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which make their eyes normal. When they grow old, they are compelled to have two pairs of glasses, one for distant objects and one for near objects; one for walking and one for reading. The hypermetropic eye may be compared to a camera which, when entirely pushed up, is too short for the imaging of any objects whatever. By drawing, it may be adjusted for distant objects, but not for near objects.
Astigmatism.—The form of a perfect eye is that of a spheroid of revolution about the optic axis. Its refraction in a horizontal and a vertical plane will be equal. This is necessary to bring all rays to a perfect point at the same distance. But eyes are found in which the horizontal curvature of the cornea or of the crystalline, or both, is different from the vertical curvature. Such eyes are said to be astigmatic, because the rays from any radiant are brought to a focal line, instead of a focal point. A very slight degree of astigmatism is not uncommon, and often exists unknown to the patient.
EXPLANATION OF PHENOMENA OF MONOCULAR VISION.
SECTION I.--STRUCTURE OF THE PETINA.
We have thus far treated of the eye, and compared it with the camera, purely as an optical instrument, contrived to form an image upon a receiving screen suitably placed. We have also treated of the defects of the eye, as much as possible, from the same physical point of view as defects of an instrument. But in both the camera and the eye the image is only a means to accomplish a higher purpose, viz., to make a photographic picture in the one case and to accomplish vision in the other. We have thus far spoken as much as possible only of an insensitive screen, the ground-glass plate in the one case and the dead retina in the other. But in both, when accomplishing their real work, we have a sensitive screen, in which wonderful changes take place, viz., the iodized plate in the one and the living retina in the other. In order to understand the real function of the eye in the living animal, it is necessary that we study the structure and functions of the retina.
Structure of the Retina.—The retina, as already stated, page 22, is a thin membranous expansion of the optic nerve. These nerves, arising from the optic lobes of the midbrain, appear first beneath the base of the brain as the optic roots, r p', Fig. 20, converge, unite, and partially cross their fibers at the optic chiasm, ch; then, again diverging, enter the conical eye-sockets a little to the interior of the point; then pass through the midst of the fatty cushion behind the eye, surrounded
c7 A VIEW OF THE Two EYES, WITH OPTIC NERVES.-ch, optic chiasm; r ro', nerve
roots ; n and n', right and left optic nerves. After Helmholtz.)
by the diverging recti muscles, and finally penetrate the sclerotic at a point about one eighth of an inch to the inside of the axes; then spread out all over the interior of the ball as an innermost coat, immediately in contact with the vitreous humor, and extend as far forward as the ciliary processes, or nearly to the iris. The wide extent of this expansion and its hollow con
STRUCTURE OF THE RETINA.
cave form are necessary to give wideness to the field of view. By this means rays from objects, not only in front but far to the right and left, above and below, fall upon and impress the retina.
The thickness of this nervous expansion is about one hundredth of an inch, or about the thickness of thin cardboard, at the bottom or thickest part, but thins to one half that amount on the anterior margins; yet, under the microscope, a section through the thickness shows that it is very complex in its structure, being composed of several very distinct layers. We may first represent it on a smaller scale as composed of three principal layers : First, the innerinost layer, f, Fig. 21,
GENERALIZED SECTION OF RETINA, ETC.-0, optic nerve; S, sclerotic; ch, choroid; R,
retina; b, bacillary layer; g, granular and cellular layer; f, fibrous layer; V, vitreous humor; C, central spot.
in contact with the vitreous humor, V, is composed wholly of fine interlaced fibers of the optic nerve. This nerve, o, is seen to pierce the sclerotic and the other layers of the retina, and then to spread out as an innermost layer. Second, outermost of all, and therefore in contact with the choroid, ch, is a remarkable layer, composed of cylindrical rods, like pencils set on end. This is called the bacillary layer (bacillum, a small rod), or
layer of rods, b. Third, between these is found a layer composed of granules and nucleated cells, g. This may be called for the present the granular and nuclear layer.
ENLARGED SECTION OF RETINA (after Schultze).-A, general view; B, nervous ele
ments; (1, bacillary layer; C, external nuclear layer; d, external granular layer; e, internal nuclear layer; f, internal granular layer; g, ganglionic layer ; h, fibrous layer, consisting of fibers of optic nerve.