icse class 10 biology

1. Sense organs
By karthik

2. The eyes

3. External parts of the eye
● Eyelids
○ The upper and lower moveable eyelids protect the outer surface of the eye and can shut out light
○ Each eyelid has outward curved eyelashes which prevent falling of dust particles into the eyes
● Orbits
○ The 2 eyes are located in deep sockets called orbits
○ Each eye is in the form of a ball and can be rotated with the help of 6 muscles
● Eyebrows
○ They prevent raindrops or trickling perspiration from falling into the eye
● Tear glands / lacrimal glands
○ They are also called lacrimal gland
○ They are located on the upper sideward portion of the orbits
○ 6-12 ducts from the glands pour their secretion into the front surface
○ The movements of the eyelid spread the liquid which serves as a lubricant
○ The tears also help wash away dust particles
○ They also have an antiseptic property due to the enzyme lysozyme which kills the germs

4. External parts of the eye cont...
● Tear ducts
○ These ducts drain off the liquid into a sac lying at the inner
angle of the eye
○ A nasolacrimal duct guides the secretion into the nasal cavity
○ Sometimes medicines dropped in the eye can come into our
nose or throat this is due to the nasolacrimal duct
○ During certain emotional states or irritation the tear glands
release the liquid onto the eyes which overflows as tears
● Conjunctiva
○ It is a thin layer covering the surface of the eye
○ It is continuous with inner lining of the eyelids
○ Over the cornea it is reduced to a single layer of transparent
epithelium
○ ‘Conjunctivitis’ a common eye disease is caused due to this
layer turning red due to a viral infection

5. Internal Structure of the eyeball
The wall of the eyeball is composed of 3 concentric layers - The outer sclerotic, The middle choroid, The inner retina
● The outer sclerotic layer
○ It is made of a tough fibrous tissue and is white in
color
○ The white portion in the front of the eye is called
the sclerotic layer
○ It is visible through the conjunctiva
○ It bulges out and becomes transparent in the front
region where it covers the coloured part of the
eye which is called cornea

6. Internal Structure of the eyeball cont...
The wall of the eyeball is composed of 3 concentric layers - The outer sclerotic, The middle choroid, The inner retina
● The middle choroid
○ It is supplied with blood vessels for providing nourishment to the eye.
○ It contains a dark black pigment called melanin which prevents light rays from reflecting and
scattering in the eye.
○ In front of the eye, the choroid expands to form the ciliary body.
○ The smooth muscles in the ciliary body alter the shape of the lens.
● Iris (colour of the eye refers to iris)
○ It is an extension of the choroid
○ It partly covers the lens and leaves a small circular opening in the centre called the pupil
○ The iris contains radial muscles to widen and circular muscles to constrict the pupil
○ The adjustment of size of pupil regulates amount of light entering the eye
○ In dim light the pupil is dilated (widens)
○ In bright light the pupil is constricted (narrow)

7. Internal Structure of the eyeball cont...
The wall of the eyeball is composed of 3 concentric layers - The outer sclerotic, The middle choroid, The inner retina
● The inner retina
○ The retina is sensitive to light
○ It contains 2 types of cells -- rods and cones

8. The difference between rods and cones
● Distributed throughout the retina
● Very sensitive to low levels of light
● More numerous
● They are sensitive to dim light but don't
respond to colour
● They contain the pigment rhodopsin/visual
purple
● Mostly confined to the yellow spot
● Only stimulated by bright light
● Less numerous
● They are sensitive to bright light and are
responsible for colour vision
● The contain the pigment iodopsin/visual
violet
Rods Cones

9. The difference between yellow spot and
blind spot
● This spot contains maximum amount of
sensory cells and particularly the cones and
hence is the region of brightest vision
● It is present at the centre of the horizontal
axis of the eyeball
● The rest of the retina has fewer cones and
more rods
● There are no sensory cells present here so
it is the point of no vision
● Lateral to the yellow spot on the nasal side
is the blind spot
● This is the point where all the sensory cells
of the retina converge and bundle to leave
the eyeball in the form of the optic nerve
Yellow spot (macula lutea) Blind spot

10. Lens
→ The lens is a transparent, flexible, biconvex crystalline body located just
behind the pupil.
→ It contains transparent nerve fibres.
→ The lens is held in position by fibres called suspensory ligaments and which
attaches it to the ciliary body.
→ The ciliary muscles contains muscles which on contraction and relaxation
change the shape of the lens to view objects and different distances.

