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this path, none of these reflected rays can reach it to enable him to see the fundus.
That is to say, the lens and other refractive media that bend the rays of the ingoing cone of light to a focus on the retina also bend those of the outgoing cone reflected from the retina to a focus at the point of illumination.
The fact that the blackness of the interior of the eye is caused by the lens, etc., can be shown by a simple experiment.
Diagram showing the effect of a lens on the rays of light reflected from the paper (retina) in
the experiment given in the text. E. Observer's eye. C. Point of illumination. On the left the reflected rays diverge, and some pass to E. On the right they are refracted by the lens to form a cone.*
Blacken the inside of a pill box (about an inch deep), paste printed paper on the bottom, and cut a round hole half an inch in diameter in the lid. By illuminating the interior of the box obliquely the print can be easily recognized. If a convex lens
a of one inch focus be placed behind the opening, the paper cannot even be seen, and the opening looks black like the pupil with any position of the light. The paths traversed by the rays in this experiment may be seen in Fig. 229.
In the left-hand figure of the above wood-cut the first case is illustrated. Here the divergent rays passing from the candle C to the surface P P are reflected in various directions from it; those which strike the blackened interior of the box are absorbed; others emerge through the hole in the lid, and reaching the eye placed at E, enable the print at P to be seen.
The second case is shown in the right-hand figure. Here, instead of diverging till they reach the bottom of the box, the rays are refracted by the lens to a focus at the point P’, from which they pass back through the lens, and are thereby bent to a cone converging to the source of light. No rays pass in the direction E, so the interior of the box looks quite black.
In attempting, then, to view the fundus, the observer must either place his head in the line of light, or the light in the line between the observed eye and his own ; in short, his eye must lie in the line of reflection, in order to see the fundus. If we could see through the source of light, the above object would be accomplished. Helmholtz, by reflecting light into the eye by means of transparent glass plates, originally succeeded in seeing through the plates some of the rays reflected from the fundus. In this method, however, the power which enables the glass plates to reflect the luminous rays toward the eye also robs the observer of much of the light sent back from the retina by reflecting it toward the source of light, and the remaining rays which penetrate the glass cannot give a clear image of the retina.
A simple instrument, the ophthalmoscope, is now in general use for examining the retina. This consists of a concave mirror of short focal distance, which is substituted for the transparent reflecting plates. The rays converging from the mirror to the eye are brought to a focus on the retina, and thence some are reflected outward, and converged by the dioptric media to the hole in the centre of the mirror, behind which the observer's eye is placed to receive the cone of converging rays.
If the observer place his eye and the mirror at a distance of about 3 cm. from the observed eye, and the refraction of both eyes be normal, he can see an enlarged virtual image of the fundus. If the refraction of either eye be abnormal, it must be corrected by a suitable lens placed behind the aperture in the mirror. This is called the direct method of examination.
To overcome the inconvenience and difficulty of this mode of examination the indirect method is usually employed. In it a convex lens of 20 or 40 diopters is used in addition, enabling the observation to be made at a more convenient distance. When
Ophthalmoscopic view of fundus of eye, in which the central artery (8 and c) and the
corresponding veins (h and d) are seen coursing through the retiua' from the optic disc (A).
the eye has been illuminated, the lens is placed at its proper focal distance (2 or 1 inches respectively) in front of the eye. By the converging power of the lens a real inverted image of the fundus is formed in the air a couple of inches to the observer's side of the lens, and can be seen by him through the aperture in the mirror, if he hold his head at a distance to suit his refraction.
With this instrument a round whitish part is seen a little to the nasal side of the axis of the eye, where the nerve pierces the
dark choroid coat. This is called the optic disc. The fundus now, when lighted up, does not look black, but is of a lurid red color, owing to the great vascularity of the choroid coat. Over this red field are seen a number of blood vessels, which start from the centre of the optic disc, and radiating over the fundus send branches to the most anterior parts that can be seen. These are the branches of the vessel which runs in the centre of the nerve. In the very axis of the eye a peculiar depression, free from branches of the blood vessels, can be seen. This central depression (fovea centralis) differs a little in color from the neighboring parts during life, and turns yellow at death, and hence has been called the “yellow spot.” The retina is so transparent that we cannot see it with the ophthalmoscope, but the radiating vessels (central arteries and veins of the retina) lie in it and belong to the nervous structure only.
The ophthalmoscope has proved of inestimable value not only to the ophthalmologist, but also to the physician, as a means of arriving at an accurate knowledge of disease. Hence, it has become more a pathological than a physiological instrument.
LIGHT IMPRESSIONS. The retina is that part of the eye by which the physical motions called light are changed into what are known physiologically as nerve impulses, by means of which the impression of light is excited in the brain. In reaching the retina the light is not altered from the light with which physicists experiment, but at the retina this physical motion is stopped. The optic nerves no more convey the light waves from the eye' to the brain than the tactile nerves carry the objects that stimulate their endings. They only send a nerve impulse which the retina, on its exposure to the light, excites in the terminals of the optic nerve. Any form of stimulation, if applied to the optic nerve, will cause an impulse to pass to the brain, which there sets up the sensation of light. Thus, we are told by persons who have had their optic nerves cut that the section was accompanied by the sensation of a flash of light but not pain. Any violent injury of the eyeball causes a flash of light to be experienced. This fact has long since been recognized in a practical manner, for a blow implicating the eyeball is vulgarly said to “ make one see stars." Also, without violent injury, if we close the eyes and turn them to the one side and then press through the lid with the point of a pencil on the other side of the eyeball, we have a sensation of a point or ring of light from the retinal stimulation. Thus we say that the specific energy of the optic nerves excites a sensation of light, and the adequate stimulus of the nerve terminals of the organ vision is light. The first question that arises is, What part of the retina does this important work of stimulating the optic nerve when light impinges on its terminals ?
THE FUNCTION OF THE RETINA. The retina is a complex peripheral nervous mechanism composed of many elements, the special functions of which are not adequately known. It spreads over the fundus of the eye, but where the nerve pierces the coats of the eyeball there is nothing but nerve fibres, and hence no retina, properly so called, exists at the optic disc.
The structure of the retina varies in different parts, but the following layers can be recognized in most regions (Fig. 231). The exceptions will be mentioned afterward.
Lying next to the hyaloid membrane is the layer of nerve fibres which radiate from the optic disc to the ora serrata near the ciliary region. The fibres spread evenly over the fundus except at the central point (fovea centralis), which they avoid by passing above and below it. These fibres form the inner layer of the retina.
Next to the fibres is a layer of nerve cells, which seem to have one pole connected with a fibre from the optic nerve, while from the other side two or three poles send processes into the adjacent layers of the retina. The cells are numerous near the yellow spot.
Outside the foregoing are four less distinctive layers. The first is broad and granular; next, two layers of peculiar nuclear bodies are found, with a thin, dense one of granular material between them.