Retina

“ We may say indeed that the function of the greater part of the retina is merely to give us a general impression of the environment of the object which is being looked at; an impression, that is to say, which will enable us to judge of its relationship to other things. It tells what else there is to look at, and subconsciously we shift our gaze so that, piece by piece, the whole landscape comes to be focused on the central portion. We regard with the central portion what we know exists to be regarded on account of the duller image thrown on the rest of the retina. ”

“ Contrast is occasioned, not by a false idea resulting from unconscious conclusions, but by the fact that the excitation of any portion of the retina—and the consequent sensation—depends not only on its own illumination, but on that of the rest of the retina as well.
"If this psycho-physical process is aroused, as usually happens, by light-rays impinging on the retina, its nature depends not only on the nature of these rays, but also on the constitution of the entire nervous apparatus which is connected with the organ of vision, and on the state in which it finds itself." ”

“ In the normal human eye, only the central parts of the retina are sensitive to colour, the peripheral parts are practically colour-blind. Anæmia of the retina, which may be produced by pressure on the eye-ball, will render the retina, first colour-blind and then insensitive to light. ”

“ If we call these eyes, we must remember that in all probability they have no power of what we call vision—only a power of distinguishing light from dark. Where, however, there exists beneath the lens a so-called retina, that is, a layer of rod-like endings of a nerve, it might, at first sight, be thought that there, at any rate, we have true vision. But in all probability, in a great number of cases the retinal rods are simply for the purpose of rendering the organism sensitive, not only to the presence of light, but to its direction. ”

“ Its finest fibers are ultimately connected with numerous elongated cells which are stimulated by light. The retina is dark purple in color, this color being caused by a layer of cells next to the choroid coat. This [Pg 361] accounts for the black appearance of the pupil of the eye, when we look through the pupil into the darkened space within the eyeball. The retina acts as the sensitized plate in the camera, for on it are received the impressions which are transformed and sent to the brain as sensations of sight. The eye, like the camera, has a lens. ”

“ In framing a theory of color-perception on the basis we have indicated, we would suppose the retina to be a body whose molecular structure is such that it will respond with promptness to all or nearly all the wave-lengths of perceptible light. This molecular change produced in the retina is carried by the optic nerve to the center of vision in the brain, and is there converted into a sensation. ”

“ Unnatural impressions are made on the retina; flashes of strong light, or luminous sparks, appear to dart across the eyes; darkened spots are seen where none exist; gnats, flies, or other minute bodies, various in colour and brilliancy, seem to flutter before the face; or a single dark speck intercepts the vision. Usually the pupil is dilated and the iris insensible to the stimulus of light ”

“ The retina is easily fatigued or deprived of its sensibility. After looking steadfastly at a bright light, or at a white object on a black ground, a dark spot, corresponding in shape to the bright object, {209}presents itself in whatever direction we look. ”

“ It is well known that when the eye is struck a cloud of sparks is seen, which is caused by the mechanical concussion of the retina. These luminous images, often perceived involuntarily, take, speaking scientifically, the form of the body producing the pressure; at the same time we observe the relation between the position of the irritation and the position of the sensation of sight. We transpose a point on the left side of the retina to the right, because we imagine that a ray of light has penetrated the eye from the right, which must fall upon the left half of the retina. ”

“ Isolating the retina from the other tissues requires considerable patience and dexterity. When the retina has been removed and placed in a special receptacle, it will be found that the specimen is well worth the little amount of time spent in making it. ”

“ Were it not for the retina there would be no color; and when the sensory nerves of the retina are partially paralyzed or deficient, as in the case of the color-blind, nature appears to the eye in her true monochromatic garb.
The human eye resembles closely the photographic camera, both in structure and in its manner of functioning. ”

“ The presence of the iris greatly diminishes the extent of surface designed for the reception of light; the whole pencil of rays that falls on the cornea, as supposed, in Fig. 3, not reaching the retina, but only as shown in Fig. 4, on that section of it which enters the pupil. Though much of the volume of light is thereby lost, the restriction is highly beneficial, by sharpening the image on the retina; for the refraction of the rays is much more equal in the centre than toward the margin. ”

“ In the retina the displacement, derangement, or covering up of the cones and rods necessarily interferes with or abolishes sight, the opacity of the cornea, lens, capsule, or vitreous interrupts the rays of light, and the destruction, or coating over of the pigment of the choroid leads to undue reflection and destroys vision. ”

“ Even in the case of the most refined of all the sense organs, that of vision, the receptive apparatus departs but little from the general type. The only essential constituent of the visual sense organ is the retina, which forms so small a part of the eyes of the higher animals; and the simplest eyes are nothing but portions of the integument, in which the cells of the epidermis have become converted into glassy rod-like retinal corpuscles. ”

“ In making a general retrospect, finally, of the effects produced by stimulus of light, we find that there is not a single phenomenon in the responses, normal or abnormal, exhibited by the retina which has not its counterpart in the sensitive cell constructed of inorganic material. ”

“ Whence it is reasonable to conclude, that the light of the day must be much too weak in its dilute state to make any mechanical impression on so tenacious a substance as the retina of the eye.—Add to this, that as the retina is nearly transparent, it could therefore make less resistance to the mechanical impulse of light ”