[BIO] Chapter 12 - Human Eyes

© Hee Xin Wei (Copyrighted) Topic 1 2: Human Eye

Chapter Analysis FOCUS EXAM WEIGHTAGE •Constitute to around 2.5% in Paper 2 in the past 5 years •commonly tested in MCQ and structured questions •tested once in section B in the past 5 years •one of the three chapters under umbrella topic of Co-ordination and Response in Humans •Linked to nervous system

structure of eyes accommodation pupil reflex Key Concept

structure of eyes external Tear gland Secretes tears which lubricate the eye, nourish the cornea and keeps it free from dust. Eyelid •Squinting is the partial closure of eyelids. This prevents excess light from entering the eye and damaging the light-sensitive tissues •Blinking spreads tears over the cornea and conjunctiva and wipes dust particles off the cornea Eyelashes Shield eyes from dust particles Iris •Contain radial and circular muscles that control the size of pupil •Pigment of iris gives the colour of eyes Pupil Allows light to enter the eye Sclera •Tough white outer layer of connective tissue •Continuous with cornea Conjunctiva •Thin, transparent mucous membrane that covers the sclera •Secretes mucous to lubricate the eye Cornea •Transparent refractive layer covering the iris and pupil. •continuous with the sclera. Ciliary body Eyelid Eyelashes Tear gland

structure of eyes internal Choroid •Middle layer of the eyeball, between the sclera and retina. •Contains blood vessels that supply oxygen and nutrients, and remove metabolic waste products. •Pigmented black to prevent an internal reflection of light. Retina Innermost layer of the eyeball which contains photoreceptors, which are connected to the optic nerve. Lens •Transparent biconvex structure that refracts light onto the retina. •The lens can change its curvature to focus light onto retina Fovea •Also called yellow spot, a small depression in the retina where images are usually focused •The fovea contains the greatest concentration of cones, but no rods. Ciliary body Contains ciliary muscles which control the curvature of the lens. It is also responsible for producing aqueous humour. Suspensory ligament Connects the ciliary body to the lens Aqueous chamber •The space between the lens and the cornea. •Transparent aqueous humour keeps the front of the eyeball firm and helps to refract light into the pupil. Vitreous chamber •The space behind lens •Transparent vitreous humour keeps the eyeball firm and helps to refract light onto the retina. Optic nerve •Transmits signal from the retina to the brain. •There are no photoreceptors in the area of the retina where the optic nerve leaves. This area is called the blind spot. Ciliary body Eyelid Eyelashes

photoreceptors •Photoreceptors in the retina consist of rods and cones. •The photoreceptors are connected to the nerve endings from the optic nerve. CONES •Cones enable us to see colours in bright light. •There are three types of cones, red, blue, and green (RBG)