What is the retina?
The retina is the light-sensitive, colour-appreciative tissue which converts light to electrical impulses for subsequent conduction via the optic nerve to the brain. It is not only the film of the camera, but also helps in adapting to the various stages of light and darkness.
The retina is also the seat of fusion of images, to form a continuous picture (for example, watching a film, the individual images are fused to form a continuous movement).
The sensitivity of the cells of the retina is so high that they can see the difference between two objects placed less than 2 seconds of an arc apart.Function of the retina.
Light enters the eye, is focused, and falls on the photo-sensitive cells of the retina—the rods and the cones. Rods are for dark vision and cones are for light and colour vision.
The light stimulates a photochemical change which converts it into electrical impulses. These impulses are conveyed after suitable amplification and modulation via the cells in the retina to the pipeline or electrical conduit, the nerve fibre layer, which forms the innermost layer of the retina. (The retina is designed inside out—the layer farthest away from the light is the rods and cones with the conductive pathway innermost. However since all the superficial layers of the retina are tranparent to light, it does not matter).
The nerve fibres collect together and pass out of a hole in the back of the eye to form the optic nerve.
The fibres of the two optic nerves from the two eyes pass into the brain, partially cross and continue as optic tracts and later as optic radiations to terminate in the back of the brain at the occipital cortex, the part of the brain concerned with sight.
How is the retina examined?
By an instrument which has a coaxial light system and coaxial observations (i.e. the light passes out from, and the observer sees through, the same hole).
The direct ophthalmoscope has been the standby for many years. With its new halogen lamp system, it has become a sophisticated instrument.
The head-worn, indirect ophthalmoscope is specially designed for retinal examination and gives a clear view of a large area of the retina with the added advantage of a three-dimensional view. Here a magnifying lens is interposed between the instrument and the eye to produce a large aerial image which is unsurpassable for its clarity.
What diseases can cause visible changes in the retina?
Many illnesses of the body, notably blood pressure and diabetes, can cause easily visible changes. The changes are so clear-cut, that often the stage of the disease process, and improvement if any occurring from treatment, can be told by looking at the retina.
The deficiency of iron in the blood, and kidney problems, can be easily told as can other changes in the blood cells.