presentation

4. The Human Eye, Its Functions, And Visual
Impairment
How the Human Eye Works
The human eye can be compared to a camera
which gathers, focuses, and transmits light though a lens to
create an image of the environment. In a camera, the image
is created on film; in the eye, the image is created on the
retina, a thin layer of light sensitive cells at the back of the
eye. The lens of the eye bends, or refracts, light that enters
the eye.
The cornea, which is a clear, transparent
covering in the front portion of the eye also contributes to
focusing light on the retina. Nerve fibers extending back from
the retina’s nerve cells come together behind the retina to
from the optic nerve, a “cable” of nerve fibers connecting the
eye with the brain,

5. The optic nerve transmits messages about what we
see from the eye to the brain. Like a camera, the human eye
controls the amount of light that enters the eye through the
lens under various lighting conditions.
The orbit is the bony cavity that contains the
eyeball, muscles, and blood vessels, as well as the structures
that produce and drain tears. Each orbit is a pear-shaped
structure that is formed by several bones.

6. An Inside Look at the Eye

7. The outer covering of the eyeball consist of a relatively
tough, white layer called the sclera(or white of the eye). Near
the front of the eye, in the area protected by the eyelids, the
sclera is covered by a thin, transparent membarance (
conjunctive), which runs to the edge of the cornea. The
conjunctive also covers the moist back surface of the eyelids
and eyeballs.
 Exterior is smooth and white
 Interior is brown and grooved
 Extremely durable
 Flexibility adds strength
 Continuous with sheath of optic nerve
 Tendons attached to it

9. The cornea is the clear bulging surface in front of the eye.
It is the main refractive surface of the eye.
 Primary refractive surface of the eye
 Index of refraction: n = 1.37
 Normally transparent and uniformly thick
 Nearly avascular
 Richly supplied with nerve fibers
 Sensitive to foreing bodies, cold air, chemical irritation
 Nutrition from aqueous humor and
 Tears maintain oxygen exchange and water content
 Tears prevent scattering and improve optical quality

11. The anterior segment is divided into two chambers. The
front (anterior) chamber extends from the cornea to the iris. The
back (posterior) chamber extends from the iris to the lens.
Normally, the equeous humor is produced in the posterior
chamber, flows slowly through the pipil into the anterior
chamber, and then drains out of the eyeball through out flow
channels located where the iris meets the cornea.
 The anterior chamber is between the cornea and the iris
 The posterior chamber is between the iris and the lens
 Contains the aqueous humor
 Index of refraction : n =1.33
 Specific viscosity of the aqueous just over 1.0 (like water, hence
the name)
 Pressure of 15-18 mm of mercury maintains shape of eye and
spacing of the elements

12. Light enters the eye through the cornea, the clear,
curved layer in front of the iris and pupil. The cornea serves
as a protective covering for the front of the eye and also helps
focus light on the retina at the back of the eye. After passing
through the cornea, light travels through the pupil. The iris-the
circular, colored area of the eye that surrounds the pupil –
controls the amount of light that enters the eye. The pupil
dilates and constricts like the aperture of a camera lens as
the amount of light in the immediate surroundings changes.
The iris allows more light into the eye when the environment
is dark and allows less light into the eye when the
environment is bright. The size of the pupil is controlled by the
action of the pupillary sphincter muscle and dilator muscle.

13.  Iris is heavily pigmented
 Sphincter muscle to constrict or dilate the pupil
 Pupil is the hole through which light passes
 Pupil diameter ranges from about 3 - 7 mm
 Area of 7 – 38 square mm
 Eye color (brown, green, blue, etc.) dependent on amount
and distribution of the pigment melanin

15.  Transparent body enclosed in an elastic capsule
 Made up of proteins and water
 Consists of layers, like an onion, with firm nucleus, soft cortex
 Gradient refractive index (1.38 -1.40)
 Young person can change shape of the lens via ciliary muscles
 Contraction of muscle cause lens to bulge
 At roughly age 50 the lens can no longer change shape

17. o Fills the space between lens and retina
o Transparent gelatinouse body
o Specific viscosity of 1.8 – 2.0 (jelly – like consistency)
o Index of refraction, n = 1.33
o Nutrition from retinal vessels, ciliary body, aqueous
o Floaters, shadows of sloughed off materials/debris in the
vitreouse
o Also maintains eye shape

19. The light has to pass through many layers of cells
before finally reaching the photoreceptors. The
photoreceptors are where the light is absorbed and
transformed into the electrochemical signals used by the
nervous system. This change is called transduction
The interior of the eyeball is the “inner” side and the
exterior is the “ outer” side. The nuclear layers contain cell
bodies. The plexiform layers contain the connection between
cells in the retina.

21. The location where the optic nerve is bundled and leaves the
retina is known as the optic disk. There are no photoreceptors at
the location of the optic disk and hence there is a blind spot. The
scientific term for a blind spot is a scotoma. So the blind spot
due to the disk is a natural permanent scotoma in normal vision.
Here is a demonstration of the natural permanent scotoma.

23. The fovea is the location on the retina of central gaze.
when you look directly, or fixate, at a stimulus you the retina
locus of this central fixation is the fovea. There are only cones in
the human fovea (no rods). They are thinner, elongated, any
very tightly packed. Because of this, the fovea is the location of
highest visual acuity and best color vision.

25. Covering the fovea is a pigment called the macula. It is
thought that macula serves a protective filter over the foviea that
absorbs blue and ultraviolet radiation. This pigment varies from
observer to observer and is a source of individual variation in
color vision. Usually we do not notice the filtering of the macula
but under special conditions we can notice its presence causing
what is known as Maxwell’s spot.