CHAPTER III.

SIGHT.

The Eye's Structure is described in all the books on anatomy. I will only mention the few points which concern the psychologist.* It is a flattish sphere formed by a tough

* The student can easily verify the coarser features of the eye's anatomy upon a bullock's eye, which any butcher will furnish. Clean it first from fat and muscles and study its shape, etc., and then (following Golding Bird's method) make an incision with a pointed scalpel into the sclerotic half an inch from the edge of the cornea, so that the black choroid membrane comes into view. Next with one blade of a pair of scissors inserted into this aperature, cut through sclerotic, choroid, and retiną (avoid wounding the membrane of the vitreous body!) all round the eyeball parallel to the cornea's edge.

The eyeball is thus divided into two parts, the anterior one containing the iris, lens, vitreous body, etc., whilst the posterior one contains most of the retina. The two parts can be separated by immersing the eyeball in water, cornea downwards, and simply pulling off the portion to which the optic nerve is attached. Floating this detached posterior cap in water, the delicate retina will be seen spread out over the choroid (which is partly iridescent in the ox tribe); and by turning the cup inside out, and working under water with a camel's-hair brush, the vessels and nerves of the eyeball may be detected.

The anterior part of the eyeball can then be attacked. Seize with forceps on each side the edge of the sclerotic and choroid (not including the retina), raise the eye with the forceps thus applied and shake it gently till the vitreous body, lens, capsule, ligament, etc., drop out by their weight, and separate from the iris, ciliary processes, cornea, and sclerotic, which remains in the forceps. Examine these latter parts, and get a view of the ciliary muscle which appears as a white line, when with camel's-hair brush and scalpel the choroid membrane is detached from the sclerotic as far forward as it will go. Turning to the parts that cling to the vitreous body observe the clear ring around the lens, and radiating outside of it the marks made by the ciliary

white membrane (the sclerotic), which encloses a nervous surface and certain refracting media (lens and 'humors') which cast a picture of the outer world thereon. It is in

processes before they were torn away from its suspensory ligament. A fine capillary tube may now be used to insufflate the clear ring, just below the letter p in Fig. 3, and thus to reveal the suspenscry ligament itself.

All these parts can be seen in section in a frozen eye or one hardened in alcohol.

fact a little camera obscura, the essential part of which is the sensitive plate.

The retina is what corresponds to this plate. The optic nerve pierces the sclerotic shell and spreads its fibres 9 radially in every direction over its inside, forming a thin translucent film (see Fig. 3, Ret.). The fibres pass into a complicated apparatus of cells, 7 granules, and branches (Fig. 4), and finally end in the so-called rods and cones (Fig. 4,—9), which are the specific organs for taking up the influence of the waves of light. Strange to say, these end-organs are not pointed for. ward towards the light as it streams through the pupil, but backwards towards the sclerotic membrane itself, so

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FIG. 4.

FIG. 5.-Scheme of retinal fibres, after Küss. Nop. optic nerve: S, sclerotic; Ch, choroid; R, retina; P, papilla (blind spot); F, fovea.

that the light-waves traverse the translucent nerve-fibres, and the cellular and granular layers of the retina, before they touch the rods and cones themselves. (See Fig. 5.)

The Blind Spot.-The optic nerve-fibres must thus be unimpressible by light directly. The place where the nerve enters is in fact entirely blind, because nothing but fibres exist there, the other layers of the retina only beginning round about the entrance. Nothing is easier than to prove the existence of this blind spot. Close the right ye and look steadily with the left at the cross in Fig. 6,

holding the book vertically in front of the face, and moving it to and fro. It will be found that at about a foot off the black disk disappears; but when the page is nearer or farther, it is seen. During the experiment the gaze must be kept fixed on the cross. It is easy to show by measurement that this blind spot lies where the optic nerve enters.

The Fovea.-Outside of the blind spot the sensibility of the retina varies. It is greatest at the fovea, a little pit lying outwardly from the entrance of the optic nerve, and round which the radiating nerve-fibres bend without passing over it. The other layers also disappear at the fovea, leaving the cones alone to represent the retina there. The sensibility of the retina grows progressively less towards its periphery, by means of which neither colors, shapes, nor number of impressions can be well discriminated.

In the normal use of our two eyes, the eyeballs are rotated so as to cause the two images of any object which catches the attention to fall on the two foveæ, as the spots of acutest vision. This happens involuntarily, as any one may observe. In fact, it is almost impossible not to 'turn the eyes,' the moment any peripherally lying object does catch our attention, the turning of the eyes being only

another name for such rotation of the eyeballs as will bring the foveæ under the object's image.

Accommodation.-The focussing or sharpening of the image is performed by a special apparatus. In every camera, the farther the object is from the eye the farther forward, and the nearer the object is to the eye the farther backward, is its image thrown. In photographers' cameras the back is made to slide, and can be drawn away from the lens when the object that casts the picture is near, and pushed forward when it is far. The picture is thus kept always sharp. But no such change of length is possible in the eyeball; and the same result is reached in another way. The lens, namely, grows more convex when a near object is looked at, and flatter when the object recedes. This change is due to the antagonism of the circular 'ligament' in which the lens is suspended, and the 'ciliary muscle.' The ligament, when the ciliary muscle is at rest, assumes such a spread-out shape as to keep the lens rather flat. But the lens is highly elastic; and it springs into the more convex form which is natural to it whenever the ciliary muscle, by contracting, causes the ligament to relax its pressure. The contraction of the muscle, by thus rendering the lens more refractive, adapts the eye for near objects ('accommodates' it for them, as we say); and its relaxation, by rendering the lens less refractive, adapts the eye for distant vision. Accommodation for the near is thus