chapter 14: the human eye

Question 1

where is the eye ball?

Answer 1

• each eyeball lies in a hollow in the skull called the
  > ORBIT
• each eyeball is attached to the skull by
  >ERECTUS MUSCLES which control eye movement

Question 2

what is the iris?
(the front part of the human eye)

Answer 2

• a circular sheet of muscles
  > contains a pigment which gives the eye its colour
• the amount of light entering the eye is controlled by the two sets of involuntary muscles in the iris
  > circular muscles & radial muscles

Question 3

what is the conjunctiva?
(the front part of the human eye)

Answer 3

• a thin transparent membrane covering the sclera in front
• it is a mucuous membrane
  > it secretes mucous
  > helping to keep the front of the eyeball moist
• it is continuous with the skin of the eyelids

Question 4

what is the sclera/ sclerotic coat?
(the front part of the human eye)

Answer 4

• a tough, white outer covering of the eyeball
• it is continuous with the cornea
  > protects the eyeball from mechanical damage

Question 5

what is the pupil?
(the front part of the human eye)

Answer 5

• a hole in the centre of the iris
• the pupil allows light to enter into the eye

Question 6

what do the eyelashes do?
(the front part of the human eye)

Answer 6

• they help to shield the eye from dust particles

Question 7

what is the tear gland?
(the front part of the human eye)

Answer 7

• a gland lying at the corner of the upper eyelid

secretes tears which:
- wash away dust particles
- keep the cornea moist for atmospheric oxygen to dissolve
> dissolved oxygen diffuse into the cornea
- lubricate the conjunctiva, helping to reduce friction when the eyelids move

Question 8

what are the functions of eyelids?
(the front part of the human eye)

Answer 8

• they protect the cornea from mechanical damage
• the eyelids can be partly closed
  > squinting prevents excessive light from entering the eye and damaging the light-sensitive tissues (retina) inside
• blinking spreads tears over the cornea and conjunctiva and wipes dust particles off the cornea

Question 9

what is the ciliary body?
(the internal structure of the eye)

Answer 9

• a thickened region at the front end of the choroid
  > contains ciliary muscles
  > which control the curvature or thickness of the lens

Question 10

what is the choroid?
(the internal structure of the eye)

Answer 10

• the middle layer of the eyeball between SCLERA and the RETINA

two functions:
- it is pigmented black to prevent internal reflection of the light
- contains blood vessels that bring oxygen and nutrients to the eyeball and remove metabolic waste products

Question 11

what is the vitreous chamber?
(the internal structure of the eye)

Answer 11

• the space behind the lens
  > filled with VITREOUS HUMOUR
  > a transparent, jelly-like substance
  > it keeps the eyeball firm and helps refract light onto the retina

Question 12

what is the fovea or yellow spot?
(the internal structure of the eye)

Answer 12

• a small yellow depression in the retina
• it is situated directly behind the lens
  >where images are normally focused
• the fovea contains the greatest amount of cones > no rods
• it enables a person to have a detailed colour vision in bright light

Question 13

what is the blind spot?
(the internal structure of the eye)

Answer 13

• the region where the optic nerve leaves the eye
  > does not contain any rods or cones
  > not sensitive to light

Question 14

what is the optic nerve?
(the internal structure of the eye)

Answer 14

• a nerve that transmit nerve impulses to the brain when the photoreceptors in the retina are stimulated

Question 15

what is the retina?
(the internal structure of the eye)

Answer 15

• the innermost layer of the eyeball
• light sensitive layer which images are formed
• it contains light-sensitive receptors or photoreceptors
  > photoreceptors consists of rods and cones
• cones enable us to see colours in bright light
• rods enable us to see in black and white in dim light
• photoreceptors are connected to the nerve endings of the optic nerve

Question 16

what is the lens?
(the internal structure of the eye)

Answer 16

• it is transparent, circular, biconvex structure
  > elastic and changes its shape or thickness in order to focus light onto the retina

Question 17

what is the aqueous chamber?
(the internal structure of the eye)

Answer 17

• the space between the lens and cornea
  > filled with aqueous humour, a transparent watery fluid
• aqueous humour keeps front of the eyeball firm and helps to refract light into the pupil

Question 18

what is the cornea?
(the internal structure of the eye)

Answer 18

• a dome-shaped transparent layer continuous with the sclera (white of the eye)
  > it refracts/ bends light ray into the eye
  > cornea causes most of the refraction of light in the eye

Question 19

what is the suspensory ligament?
(the internal structure of the eye)

Answer 19

• a connective tissue that attaches the edge of the lens to the ciliary body

Question 20

what are the functions of cones?

