human eye

• The retina contains photoreceptors, consisting of rods and cones connected to the nerve ending from the optic nerve, which detect light and send signals to the brain via the optic nerve.
• Sclerotic coat (sclera) is a tough, white outer covering of the eyeball which is continuous with the cornea.
• Eye muscles are attached to the sclera, facilitating the movement of the eyeball.
• The sclera protects the eyeball from mechanical damage.
• Cornea is a dome-shaped transparent layer that is able to refract light rays into the eye and is continuous with the sclera.
• Conjunctiva is a mucous membrane, covering the sclera and secretes mucus to keep the front of the eyeball moist.
• Iris is a circular sheet of muscles, consisting of two involuntary muscles (circular and radial muscles). Muscles contract and relax to control the size of the pupil, which then controls the amount of light entering the eye.
• Iris also contains a pigment which gives the eye its colour.
• Pupil is a hole in the centre of the iris, allowing light to enter the eyes.
• Eyelid protects the cornea from mechanical damage. squinting prevents excessive light from entering the eye to protect the retina from getting damaged
• blinking spreads tears over the cornea and conjunctiva, and wipes away dust particles off the cornea
• Eyelash shields the eye from dust particles.
• Tear gland secretes tears to wash away dust particles, keep the cornea moist for atmospheric oxygen to dissolve, and lubricate the conjunctiva, reducing friction when the eyelids move.
• Choroid is a black pigmented middle layer which prevents the internal reflection of light and contains blood vessels that carry oxygen and nutrients to the eyeball & remove metabolic waste products from the eyeball.
• Retina is the innermost layer of the eye wall and contains light-sensitive cells, photoreceptors, which consist of “rods” and “cones”.
• Cones: 3 types: red, blue, green, each type contains different pigment, which absorbs light of different wavelengths, and they work together to enable us to see a variety of colours in bright light.
• • Rods are stimulated even by very dim light and enable us to see in dim light, but only in black or white, and contain a pigment → visual purple. The pigment is bleached when exposed to bright light & impulses cannot be sent to the brain.
• Fovea (yellow spot) is a small yellow depression where images are focused and contains cones but not rods, enabling a person to have detailed colour vision in bright light.
• Blind spot is a region where the optic nerve leaves the eye and does not contain photoreceptors → not sensitive to light.
• Optic nerve transmits nerve impulses to the relay neurone in the brain when photoreceptors are stimulated.
• Nerve fibres of sensory neurones are located in the optic nerve.
• Lens is a transparent, circular & biconvex structure that can be changed in shape and thickness to refract/focus light onto the retina.
• Ciliary body contains ciliary muscles that control the curvature and thickness of lens.
• Suspensory ligament attaches the edge of the lens to the ciliary body.
• Aqueous chamber is the space between the lens and cornea filled with aqueous humour, a transparent, watery fluid that keeps the eyeball firm and refracts light into the pupil.
• Vitreous chamber is the space behind the lens filled with vitreous humour, a transparent jelly-like substance that keeps the eyeball firm and refracts light onto the retina.
• Pupil Reflex is a reflex action, which is a response to changes in light intensity.
• The pupil DILATES or CONSTRICTS to regulate the amount of light entering the eyes, protecting the eye from excessive light exposure, which could damage the retina.
• Pupil Reflex involves circular & radial muscles, which are antagonistic muscles.
• Cranial reflex involves a reflex centre in the brain.
• Focusing, also called accommodation, is the adjustment of the lens of the eye so that clear images of objects at different distances are formed on the retina.
• FOCUSING ON NEARBY OBJECTS:

  1. Ciliary muscles contract, relaxing their pull on suspensory ligaments
  2. Suspensory ligament slackens, relaxing their pull on the lens
  3. Lens become thicker and more convex, decreasing the focal length
  4. Light rays are sharply focused on the retina, stimulating the photoreceptors
  5. Nerve impulses produced are transmitted to the brain via the optic nerve
  6. The brain interprets the impulses → the person sees the nearby object
• FOCUSING ON DISTANT OBJECTS (est. 7m of further): 

  1. Ciliary muscles relax, pulling on the suspensory ligaments
  2. Suspensory ligaments become taut, pulling on the edge of the lens
  3. Lens become thinner & less convex, increasing the focal length
  4. Light rays are sharply focused on the retina, stimulating the photoreceptors
  5. Nerve impulses produced are transmitted to the brain via the optic nerve

  The brain interprets the impulses → the person sees the nearby object