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optic nerve. These nerves, arising from the optic lobes of the midbrain, appear first beneath the base of the brain as the optic roots, rr', 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

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A VIEW OF THE TWO EYES, WITH OPTIC NERVES.-ch, optic chiasm; rr', nerveroots; 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

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 innermost layer, f, Fig. 21,

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GENERALIZED SECTION OF RETINA, ETC.-O. optic nerve; &, 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.

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ENLARGED SECTION OF RETINA (after Schultze).-A, general view; B, nervous elements; a, 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.

Further, it will be seen that these layers exist, all three, in every part of the retina except two spots. These are the spots where the optic nerve, o, enters, and the central spot, c, which is in the axis of the eye. Where the optic nerve enters, of course, no other layer can exist except the fibrous layer. In the central spot the fibrous layer is wholly wanting, and the granular and nuclear layer is almost wanting, so that the retina is here almost reduced to the bacillary layer. For this reason this spot forms a depression in the retina.

But the extreme importance of the retina requires that these layers be examined more closely. For this a much greater enlargement is necessary. Fig. 22 represents such enlargement. The fibrous layer, h, requires no further description; but the granular and nuclear layer is seen to be composed of two distinct layers of small granules, d and ƒ, and two layers of large nucleated cells, c and e, and a layer of very large nucleolated cells, g, from which go out branching fibers. These are multipolar cells, or ganglia. It is further seen that the bacillary layer is composed of two kinds of elements, viz., slender cylindrical rods and larger cone-like bodies. These are called rods and cones. It is seen, still further, that all these different elements of the retina are in continuous connection with each other, and with the fibers of the optic nerve.

The bacillary layer is of the extremest interest. It consists mostly of rods, but among these are distributed the larger cones, as in Fig. 23, A. As we approach the central spot the cones become more numerous, as seen in B. In the depression of the central spot (fovea centralis) we find only cones, and these are of much smaller size than those in other parts of the retina, as seen in C. The rods are about inch in length and

T4 inch in diameter. The cones are shorter and about three times thicker than the rods, except in the central depression, where they are nearly as small as the rods, being there only inch in diameter. In this spot, therefore, there are probably no less than one million cones in a square

inch.

FIG. 28.

B

BACILLARY LAYER, VIEWED FROM THE OUTSIDE SURFACE.-A, appearance of usual surface; B, appearance of surface of the raised margin of yellow spot; C, surface of central spot.

Distinctive Functions of the Layers.-As the distinctive functions of the several sub-layers of the middle layer (granular and nuclear) are unknown, we will treat of only the three layers-inner, middle, and outer. The outer layer of rods and cones (bacillary) is undoubtedly the true receptive layer, which corresponds to the iodized film of the sensitized plate of the camera. These rods and cones receive and respond to the vibrations of light; they co-vibrate with the undulations of the ether. The inner or fibrous layer conducts the received impression to the optic nerve; for each rod and cone is connected by a slender thread, continuous with nucleated cells of the granular layer and a fiber of the fibrous layer. The fibrous layer may, in fact, be regarded as a layer of conducting threads coming from the rods and cones, which threads are then gathered into a cord or cable, the optic nerve, which in its turn finally conducts the impression to the brain. The function of the middle layer is more obscure; but nucleated nerve-cells, and especially multipolar cells, are always generators or

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