Answer 20

• enable us to see colour in bright light
• three types of cones: red, blue and green
• each cone contains a different pigment which absorbs light of different wavelengths
  > all the cones together enable us to see a wide variety of colours
  > cones do not work well in dim light

Question 21

what are rods?

Answer 21

• more sensitive to light than cones
  > enable us to see in dim light > only in black and white
• rods sensitive to light of low intensity (dim) because they contain a pigment called visual purple
• when a person is exposed to bright light,
  > visual purple is bleached
  >visual purple must be reformed for a person to see in the dark
• when you move from a brightly lit room to a dimly lit room
  > may not be able to see objects arnd you for sometime
  > takes time for visual purple to be formed again in rods
• formation of visual purple requires vitamin A
  > person deficient in vitamin A > may not be able to see in the dark
  > night blindness

Question 22

how does the iris control the amount of light entering the eye?

Answer 22

• to see clearly, only the right amount of light should enter the eye
• eg. more light must enter the eye in dim light
• the size of the pupil determines how much light enters the eye

-the size of the pupil controlled by two sets of involuntary muscles
> CIRCULAR MUSCLES and RADIAL MUSCLES
- circular muscles are arranged in a circle round the pupil
> radial muscles arranged radially
>circular and radial muscles are antagonistic muscles

Question 23

how does the iris control the amount of light entering the eye?
in a brightly lit room and dimly lit room.

Answer 23

bright light:
- the circular muscles of the iris contract
- the radial muscles of the iris relax
- the pupil becomes smaller / constricts
> reduces the amount of light entering the eye

dim light:
- radial muscles of iris contract
- circular muscles of iris relax
- the pupil enlarges/dilates
> increases the amount of light entering the eye

Question 24

what is the pupil reflex?

Answer 24

• a reflex action
  > the pupil changes size in a result of changes in light intensity
  > pupil becomes larger when light intensity is low
  >smaller when light intensity is high
• sometimes light may be so bright that decreasing the size of the pupil is not enough
  > eyelids have to come closer together to screen off part of the light
  >prevents excessive light from entering the eye and damaging the retina

-receptor: the retina
- effector: iris

Question 25

how do we see?

Answer 25

1. the light rays are refracted through the cornea and the aqueous humour onto the lens
2. the lens causes further refraction and the rays are brought to a focus of the retina
3. • image on the retina stimulates either the rods or cones
     > depending on light intensity
     the image formed on the retina is :
     - upside down (inverted)
     - laterally inverted
     - diminished (smaller than the actual object)

• nerve impulses are produced when light falls on rods and cones
  > impulses are transmitted via OPTIC NERVE to the brain
• brain interprets the impulses so that we see the object the right way up, front to back and the right size

Question 26

what is focusing?

Answer 26

• the adjustment of the lens of the eye so that clear images of objects at different distances are formed on the retina

> during focusing, the thickness or curvature of the lens is adjusted
allows light to be focused on the retina

Question 27

what are the changes in the eye when focusing on a distant object?
( 7 meters or more)

Answer 27

1. ciliary muscles relax, pulling on suspensory ligaments
2. suspensory ligaments become taut, pulling on the edge of
   the lens
3. lens become thinner and less convex, increasing its focal length
4. light rays from the distant object are sharply focused on the retina
5. photoreceptors are stimulated
6. nerve impulses produced are transmitted by the optic nerve to the brain
   > the brain inteprets the impulses and the person sees the distant object

Question 28

what are the changes in the eye when focusing on a near object?

Answer 28

1. ciliary muscles contract, relaxing their pull on the suspensory ligaments
2. suspensory ligaments slacken, relaxing their pull on the lens
3. the lens, being elastic, becomes thicker and more convex
   > decreasing its focal length
4. light rays from the near object are sharply focused on the retina
5. photoreceptors are stimulated
6. nerve impulses produced are transmitted by the optic nerve to the brain
   > the brain interprets the impulses and the person sees the near object