The best way to learn is to observe nature and you will find everything you need.

1. UNIT 2:
SENSE ORGANS –
EYES, TONGUE AND
SKIN
•
Campbell et.al, 2010 –
CHAPTER 50

2. SENSE ORGAN: EYES
SENSE OF VISION
Photoreceptors are sensory receptors
that are sensitive to light.
Animals and humans with 2 eyes facing
forward have 3D or stereoscopic vision,
visual fields overlap and each eye is able
to view and object from a different angle.

3. THE HUMAN EYE
 Main parts of the eye found in 3 main layers
1. Outer layer (Cornea, Sclera)
2. Middle layer (Choroid, ciliary body, iris)
3. Inner layer (Retina – rods, cones, fovea)

4. THE HUMAN EYE
 Other parts of importance:
1. Lens (Attach to suspensory ligaments)
2. Humors (Aqueous humor and vitreous humor –
Transmit light rays and support eyeball)
3. Optic nerves – Transmits impulses to the brain

5. OUTER LAYER (STRUCTURE AND
FUNCTION)
 The sclera: white outer layer, protects
and supports eyeball
 Cornea: Front transparent part of eye.
Refracts light rays
 Conjunctiva: Transparent membrane
(front). Moistens eye surface.

6. MIDDLE LAYER STRUCTURE AND FUNCTIONS
 Pupil: Hole in middle of eye. Black. Admits
light into eye.
 The choroid: Pigmented 2nd layer. Absorb
stray light. Contain blood vessels for
nutrition and oxygen.
 The iris: Coloured part of the eye that
regulates the size of the pupil and therefore
light entrance
 The lens: Transparent biconvex structure
that refract and focuses light on the retina

7. INNER LAYER, STRUCTURE AND
FUNCTION
 The retina: contains photoreceptors (cones and rods)
- Rods: Make black and white vision possible/ overview sight
- Cones: Make colour vision possible
- Fovea centralis: Best image possible
- Blind spot: place where nerves and blood vessels enter and
leave the eyeball. No image can be seen if light is focused on this
point.

8.  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.

9. Fig. 50-18
Optic
nerve
Fovea (center
of visual field)
Lens
Vitreous humor
Optic disk
(blind spot)
Central artery and
vein of the retina
Iris
Retina
Choroid
Sclera
Ciliary body
Suspensory
ligament
Cornea
Pupil
Aqueous
humor

10. PHOTORECEPTORS
Found in the retina of the eye.
2 Types: Rods and Cones
They consist of an outer segment and an inner
segment joined by a stalk.
Visual pigment found in rods, is a deep-purple
pigment called Rhodopsin.
Rods are sensitive to light and are therefore
suited for night vision (black and white) and
peripheral vision.
Rods found on either side of fovea- the further
away from the fovea – the more rods present.

11. The cones, are primary located in the fovea
centralis, the further away from the fovea the
less the cones.
Activated by bright light
Cones allow us to detect fine detail and
colour.
Cones contain 3 pigments –B (blue), G
(green) and R (red)
In colour blindness, a person lack certain of
these pigments.

12. https://www.youtube.com/watch?v=eDIma_
Ai1Rc

13. INTEGRATION OF VISUAL SIGNALS IN
THE RETINA
Light enter the eye through the cornea,
aqueous humor, pupil, lens, vitreous humor
and focuses on the retina.
Light stimulates the photoreceptors (rods and
cones) in the retina.
Retina sends an impulse to the optic nerves
Optic nerves send an impulse to the occipital
lobe of the cerebrum
Where sight is integrated.

14. Fig. 50-18
Optic
nerve
Fovea (center
of visual field)
Lens
Vitreous humor
Optic disk
(blind spot)
Central artery and
vein of the retina
Iris
Retina
Choroid
Sclera
Ciliary body
Suspensory
ligament
Cornea
Pupil
Aqueous
humor

15. ACCOMMODATION
The ability of the lens to change shape to
enable the eye to focus on near and far
objects.= accommodation
 Accomodation in the eye (focussing) for IGCSE
Biology.mp4

16. NEAR VISION
 Longitudinal Cilliary
muscle contracts
 Suspensory ligaments
relax
 Lens becomes more
concex (rounded)
DISTANCE VISION
 Longitudinal Cilliary
muscle relax
 Suspensory ligaments pull
tight
 Lens becomes flat (less
convex)
Fig. 50-19
Ciliary muscles
relax.
Retina
Choroid
(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.

17. VISUAL DISORDERS
Nearsightedness/
Myopia
 Can see objects closer than
6m, but not far objects.
 Have an elongated eyeball.
 When looking at a far object,
the image is focused in front
of the retina.
 These people can wear
concave lenses, which
diverge the light rays so that
the image can be focused
on the retina.
Farsightedness/
Hypermetropic
 Can see objects far but not
near objects.
 Have a shortened eyeball.
 When looking at a near
object, the image is focused
behind the retina.
 These people can wear a
convex lens, to increase the
bending of light rays so that
the image can be focused on
the retina.

18. MYOPIA OF THE EYE

19. HYPERMETROPIA

20. VISUAL DISORDER: ASTIGMATISM
When the cornea or
lens is uneven and
the image that you see
is fuzzy.
Can be corrected by
wearing glasses with
an unevenly round
lens to compensate for
the uneven cornea.