Right now at this very moment as you watch these light rays striking the magnified
by similar tiny beams of light are entering your own eyes and it's by my eyes
that we are able to gain a great part of our knowledge.
Nature has located the eye close to the brain so
that its messages may arrive there quickly.
Nature is also provided ample protection for this very delicate organ here with the
outer covering is removed.
We see the eyeball completely surrounded by a layer of salt that it tissue
and placed within the bony orbit where it lies protected against sudden jolts
seen from the side a protected position of the eyeball within its final shape.
I suck at the show and still more clearly.
Once again we see the fetid cushion which protected on all sides know this white
stalk which is the optic nerve and also these muscles which move the eyeball.
This is the empty. I suck at within the skull with its bony walls inside
and the rim of the bony or butt in front.
By gradually restoring the outer portions of the skull
and also the covering tissues we can now realize the location of the entire in
relation to the outline of the face another important safeguard to the eye
is the tear gland which secretes the tear fluid.
This is an effective germicide which drains through the tear ducts into the nose
after flushing and cleaning the entire I surface.
The eyeball itself has a white glistening surface.
Its front part bulges and forms a highly transparent window.
In this sectional view the capstone of the eyeball the seem to have three layers
this tough outer layer is called the sclera
and serves to protect the delicate structures within this transparent bulging
portion is called the cornea Notice also the presta lens which is one of the main
features of the eye mechanism.
The second layer is called the choroid it consists of three different belts
or zones. The first stone is called the choroid proper and is the part
that carries nourishment to the tissues of the eye.
The next zone is called the ciliary body.
This is a broad ring shaped band of thin muscle fibers which play a very active
and vital part in the visual adjustment of the eye.
The third zone is the well known Iris which expands
and contracts the people much like the diaphragm of the camera the iris will soon
be described in greater detail.
Here is an outside view of the entire choroid which shows the dense network of
arteries
and veins carrying nourishment to the eyeball You can also see the shape of the
ciliary body
and the iris.
The innermost layer of the eyeball is the retina a very delicate membrane the
retina is actually a part of the optic nerve which transmits the light impulses to
our brain. The retina is the most important and complex structure in the eyeball.
Magnified many hundred times the retina is seen to consist of this complicated
arrangement of rods and cones which convert light waves into nerve impulses.
In some manner which even science of today cannot fully explain.
Between the Lines in the cornea is the most humor consisting mostly of water
and a little salt this large a space within the eye is filled with the vitreous
humor
or body consisting certainly of water with some songs an album the vitreous humor
is really a highly transparent jelly
and plays a very important part in the act of if you want to just.
Thus light rays entering the I must pass in succession throw first the cornea
second the ECK museum or third the people.
Fourth the crystal lens
and fifth leave a trace humor in order to reach their destination.
They are right in the.
Now just how do the light rays active form a clear picture within the eye.
First let us realize
that light rays reflected from any object radiate in all directions and
that as an object moves farther away from us the angle between the rays
that enter our eyes grows smaller and smaller.
And finally when the object is reached a distance of fifteen feet
or more the ray is entering our eyes are practically parallel.
Nature has provided the parallel ratio produce a sharp image upon the retina
without any effort.
And as one may say with the eye in a state of complete relaxation.
But if a distant object approaches the eye the angle between entering rays again
grows larger.
This increasing angle changes the direction of the light rays as they pass through
the lens so that the sharp image falls behind the right.
As a result the image upon the retina tends to become blurred.
This blurring causes a response
or reflects within our brain which brings about our money as internal adjustments
of all parts of the eye that are just one is called accommodation.
Accommodation restores the sharply focused image with marvelous speed and accuracy.
This is all the more remarkable because the distance between lens
and retina is fixed and the lens itself must change its shape.
Now compare this with the focal adjustment of a camera.
Here the lens remains unchanged
but the film itself is moved until it reaches a sharp local image.
It is also necessary that both our eyes be fixed upon the same point in order
that we may see an object clearly.
The US as an object approaches us from a distance or eyeballs turn accordingly.
This movement is called the convergence of visual axes.
This action is shown here as the object approaches and receives.
Here in addition we see the lenses accommodating their optical shape under the
combined influence of blurred image and eyeball movement.
And now let us look at this highly enlarged view of the lens itself.
It is some of the yielding body and it is transparent as glass.
But it also has a firm ball shaped central core or nucleus.
It is contained within a thin elastic membrane.
And this is spend it all around by a spoke like a arrangement of delegates threads
or ligaments lends itself lies between to make words these liquids can
exert pressure Golick which cannot be compressed. You may ask.
Then how is the soft pliable body of the limbs made to take on the different
degrees of convexity necessary for accommodation in order to explain this.
That is again called attention to the important ciliary muscles which we see here
in the diagram.
This muscle is shaped like a broad ring which going to expand and contract.
Whenever it does contract. It was pressed upon the non-elastic vitreous humor.
Let's make what is confined within the rigid capstone of the eyeball
and must therefore exert pressure upon the limbs.
And the lens as a whole cannot give way against the equally non-elastic it with
humor and its confining quantity.
So the only part which can possibly yield to the pressure of the vitreous humor
or body is the soft substance of the limbs.
Let's lend substances that's molded into an even curved against the rigid ball
shaped core that's forming this highly convex boat which science is called Letter
con is.
So whenever the image upon the retina becomes blurred the brain automatically
causes the ciliary muscles to contract
or to relax until the lens is attained just the right shape to restore a sharply
defined image upon the retina.
This is the remaining portion of the choroid goal the iris with its central opening
the people like the diaphragm of the camera the iris regulates the amount of light
and sharpens the image upon the retina the iris acts automatically.
And so is an important link in the process of accommodation
its action is caused by two sets of delicate my. So fibers within the iris itself.
These muscles are shown here and simplified diagram.
They spoke like radio by a Bers both of the people open.
This ring shape muscle around the pupil closes it.
In the side view we see the opening and closing of the people so
that we may more clearly understand another important part played by the iris.
Whenever demanded by conditions the lens acquires this peculiar bones called
Linthicum is this cone shaped curve at first causes light rays to become distorted
into aberrations upon the retina
as a result of these distortions. The purple now contracts.
Don't they die if you Israelis have been shut away
and only a clear image remains upon the retina.
These interesting mechanics of the lens are not generally known in the facts
presented here. I've only recently been accepted by science.
It is well first and know that our eyes are one of our greatest gifts of nature
and are indeed were they about our constant care and protection.