The Sense Organs and Other Sense Receptors

Question 1

The Protection of the Eye

Answer 1

The eyeball is protected in the following ways:
It lies in a bony cavity in the skull called the orbit
fat and connective tissue between the eyeball and bone of the socket protect they eye
The exposed part of the eye is protected by a thin membrane, the conjunctiva
The front of the eye is protected by the upper and lower eyelids
The eyelids have eyelashes which prevent foreign particles such as dust and insects from entering the eye
Tears secreted by the lachrymal glands keep the conjunctiva moist, remove dust particles and destroy bacteria

Question 2

The movement of the eye

Answer 2

Each eye is held in position by three pairs of muscles
These muscles move the eyes about in the orbit as required

Question 3

Structure of the eye

Answer 3

The wall of the eyeball consists of three layers viz. the sclera, choroid and retina

Question 4

The Sclera

Answer 4

The Sclera is a tough, non-elastic, opaque outer covering
In front, the opaque sclera becomes specialised as a transparent disc called the cornea, which is a part of the sclera

Question 5

The Functions of the Sclera

Answer 5

The Sclera has the following Functions:
Protects the inner structure
Maintains the round shape of the eye
Permits light to enter the eye
Refracts light rays so as to focus them on the retina

Question 6

The Choroid

Answer 6

The Choroid is the middle layer, which contains blood vessels and a brown pigment.
The choroid is continuous with the ciliary body and the iris
The ciliary body contains the ciliary muscles
The iris may be coloured blue, green or brown etc, and gives the eye its colour
The opening in the centre of the iris is the pupil

Question 7

The Functions of the Choroid

Answer 7

The Choroid has the following Functions:
The pigment within the choroid prevents reflection of light within the eye by absorbing light rays
Suspensory ligaments arising from the ciliary body hold the lens in position
The ciliary muscles help to alter the shape of the lens for rear and distant vision The iris has radical and circular muscles, which control the size of the pupil
The pupil is able to control the amount of light entering the eye

Question 8

The Retina

Answer 8

The Retina is the innermost layer of the eye
It is made up of two types of light receptors cells viz. rods and cones
The nerve fibres from the rods and cons leave the eye as the optic nerve, which carry impulses to the brain
The yellow spot, a small depression in the centre of the retina, has a very high concentration of cones without rods
It is the centre of the clearest vision
the blind spot, the area of the retina from which the optic nerve arises, contains no rods and cones and is therefore not sensitive to light

Question 9

The Functions of the Retina

Answer 9

The Retina has the following Functions:
The Retina is the light sensitive area of the eye, images are formed on the retina
The rods are responsible for vision in dim light while the cones are responsible for bright light vision and colour vision

Question 10

The Inside of the eye

Answer 10

The inside of the eye contains the aqueous humour and the vitreous humour

Question 11

The Lens

Answer 11

The lens is an elastic, transparent, biconvex structure situated behind the iris
It is held in position by the suspensory ligaments

Question 12

The Functions of the Lens

Answer 12

The shape of the lens can be altered for near or distant vision
The lens refracts light rays entering so as to focus them on the retina

Question 13

The Aqueous Humour

Answer 13

The Aqueous Humour is the liquid that fills the space between the cornea and the lens

Question 14

The Functions of the Aqueous Humour

Answer 14

The Aqueous Humour has the following Functions:
It maintains the shape of the cornea
It supplies the lens and cornea with food and oxygen
It plays a minor role in the refraction of light

Question 15

Functioning of the eye

Answer 15

Light reflected from objects passes through the cornea, aqueous humour, pupil, lens and vitreous humour and falls on the retina where an image is formed
The rods and cones pick up stimulus and convert it to a nerve impulse
These impulses are conveyed to the optic nerve which transmits the impulse to the cerebrum of the brain
Here the impulse is interpreted
The eye is able to adjust to the distance of the object from the eye as well as to the light intensity

Question 16

Binocular Vision

Answer 16

Binocular Vision is vision in which both eyes are used together
Since each eye forms an image, the two images are combined to form a three dimensional (stereoscopic) presentation of that object

Question 17

Accommodation for near vision (objects less than 6 metres away)

Answer 17

The ciliary muscles contract
The suspensory ligaments become slack
The tension on the lens decreases
The lens become more convex
The refractive power of the lens is increased
A clear image of the object is now formed on the retina

Question 18

Accommodation for distant vision (objects more than 6 metres away)

Answer 18

Ciliary Muscles relax
Suspensory ligaments become tout
Tension on the lens capsule increases
The lens becomes flattened (less convex)
The refractive power on the lens is decreased
A clear image of the distant vision is now formed on the retina

Question 19

Pupillary Mechanism

Answer 19

Pupillary Mechanism refers to the process by which the diameter of the pupil is altered so as to control the amount of light entering the eye

Question 20

In bright light

Answer 20

The circular muscles of the iris contract
The radial muscles relax
The pupil constricts
The amount of light entering the eye is reduced