objects at different distances.

Cornea

The cornea lets (refracts light) into the eye, and is at the front part of the sclera, it is rounded and transparent.

Retina (description)

The lining of the back of eye containing two types of photoreceptor cells- rods - sensitive to dim light and black and white. cones - sensitive to colour. A small area called the fovea in the middle of the retina has many more cones than rods.

Blind spot

The one area of the retina cannot be formed, this is where the optic nerve leaves the eye. At this position there are no rods or cones, hence why it is called a BLIND SPOT.

Sclera

The visible, white part of the eye, a tough outer layer.

Lens

Transparent, bi convex, flexible disc behind the iris. Attached to the ciliary muscles by the suspensory ligaments. REFRACTS LIGHT TO FOCUS IT ONTO THE RETINA. The amount of refraction can be adjusted by altering the thickness and curvature of the lens.