EYE COMPARED TO CAMERA

Back of the iris, in a little capsule of fluid, floats the crystalline lens. This lens is much like that in a camera except that, in a camera, the lens is a solid piece of glass, whereas in the normal adult eye, the lens is a series of layers of transparent tissue, through which the light admitted by the pupil may be directed to the retina at the back of the eye.

A clear watery fluid fills the aces under the cornea, in front of and behind the iris and around the crystalline km. This fluid-is called the aqueous and aids in holding these parts in shape and position. The greater hollow inside the eye, between the lens at the front-and the retina at. the rear, is filled with a heavier fluid more like the white of an egg. This thickish fluid, called the vitreous, maintains the spherical contour of two-thirds of. the eye, keeping it inflated as air inflates a balloon. The vitreous is a further protection to the retina.

So we find the precious retina (the extension of the optic nerve): protectively. placed as an innermost lining, deep inside the eyeball. The cornea, the lens and two spas of fluid protect it from. The front; two strong tunics or coatings completely envelop it. With such pro intention for the, nerves of sight, we have the right to, hope for vision as, long as we live.

The mechanical work of the eye is done by the six biportant muscles attached to the putside of the eye. These are fastened. to the white of the eye or sclera. Four of these muscles reach from front to back, near the cornea and extending to the bony structure the rear of the eyeball. There is one above, one and one on-each side, of the eyeball. These are other two muscles, the obliques, circle the eye, obliques one attaching itself to the sclera underneath, the other to the upper side of the eyeball.

All six of these muscles am long, and striated striped except where they attach to the sclera. At these important junctures they are smooth. The striped portion of any muscle is voluntary and can be commanded. The smooth portions are involuntary which means that they operate without conscious direction.

It is with the long striped portion of these outside or extrinsic muscles that we roll our eyes or turn them up, down or from side to side.

The smooth portions of the muscles involuntarily lengthen or flatten the eyeball to accommodate for near or far vision. These two groups of smooth muscles will do their work perfectly, lengthening and flattening the eye, in full co-operation if tension does not interfere; that is, if they remain relaxed, just as any opposing group of muscles in the rest of our bodies permit us to move by co-operating.

Chemical protection for the outside of the eyeball is furnished by the lubricating and disinfecting machinery, the tear glands. The tear glands lie above the eyeball and under the upper lid, and the eyelid is equipped to spread the moisture they provide over the surface of the eye. They put forth a chemical called lysozyme, so Powerful against eye bacteria that scientists claim one tablespoonful is equivalent to gallons of salty water against germs that attack the eye. Is it any wonder that, of .the thousands of eyes we meet, we so seldom see an infected eyeball?

The eye has an extensive and rapid circulatory system. The, optic nerve has a blood vessel all its own. For that reason, when we get eye relaxed and the circulatory channels opened, the circulation improves. Thus, the eye has a better chance to normalise itself.

We have described the most important parts of this camera eye. Let us see how it works when it functions properly.

The voluntary portions of the extrinsic or outside muscles turn the eyeballs so that the pupils may face, or focus on, the object to be viewed. The involuntary portions of these muscles lengthen the globe if the object is close by, flatten the eye if the object is far away. Proper lengthening and flattening of the eyeball itself enables the shadow image to fall on the proper point of the retina, the macula.

The picture enters through the watch crystal cornea, through the clear fluid behind that, through the pupil formed by the iris, through the clear crystalline lens that concentrates the light, then through the thicker fluid in the dark chamber and on to the rods and cones of the retina at the rear. The light beam on which the picture rides stimulates the macula and frees the cones of the fovea into activity. When the eye is relaxed, these foveal cones vibrate with lightning rapidity, hunting light all about and in the object viewed, literally etching it in bold relief, flashing out most minute details into vividly contrasting contours of light and dark.

This etched shadow picture is taken by the optic nerve transmission line to the visual centres of the brain where the mind has the opportunity to study the picture and interpret it. The shadow picture becomes conscious vision by a brain process strictly mental. Just how the retinal impressions are registered so that they become conscious vision is not known.

This interpretation of a new object is possible because of former impressions stored away which are called forth from memory. Then, by aid of the imagination, the new picture can be "made out" or pieced together from past visual experience. Eye-mind co-ordination is then complete and good vision has taken place.