human eye (structure)

eyelid

protects the eye from mechanical damage, prevents excessive entry of light by squatting, spreads tears over the eyes so dust can be wiped off by blinking

eyelash

shields eye from dust particles

tear gland

secrets tears that wash away dust particles, keeps cornea moist and lubricartes conjunctiva to reduce friction when eyelids move

sclera

tough white outermost layer of the eyeball

protects eyeball from mechanical damage

have rectus muscles to facilitate movement of eyeball;;

pupil

hole in the centre of the iris which allows light to enter the eye

conjunctiva

thin transparent mucous membrane covering the sclera

secretes mucus to keep the front of the eyeball moist

iris

sheet of muscles consisting of involuntary muscles that are antagonistic: circular and radial muscles

controls amount of light entering eye

contains pigment giving eyes colour

choroid

black pigments middle layer which prevents the internal reflection of light

contains blood vessels to carry oxygen and nutrients to eyeball and remove metabolic waste from eyeball

retina

innermost layer of the eye wall on which images are formed

contains photoreceptors that detect light

optic nerve

consist of many sensory and motor neurones

transmit nerve impulses from photoreceptors in retina to the brain

rectus muscle

muscles attached to sclera, controls movement of eyeball

cornea

dome shaped transparent layer that refracts light rays into eye

causes most of the refraction of light entering eye (greater difference of refractive index with air)

aqueous chamber

space between lens and cornea

filled with aqueous humour: transparent watery fluid that keeps the front of the eyeball firm and refracts light into pupil

lens

transprent, circular and biconvex

thickness of lens can be changed in order to refract and focus light rays onto retina

vitreous chamber

space behind lens

filled with vitreous humour, a transparent, jelly like substance

virtuous humour keeps eyeball firm and refracts light onto retina

blind spot

region where optic nerve leaves eye

does not not contain photoreceptors, therefore not sensitive to light

fovea (yellow spot) directly below lens

small yellow depression where images are most sharply focused

suspensory ligaments

attach the edge of the lens to the ciliary muscles

slacken or become taut to change thickness of lens

ciliary body

contain ciliary muscles

control the curvature and thickness of lens with help of suspensory ligaments