2. Structure and function of eye
and ear

3. Eye

4. Components The eye is made up of three coats, enclosing three
transparent structures.:
 The outermost layer, known as the fibrous tunic, is
composed of the cornea and sclera.
The middle layer, known as the vascular tunic or uvea,
consists of the choroid, ciliary body, and iris.
 The innermost is the retina, which gets its circulation
from the vessels of the choroid as well as the retinal
vessels, which can be seen in an ophthalmoscope.
 Blood vessels can be seen within the sclera, as well as a
strong limbal ring around the iris.

5.  Within these coats are the aqueous humour, the vitreous
body, and the flexible lens.
 The aqueous humour is a clear fluid that is contained in
two areas: the anterior chamber between the cornea and
the iris, and the posterior chamber between the iris and
the lens. The lens is suspended to the ciliary body by the
suspensory ligament (Zonule of Zinn), made up of fine
transparent fibers.
 The vitreous body is a clear jelly that is much larger than
the aqueous humour present behind the lens, and the
rest is bordered by the sclera, zonule, and lens. They are
connected via the pupil.

6. Working of the eyes
 Vision begins when light rays are reflected off an object
and enter the eyes through the cornea, the transparent
outer covering of the eye.
 The cornea bends or refracts the rays that pass through a
round hole called the pupil.
 The iris, or colored portion of the eye that surrounds the
pupil, opens and closes (making the pupil bigger or
smaller) to regulate the amount of light passing through.
 The light rays then pass through the lens, which actually
changes shape so it can further bend the rays and focus
them on the retina at the back of the eye.

7.  The retina is a thin layer of tissue at the back of the eye
that contains millions of tiny light-sensing nerve cells
called rods and cones, which are named for their distinct
shapes.
Cones are concentrated in the center of the retina, in an
area called the macula. In bright light conditions, cones
provide clear, sharp central vision and detect colors and
fine details.
Rods are located outside the macula and extend all the
way to the outer edge of the retina. They provide
peripheral or side vision. Rods also allow the eyes to
detect motion and help us see in dim light and at night.
.

8. Some important points
 It is estimated that the human eye contains about 120 million
rods and about 6 million cones.
 The blind spot is the region of the retina where the optic
nerve fibers leave and where the blood vessels enter and leave
the retina.
 Cones are the most numerous in a specialized region of the
retina known as fovea.
 Rods occur more abundantly 20 degrees around the back of
the eyeball.
 Visual acuity is greatest at the fovea, non-existent at the
blind spot .

9.  It is said that we have two visual systems- a cone system and a
rode system. It is known as duplicity theory of vision.
 Cones are the retinal elements active in bright light.
 Rods are the retinal elements active in very dim light.
 Cones are necessary for color vision.
 Color-blind persons have deficiencies in their cone
functioning.

10. Transduction in vision
 The rods and cones contain photosensitive pigments.
 When the electromagnetic energy in the visible spectrum
strikes these pigments, some of the light energy is absorbed
by the pigments and causes them to change their
configuration and thus creates electrical energy.
 This results in receptor voltage which can be recorded.
 Through a series of further electrical steps involving the
horizontal, bipolar and amacrine cells of the retina,
electrical activity is passed from the rods and cons to the
ganglion cells of the retina.
 The electrical events that have travelled across the retina
trigger nerve impulses in the ganglion cells..

11. The visual pathway in the brain Ganglion cells have long axons that leave the retina through
the optic disc to make up the optic nerve.
 The patterns of nerve impulses in these fibers carry
information about the light that struck the rods and cones.
 The axons in the optic nerve reach the lateral geniculate body
of the thalamus.
 There they make synapses with the lateral geniculate body.
 Then fibers from the lateral geniculate cells carry nerve
impulses to the primary visual sensory area at the back of the
brain.

12. Ear
Broken into three sections:
 outer
 Middle
 inner.

13. Outer Ear
 The auricle (pinna) is the visible portion of the outer
ear. It collects sound waves and channels them into the
external auditory meatus (ear canal) where the sound
is amplified.
 The sound waves then travel toward a flexible, oval
membrane at the end of the external auditory meatus
called the tympanic membrane (eardrum). The
tympanic membrane begins to vibrate.

14. Middle Ear
 The vibrations from the eardrum set the ossicles into
motion. The ossicles are three tiny bones (smallest in
the human body): malleus (hammer), incus (anvil)
and stapes (stirrup) which further amplify the sound.
 The stapes attaches to the oval window that connects
the middle ear to the inner ear. The Eustachian tube,
which opens into the middle ear, is responsible for
equalizing the pressure between the air outside the ear
to that within the middle ear.

15. Inner Ear
 The sound waves enter the inner ear and then into the
cochlea, a snail shaped organ.
 The cochlea is filled with a fluid that moves in response to
the vibrations from the oval window.
 As the fluid moves, 25,000 nerve endings are set into motion.
 These nerve endings transform the vibrations into electrical
impulses that then travel along the VIII cranial nerve
(auditory nerve) to the brain.
The brain then interprets these signals and this is how we
hear. The inner ear also contains the vestibular organ that is
responsible for balance.

16. THANK YOU