11. The difference between aqueous and
vitreous chambers
● It is the front chamber between the lens
and the cornea
● It is filled with a watery liquid called the
aqueous humour
○ The aqueous humour keeps the lens moist
and protects it from physical shock
○ It refracts light
● It is a large cavity of the eyeball behind the
lens
● It is filled with a transparent jelly like
substance called the vitreous humour
○ Helps in keeping the shape of the
eyeball
○ It protects the retina and its nerve
endings
Aqueous chambers Vitreous chambers

12. Common defects of the eye
● Myopia [short-sightedness]
○ Near objects can be seen clearly whereas far away objects are
blurred
○ The image of distant objects is formed in front of retina
○ Reasons for myopia
■ Eyeball is lengthened from front to back
■ Lens is too curved
○ Correction for myopia
■ Use of concave lens (causes lens to diverge)
■ Power of glass is minus (-)

13. Common defects of the eye cont….
● Hyperopia [farsightedness]
○ Difficulty in seeing near objects
○ Image of near objects falls behind the retina
○ Reasons for hyperopia
■ Lens is too flat
■ Shortening of eyeball
○ Correction of hyperopia
■ Convex lens is required (lens needs to converge light
rays)
■ Power of glass is plus (+)

14. Common defects of the eye cont….
● Astigmatism
○ Some parts of object are in focus while others are blurred
○ Arises due to uneven curvature of the cornea
○ Corrected by cylindrical lenses
● Presbyopia
○ Older people cannot see nearby objects clearly
○ Their lens loses flexibility resulting in a kind of farsightedness
○ It is corrected by a convex lens
● Cataract
○ It is the condition in which lens turns opaque and vision is cut down to total blindness
○ It can be corrected by surgically removing the lens
○ And Using a highly convex lenses
○ Implanting a small plastic lens behind or infront of the iris

15. Stereoscopic vision
● All monkeys/apes and humans can perceive depth or the relative depth of
the objects.
● This is due to the simultaneous focusing of an object in both eyes and their
images by a kind of overlapping in the bran giving a 3 dimensional effect.

16. The ears

17. Parts of the ear
● Outer ear
○ Consists of the projecting part pinna(auricle) and the passage of the auditory canal leading to the
eardrum(tympanum)
● Middle ear
○ Contains 3 tiny bones malleus(hammer), incus(anvil), stapes(stirrup)
○ Eustachian tube which connects the cavity of the middle ear of the throat
○ The 3 bones are collectively called the ear ossicles
○ The handle of the ear bone is attached to the inner surface of the eardrum
○ Its opposite end is attached to the anvil which in turn is joined to the stirrup
○ The flat part of the stirrup fits on the so-called oval window ,a membrane covered opening to the inner ear
○ The round window also covered by a thin membrane connects the middle and inner ear

18. Parts of the ear (cont...)
● Inner ear
○ It is also called membranous labyrinth
○ Has 3 parts
■ the cochlea,
● Spiral shape and looks like a snail shell
● It has 2 and a half turns
● The inner winding cavity is divided into 3 parallel canals
■ The median canal (cochlear canal)
● It is filled with a fluid called endolymph
● The vestibular canal and tympanal canal filled with a fluid called perilymph
● The middle canal contains areas containing sensory cells spiral organs called organs of corti
for hearing
● The nerve fibres arising from these cells join the auditory nerve and the sensory cells lie on
the basilar membrane

19. Parts of the ear (cont...)
● Inner ear continued...
■ Median canal (continued...)
● The other parts of the inner ear is a set of 3 semicircular canals arranged at right angles such
that one is horizontal and the other 2 are vertical
● One end of each canal is widened to form an ampulla which contains sensory cells for
dynamic balance
● Nerve fibres from them join the auditory nerve
■ Vestibule
● The short stem joining the base of semicircular canals to the cochlea shows to parts
utriculus and sacculus. They are collectively termed vestibule
● These parts also contain a sensory cell for static balance (balancing while being stationary)

20. Functions of the ear (hearing)
● Hearing
○ The pinna collects the sound waves and conducts them through the external auditory canal
where they finally strike the eardrum which is set into vibration
○ The eustachian tube equalises the air pressure on either side of the eardrum allowing it to
vibrate freely
○ The vibration of the stirrup is amplified due to the lever like action of the hammer and anvil
○ The vibrating stirrup transmits the vibration to the membrane of the oval window which in
turn sets the fluids of the cochlear canal into vibration
○ The vibrating movements of the fluid stimulate the hair like processes of the sensory cells of
the cochlea and the impulses are transmitted the brain via the auditory nerve
● What can we hear ???
○ Our sensory endings can receive only those sounds from
■ 20Hz to 20,000 Hz
■ Most keenly heard sounds are at the frequency 1000 Hz to 4000 Hz

21. Functions of the ear (balancing)
● Balancing
○ As the head is turned in different directions the fluid inside the semicircular canals are also
shaken
○ The moving fluids in the canals pushes against the sensory hair cells sending the nerve impulses
through the nerve fibres attached to them to the brain via the auditory nerve
○ The sensory cells in the semicircular canals are concerned with dynamic equilibrium (when the
body is in motion)
○ Similarly sensory patches are also located in the utriculus and sacculus which register the static
balance with respect to gravity