THE EYES
THE EYES:
The eyes are positioned in sockets known as the orbits, the orbits are
lined with a tissue called adipose which protects the eyes from mechanical
damage.
The eyebrows and eyelashes protect the eyes against foreign bodies such as
dust and sweat, the eyelids ensure that the eyes are constantly hydrated,
blinking spreads oily secretions and tears from the lachrymal glands and
the glands of Meiboom. Tears also contains lysosomes which help prevent
microbial infections. The eye is held in place by three pairs of muscles
that also allow for voluntary eye movement but also require control for
coordination, convergence and accommodation.
Pathway of light through the eyes:
1. Light rays enter the conjunctiva and cornea, the first refraction occurs
here.
2. Light rays then go through the aqueous humour and the transparent lens
to be refracted again.
3. The lens are the only part of the eye that can alter refraction power,
this is done through accommodation.
4. The light then passes through the vitreous humour.
5. Refraction converges the light rays onto the fovea of the retina.
6. Light causes the pigment found in the rods and cones to breakup, this
causes an impulse to be transmitted to the cerebrum of the brain by the
optic nerve.
* Rods are black and white pigment that is activated in dim lighting and
are found around the fovea, Cones are colour pigment that is activated in
bright lighting and are found on the fovea.
,Binocular vision:
The coordination of the eye muscles makes it possible for the eye to focus
of objects that are far away from you.
The process of Binocular vision is to see depth and distance.
1. Both eyes see a scene from different angels
2. The image is overlapped in the middle field
3. The image from both eyes are fused into one
4. The image is processed by the cerebrum as one.
Pupillary mechanism:
The pupillary mechanism is based on light brightness, the pupil changes
size based on how much light and how bright that light is that is entering
the pupil.
In bright light; the pupil is smaller based on the fact that when bright
light enters the eye, the circular muscles contract and the radial muscles
relax causing the pupil to shrink in size.
In dim light; the pupil is bigger based on the fact that when dim light
enters the eye, the circular muscles relax and the radial muscles contract
which causes the pupil to expand.
, Accommodation:
The definition of accommodation is the process whereby the lens becomes
more convex or less convex based on the light rays coming from objects
that are closer than 6m, this process is to ensure the focus of objects.
Closer than 6m;
1. When an object is closer than 6m the rings of ciliary muscles contract
2. There is then less pull on the suspensory ligaments which therefore
slacken
3. Because there is a slack in the suspensory ligaments, they do not pull
the lens taught.
4. The elastic lens then becomes more convex (fatter)
5. This results in more light being refracted which allows for the focus
onto the retina
Further than 6m;
1. When an object is further than 6m the rings of ciliary muscles relax
2. This causes the suspensory ligaments to contract which causes them to
become tense.
3. The ligaments pull of the lens, the lens becomes less convex (flatter).
4. Less refraction occurs because the object is further away.
THE EYES:
The eyes are positioned in sockets known as the orbits, the orbits are
lined with a tissue called adipose which protects the eyes from mechanical
damage.
The eyebrows and eyelashes protect the eyes against foreign bodies such as
dust and sweat, the eyelids ensure that the eyes are constantly hydrated,
blinking spreads oily secretions and tears from the lachrymal glands and
the glands of Meiboom. Tears also contains lysosomes which help prevent
microbial infections. The eye is held in place by three pairs of muscles
that also allow for voluntary eye movement but also require control for
coordination, convergence and accommodation.
Pathway of light through the eyes:
1. Light rays enter the conjunctiva and cornea, the first refraction occurs
here.
2. Light rays then go through the aqueous humour and the transparent lens
to be refracted again.
3. The lens are the only part of the eye that can alter refraction power,
this is done through accommodation.
4. The light then passes through the vitreous humour.
5. Refraction converges the light rays onto the fovea of the retina.
6. Light causes the pigment found in the rods and cones to breakup, this
causes an impulse to be transmitted to the cerebrum of the brain by the
optic nerve.
* Rods are black and white pigment that is activated in dim lighting and
are found around the fovea, Cones are colour pigment that is activated in
bright lighting and are found on the fovea.
,Binocular vision:
The coordination of the eye muscles makes it possible for the eye to focus
of objects that are far away from you.
The process of Binocular vision is to see depth and distance.
1. Both eyes see a scene from different angels
2. The image is overlapped in the middle field
3. The image from both eyes are fused into one
4. The image is processed by the cerebrum as one.
Pupillary mechanism:
The pupillary mechanism is based on light brightness, the pupil changes
size based on how much light and how bright that light is that is entering
the pupil.
In bright light; the pupil is smaller based on the fact that when bright
light enters the eye, the circular muscles contract and the radial muscles
relax causing the pupil to shrink in size.
In dim light; the pupil is bigger based on the fact that when dim light
enters the eye, the circular muscles relax and the radial muscles contract
which causes the pupil to expand.
, Accommodation:
The definition of accommodation is the process whereby the lens becomes
more convex or less convex based on the light rays coming from objects
that are closer than 6m, this process is to ensure the focus of objects.
Closer than 6m;
1. When an object is closer than 6m the rings of ciliary muscles contract
2. There is then less pull on the suspensory ligaments which therefore
slacken
3. Because there is a slack in the suspensory ligaments, they do not pull
the lens taught.
4. The elastic lens then becomes more convex (fatter)
5. This results in more light being refracted which allows for the focus
onto the retina
Further than 6m;
1. When an object is further than 6m the rings of ciliary muscles relax
2. This causes the suspensory ligaments to contract which causes them to
become tense.
3. The ligaments pull of the lens, the lens becomes less convex (flatter).
4. Less refraction occurs because the object is further away.
