The human eye is an organ that reacts to light and allows for vision. It contains several key structures working together, including the iris, pupil, cornea, lens, retina, and optic nerve. The iris controls the size of the pupil to regulate the amount of light entering the eye. The retina contains light-sensitive rod and cone cells that convert light into electrical signals sent to the brain via the optic nerve. Common abnormalities include myopia, hyperopia, presbyopia, and cataracts, which can often be corrected using lenses or surgery.
2. Human
Eye :
The human eye is an organ which reacts to
light and pressure. As a sense organ, the
mammalian eye allows vision. A Normal Eye
looks like,
3. The human eye is an
organ that reacts to light and has
several purposes. As a sense organ,
the mammalian eye allows vision.
Rodand cone cell in the retina allow
conscious light perception.
4. 4
Eye and Associated Structures
70% of all sensory receptors are in the eye
Photoreceptors – sense and encode light patterns
The brain fashions images from visual input
Accessory structures include:
Eyebrows, eyelids, conjunctiva
Lacrimal apparatus.
5. 5
Eyebrows
Coarse hairs the overlie the supraorbital margins
Functions include:
Shading the eye
Preventing perspiration from reaching the eye
Orbicularis muscle – depresses the eyebrows
Corrugator muscles – move the eyebrows medially
7. 7
Accessory Structures of the Eye
Eyelashes
Project from the free
margin of each eyelid
Initiate reflex blinking
Lubricating glands
associated with the eyelids
Meibomian glands and
sebaceous glands
Ciliary glands
Figure 16.5a
8. 8
Conjunctiva
Transparent membrane
that:
Lines the eyelids as the
palpebral conjunctiva
Covers the whites of
the eyes as the ocular
conjunctiva
Lubricates and protects
the eye
Figure 16.5a
9. 9
Lacrimal Apparatus
Consists of the lacrimal gland and associated ducts
Lacrimal glands secrete tears
Tears
Contain mucus, antibodies, and lysozyme
Enter the eye via superolateral excretory ducts
Exit the eye medially via the lacrimal punctum
Drain into the nasolacrimal duct
13. The Camera The Human Eye
The individual components of the eye work in a manner
similar to a camera. Each part plays a vital role in providing
clear vision. So think of the eye as a camera with the cornea,
behaving much like a lens cover. As the eye 's main focusing
element, the cornea takes widely diverging rays of light and
bends them through the pupil, the dark, round opening in
the center of the colored iris. The iris and pupil act like the
aperture of a camera.
Next in line is the lens which acts like the lens in a camera,
helping to focus light to the back of the eye. Note that the
lens is the part which becomes cloudy and is removed during
cataract surgery to be replaced by an artificial implant
nowadays.
14. Parts of Human
Eye
The front part of the Eye includes:
Iris: the colored part.
Cornea: a clear dome over the iris.
Pupil: the black circular opening in the iris that
lets light in.
Sclera: the white of your eye.
16. Cornea
CORNEA
(clear lens in front of
eye)
transparent covering of the
front of the eye
Allows for the passage of
light into the eye and
functions as a fixed lens.
Function:
Allows for the passage of light
into the eye and it also focuses
the light.
18. The iris control the amount of light entering the
eyes. The iris automatically adjust the size of the
pupil according to the intensity of light received by
the eye. If the amount of light received by the eye
is large (as during the day time ),then the iris
contracts the pupil (makes the pupil small) and
reduces the amount of light entering the eye. On
the other hand , if the amount of light received by
the eye is small(as the room is dark)the iris
expands the pupil (make the pupil large ) so that
more light may enter the eyes.
The adjustment of the size of pupil takes some
time .
19.
20.
21. Pupil Expansion
When the eye needs more light to
enter (when it is dark), the pupils
get larger; allowing more light to
enter the eye
22. Pupil Contraction
When the eye needs less light to
enter (when it is very bright), the
pupils get smaller; allowing less
light to enter the eye.
23. Sclera
SCLERA – a tough white skin
(made of tissue) that covers all of
the eyeball except the cornea.
Function:
Supports eyeball and
provides attachment for
muscles
25. 25
Sensory Tunic: Retina
A delicate two-
layered membrane
Pigmented layer –
the outer layer that
absorbs light and
prevents its
scattering
Figure 16.9a
26. 26
Sensory Tunic: Retina
Neural layer, which
contains:
Photoreceptors
that transduce light
energy
Bipolar cells and
ganglion cells
Amacrine and
horizontal cells
Figure 16.9a
27. 27
The Retina: Ganglion Cells and the Optic Disc
Ganglion cell axons:
Run along the inner surface of the retina
Leave the eye as the optic nerve
The optic disc:
Is the site where the optic nerve leaves the eye
Lacks photoreceptors (the blind spot)
29. The retina of our eye has a large number of light –
sensitive cells. There are two kinds of light-
sensitive cells on the retina: rods and cones .
30. 30
The Retina: Photoreceptors
Rods:
Respond to dim light
Are used for peripheral vision
Cones:
Respond to bright light
Have high-acuity color vision
Are found in the macula lutea
Are concentrated in the fovea centralis
31. Rod cells, or rods,
are photoreceptor cells in the
retina of the eye that can
function in less
intense light than the other
type of visual
photoreceptor, cone cells.
Cone cells, or cones, are one of
the two types
of photoreceptor cells that are
in the retina of
the eye which are responsible
for color vision as well as eye
color
sensitivity; they function best in
relatively bright light, as
opposed to rod cells.
