How the Eye Functions (1941)

protection for this very delicate organ removed we see the eyeball completely surrounded by a layer of su and placed within the Bony orbit where it lies protected against sudden jolts position of the eyeball within its own still more clearly once again we see the fatty cushion which protects it on all sides note this white stock which is the op nerve and also the muscles which move the eyeball this is the empty eye socket within the skull with its bony walls inside and the rim of the Bony orbit 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 eye in Rel 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 eye surface the eyeball itself has a white glistening surface its front part bulges and forms a highly transparent window in this sectional view the capsule of the eyeball is seen to have three layers this thick tough outer layer is cor and ser protect the delicate structures within this transparent bulging portion is called the corneum notice also the crystallin lens which is one of the main features of the eye mechanism the second layer is called the choid it consists of three different belts or zones the first zone is called the choid 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 pupil much like the diaphragm of a camera the Iris will soon be described in Greater detail here is an outside view of the entire choid 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 lens and the cornea is the aquous humor fle of water and a little salt this largest space within the eye is filled with the vitous humor or body cons in cheaply of water with some salt and albumin the Victorious humor is really a highly transparent jelly and plays a very important part in the act of visual adjustment thus light rays entering the eye must pass in succession through first the cornea second the aor third the pupil fourth the crystallin lens and fifth the vitous humor in order to reach their destination the retina now just how do light rays act to 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 15 ft or more the Rays entering our eyes are practically parallel now Nature has provided that parallel rays shall 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 falls behind the retina as a result the image upon the retina tends to become blurred this blurring causes a response or reflex within our brain which brings about harmonious adjustments of all parts of the eye this adjustment 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 focal image it is also necessary that both our eyes be fixed upon the same point in order that we may see an object clearly thus as an object approaches Us From a Distance our eyeballs turn accordingly this movement is called the convergence of visual axes this action is shown here as the object approaches and recedes 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 has a soft yielding body and is as transparent as glass but it also has a firm ball-shaped Central core or nucleus it is contained within a thin elastic membrane and is suspended All Around by a spoke-like arrangement of delicate threads or ligaments the lens itself lies between two liquids these liquids can exert pressure but like all liquids cannot be compressed you may ask then how is this soft pliable body of the lens made to take on the different degrees of convexity necessary for accommodation in order to explain this let us again call attention to the important C muscle which we see here in the diagram this muscle is shaped like a broad ring which can expand and contract whenever it does contract it must press upon the non-elastic vitus humor this liquid is confined within the rigid capsule of the eyeball and must therefore exert pressure upon the lens now the lens as a whole cannot give way against the equally non-elastic aquous humor and its confining corium so the only part which can possibly yield to the pressure of the vitus humor or body is the soft substance of the lens this lens substance is thus molded into an even curve against the rigid ball-shaped core thus forming this highly convex bulge which science has called lenticonus so whenever the image upon the retina becomes blurred the brain automatically causes the ciliary muscle to contract or to relax until the lens has obtained just shape to restore a sharply defined image upon the retina this is the remaining portion of the choid called the iris with its Central opening the pupil like the diaphragm of a 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 muscle fibers within the iris itself these muscles are shown here in simplified diagram these spokelike radial fibers pull the pupil open this ring-shaped muscle around the pupil closes it in this side view we see the opening and closing of the pupil so that we may whenever demanded by conditions the lens acquires this peculiar bulge called lenticonus this cone-shaped curve at first causes light rays to become distorted into aberrations upon the retina as a result of these distortions the pupil until the defused Rays have been shut away only a clear image remains upon the retina these interesting mechanics of the lens are not generally known and the facts presented here have only recently been accepted by science it is well for us to know that our eyes are one of our greatest Gifts of Nature and are indeed worthy of our constant care and [Music] protection