FUNCTION OF THE RETINA

While we know a good deal about the retina, there is still much that we have to learn. Like protoplasm itself, the dead retina changes almost before scientists can analyse it and the dead retina, of course, offers nothing in the research of its functioning: how it carries the pictured image to the brain, how the image is interpreted or becomes conscious vision in the brain by means of memories of former images. The physical eye is but the camera which aids vision; vision is fundamentally, a mental process. For example; as in a camera, the image falls on the retina upside down, yet the brain sees the picture right side up. That this is strictly a mental process is shown by the psychologists experiment, when, with lenses, the image was thrown on the retina right side up. This should have caused the person to see it upside down. The brain, however, soon learned to interpret it correctly.

As mentioned above, the retina corresponds to the film in your camera. Like the film it is coated with a chemical which helps it to capture this shadow picture. In the eye this chemical is called visual purple or rhodopsin. When the film in a camera is exposed to a shadow image the chemical wash with which it has been sensitised is immediately used up. In the normal eye, however, when a shadow image falls on the retina, the chemical (visual purple) is replaced immediately, so that the retina is ready to receive the visual image. Scientists estimate that on the normal retina the visual purple is exposed and restored with great speed, enabling the eye to receive a continuous succession of visual !images. However, they do not agree on the purpose of the visual purple or its function in active seeing, as there is no visual purple in the most important part of the retina, the centre of sight or macula.

Strangely enough, there are no blood vessels in the macula either. Again, although the rods and cones are intermixed throughout most of the retina, there are few rods in the macula and none in the macula's keenest seeing portion, the fovea.

The rods of the retina increase in number toward the outer portion of its circumference and take care of the peripheral vision, enabling us to have visual awareness on either side and not be startled by something passing. The rods also do our night seeing, taking over when the cones of the macula and the fovea, the light finders, are insensitive.

The study of this area of greatest visual acuity, the fovea, is carried on by observing it through the pupil of the living eye with an ophthalmoscope. Dr. Bates, in his method of simultaneous retinoscopy, learned the most about its' functioning. He was able to study the retina from a distance of six feet or farther while his subject carried on the usual visual activities. He discovered that this most sensitive group of nerve ends, the fovea, when in action, could, furnish the keen vision upon which the eye depends, only if the eye was in a state of relaxed activity; that while this state of "dynamic relaxation" was maintained, the fovea could perform its function of rapidly searching out every gleam of light around and in the object observed. Then, and then only, keen vision resulted.

Conversely, if the mind is tense, the voluntary muscles of the eye tighten. This pulls the eye off focus, deflecting the light rays from the keen point of the macula to the less sensitive peripheral nerves. The macula registers sharp specific images. The peripheral nerves register blurred general images only, on which the mind strains in its attempt to interpret what is not definite, thus completing, vicious circle. The person sees but with effort, mental and visual, and macula no clarity. If the ma a is in position and the fovea functioning, there is no strain in the eye or the mind and the vision is perfect plainly, then, the retina is the most precious part of the eye and needs the most careful consideration and the greatest protection. Nature plays ever safe in the formation of our bodies and takes no chances with a single protective covering but generously furnishes - multiple layers or coatings. She has carefully encased the retina in two outer protective coatings and has filled the hollow of the eye with protective fluids,

The sclerotic outer covering, the opaque coat of armour or white of the eye, has in itself numerous layers so that if one is scratched, bruised or burned, the eye is still protected. This, covering reaches completely around the globe of the eye, pierced at the back where the optic nerve enters and changing in the front to a transparent window like a watch crystal; called the cornea. This 'cornea window, being the most exposed portion of the eye, has in itself five layers, one tough horny sheath as an added protection against puncture or injury.

Beneath the sclera, the outer coating, is a second tunic, the choroid lining, dark in colour and non-transparent in order to furnish the dark chamber for the camera eye. This choroid has also multiple layers in which lie the blood vessels that provide the nutritive fluids for the nourishment of the retina. It holds, as well, the veins which carry away the used blood and waste matters. This choroid also has one harder layer, a layer like a thin-shelled fingernail for the further protection of the delicate tissues of the precious retina that lie just within it and attached to it for their nourishment.

The choroid when it reaches two-thirds toward the front of the eye becomes the ciliary in front of which is the iris or coloured portion of the eye which makes its beauty but, more important, aids in regulating the amount of light the eye requires for taking its pictures. The iris, the ciliary body and the choroid coating are all of the same tunic. The iris has been likened to the diaphragm in a camera since it opens wider for a picture in poor light but closes to a small point for a picture. in brilliant light. The round opening of the iris is called the pupil.