32. 32
Blood Supply to the Retina
The neural retina receives it blood supply from two sources
The outer third receives its blood from the choroid
The inner two-thirds are served by the central artery and vein
Small vessels radiate out from the optic disc and can be seen
with an ophthalmoscope
34. 34
The Lens
A biconvex, transparent, flexible, avascular structure that:
Allows precise focusing of light onto the retina
Is composed of epithelium and lens fibers
Lens epithelium – anterior cells that differentiate into lens
fibers
Lens fibers – cells filled with the clear protein crystalline
With age, the lens becomes more compact and dense and loses
its elasticity
35. Optic Nerve
OPTIC NERVE
Transmits electrical impulses from
retina to the brain
Creates blind spot
Brain takes inverted image and
flips it so we can see
Function:
Transmits electrical signals from
retina to the brain
36. Functions
The eye has many parts that must work
together to produce clear vision:
The sclera, or white part of the eye, protects
the eyeball.
The pupil, or black dot at the center of the eye,
is a hole through which light can enter the eye.
The iris, or colored part of the eye, surrounds
the pupil.
37. Accommodation (Acc) is the process by which the
vertebrate eye changes optical power to maintain a
clear image or focus on an object as its distance
varies.
Accommodation acts like a reflex, but can also be
consciously controlled. Mammals, birds and reptiles
vary the optical power by changing the form of the
elastic lens using the ciliary body (in humans up to
15 dioptres). Fish and amphibians vary the power by
changing the distance between a rigid lens and the
retina with muscles.[1]
Accommodation (eye)
38. 38
Refraction and Lenses
When light passes
from one transparent
medium to another its
speed changes and it
refracts (bends)
Light passing through
a convex lens (as is in
the eye) is bent so that
the rays converge to a
focal point
When a convex lens
forms an image, the
image is upside down
and reversed right to
left
Figure 16.15
39. 39
Focusing Light on the Retina
Pathway of light entering the eye: cornea, aqueous
humor, lens, vitreous humor, and the neural layer of the
retina to the photoreceptors
Light is refracted:
At the cornea
Entering the lens
Leaving the lens
The lens curvature and shape allow for fine focusing of
an image
40. 40
Adaptation
Adaptation to bright light (going from dark to light) involves:
Dramatic decreases in retinal sensitivity – rod function is lost
Switching from the rod to the cone system – visual acuity is
gained
Adaptation to dark is the reverse
Cones stop functioning in low light
Rhodopsin accumulates in the dark and retinal sensitivity is
restored
41. 41
Visual Pathways
Axons of retinal
ganglion cells form the
optic nerve
Medial fibers of the
optic nerve decussate at
the optic chiasm
Most fibers of the optic
tracts continue to the
lateral geniculate body
of the thalamus
Figure 16.22
42. 42
Visual Pathways
Other optic tract
fibers end in superior
colliculi (initiating
visual reflexes) and
pretectal nuclei
(involved with
pupillary reflexes)
Optic radiations
travel from the
thalamus to the visual
cortex
Figure 16.22
44. Myopia
Myopia commonly known as near-sightedness
(American English) and short-sightedness (British
English), is a condition of the eye where the light
that comes in does not directly focus on the retina
but in front of it, causing the image that one sees
when looking at a distant object to be out.
45. This figure shows an defect called
myopia. In this case, the parallel rays
of light coming from the distant
object are converged to form an
image in front of the retina due to
which the eye cannot see the distant
object clearly. Myopia is corrected by
using spectacles containing
concave lenses. When concave lens is
placed in front of the myopic eye as
shown in figure then the parallel rays
of light coming from the distant
object at the far point of myopic eye.
Since the rays of light now appear to
be coming from the eye’s far
46. The defects of eye
called hypermetropia is
caused :
i. Due to low
converging power of
eye-lens,
ii. Due to eye –ball
being too small.
Hypermetropia (Hyperopia) - Long
47. Hypermetropia means long sight and is
where the image of nearby object is
formed behind the retina. This could be
because the eye is too short, or the
cornea or crystalline lens does not refract
the light enough.
A hypermetropic person may have blurred
vision when looking at objects close to
them, and clearer vision when looking at
objects in the distance. By placing a
convex (plus powered) lens in front of a
hypermetric eye, the image is moved
forward and focuses correctly on the
retina.
48. Presbyopia
Presbyopia is a condition associated with aging in
which the eye exhibits a progressively diminished
ability to focus on near objects. Presbyopia’s exact
mechanisms are not fully understood; research
evidence most strongly supports a loss of elasticity
of the crystalline lens, although changes in the lens’
curvature from continual growth and loss of power
of the ciliary muscles (the muscles that bend and
straighten the lens) have also been postulated as its
cause.
49. This defect is corrected in the same way as
hypermetropa is by using spectacles having
convex lenses.
50. Cataract
A cataract is a clouding of the lens inside
the eye which leads to a decrease in vision. It is
the most common cause of blindness and is
conventionally treated with surgery. Visual loss
occurs because opacification of
the lens obstructs light from passing and being
focused on the retina at the back of the eye.
It is most commonly due to aging, but has many
other causes.
51. Over time, yellow-brown pigment is deposited in
the lens, and this, together with disruption of the
lens fibers, reduces the transmission of light and
leads to visual problems.
Those with cataracts often experience difficulty in
appreciating colors and changes in contrast,
driving, reading, recognizing faces, and coping
with glare from bright lights.