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the macula lutea, or yellow spot (Fig. 72, m.l.; Fig. 75, 8"),—not easily seen, however, unless the eye be perfectly fresh,-and, at some distance from this, towards the inner, or násal, side of the ball, is a radiating appearance, prođuced by the entrance of the optic nerve and the spreading out of its fibres into the retina.
Fig. 72.—THE EYE-BALL DIVIDED TRANSVERSELY IN THE MIDDLE LINE,
AND VIEWED FROM THE FRONT. s, sclerotic; ch, choroid, seen in section only. r, the cut edge of the retina ; v.v. vessels of the retina, springing from o, the optic nerve or blind spot; m.l., the yellow spot, the darker spot in its middle being the fovea centralis.
3. A very thin vertical slice of the retina, in any region except the yellow spot and the entrance of the optic nerve, may be resolved into the structures represented separately in Fig. 71. The one of these (A) occupies the whole thickness of the section, and comprises its essential, or nervous, elements. The outer (or posterior) fourth, or rather less, of the thickness of these consists of a vast multitude of minute, either rod-like, or conical bodies, ranged side by side, perpendicularly to the plane of the retina. This is the layer of rods and cones (6c). From the front ends or bases of the rods and cones very delicate fibres pass, and in each is developed a granule-like body (b' c'), which forms a part of what has been termed the outer layer of granules. It is probable that these fibres next pass into and indeed form the close meshwork of very delicate nervous fibres which is seen at d d (Fig. 71, A). From the anterior surface of this meshwork other fibres proceed, containing a second set of granules, which forms the inner granular layer (ff) In front of this layer is a stratum of convoluted fine nervous fibres (ở g')—and anterior to this again numerous ganglionic corpuscles (hh). Processes of these ganglionic corpuscles extend, on the one hand, into the layer of convoluted nerve-fibres
; and on the other are probably continuous with the stratum of fibres of the optic nerve (i).
These delicate nervous structures are supported by a sort of framework of connective tissue of a peculiar kind (B), which extends from an inner or anterior limiting membrane (?), which bounds the retina and is in contact with the vitreous humour, to an outer or posterior limiting membrane, which lies at the anterior ends, or bases, of the rods and cones near the level of ý c in A. Thus the framework is thinner than the nervous substance of the retina, and the rods and cones lie altogether outside of it, and wholly unsupported by any connective tissue. They are, however, as we shall see, imbedded in the layer of pigment on which the retina rests (§ 16).
The fibres of the optic nerve spread out between the limiting membrane (?) and the ganglionic corpuscles (h'), and the vessels which enter along with the optic nerve ramify between the limiting membrane and the inner granules (f f'). Thus, not only the nervous fibres, but the vessels, are placed altogether in front of the rods and
At the entrance of the optic nerve itself, the nervous fibres predominate, and the rods and cones are absent. In the yellow spot, on the contrary, the cones are abundant and close set, becoming at the same time longer and more slender, while rods are scanty, and are found only towards its margin. The layer of fibres of the optic nerve disappears, and all the other layers, except that of the cones, become extremely thin in the centre of the macula lutea (Fig. 73).
4. The most notable property of the retina is its power
Fig. 73.-A DIAGRAMMATIC SectioN OF THE MACULA LUTEA, OR
YELLOW SPOT. aq, the pigment of the choroid ; b, c, rods and cones ; dd, outer granular
layer; ft, inner granular layer ; p g, molecular layer; h h, layer of ganglionic cells; i i, fibres of the optic nerve.
(Magnified about 60 diameters.)
of converting the vibrations of ether, which constitute the physical basis of light, into a stimulus to the fibres of the optic nerve—which fibres, when excited, have the power of awakening the sensation of light in, or by means of, the brain. The sensation of light, it must be understood, is the work of the brain, not of the retina ; for, if an eye be destroyed, pinching, galvanizing, or otherwise irritating the optic nerve, will still excite the sensation of light, because it throws the fibres of the optic nerve into activity ; and their activity, however produced, brings about in the brain certain changes which give rise to the sensation of light.
Light, falling directly on the optic nerve, does not excite it; the fibres of the optic nerve, in themselves, are as blind as any other part of the body. But just as the delicate filaments of the ampullæ, or the otoconia of the vestibular sac, or the Cortian fibres of the cochlea, are contrivances for converting the delicate vibrations of the perilymph and endolymph into impulses which can excite the auditory nerves, so the structures in the retina appear to be adapted to convert the infinitely more delicate pulses of the luminiferous ether into stimuli of the fibres of the optic nerve.
5. The sensibility of the different parts of the retina to light varies very greatly. The point of entrance of the optic nerve is absolutely blind, as may be proved by a very simple experiment. Close the left eye, and look steadily with the right at the cross on the page, held at ten or twelve inches' distance.
The black dot will be seen quite plainly, as well as the cross. Now, move the book slowly towards the eye, which must be kept steadily fixed upon the cross ; at a certain point the dot will disappear, but, as the book is brought still closer, it will come into view again. It results from optical principles that, in the first position of the book, the figure of the dot falls between that of the cross (which throughout lies upon the yellow spot) and the entrance of the optic nerve: while, in the second position, it falls on the entrance of the optic nerve itself; and, in the third, inside that point. So long as the image of the spot rests upon the entrance of the optic nerve, it is not perceived, and hence this region of the retina is called the blind spot.
6. The impression made by light upon the retina not only remains during the whole period of the direct action of the light, but has a certain duration of its own, however short the time during which the light itself lasts. A flash of lightning is, practically, instantaneous, but the
Fig. 74.—PIGMENT CELLS FROM THE CHOROID Coat. A. Branched pigment cells from the deep layer. B. Pigment epithelium. a, seen in face; b, seen in profile ; C, pigment
granules. sensation of light produced by that flash endures for an appreciable period. It is found, in fact, that a luminous impression lasts for about one-eighth of a second ; whence it follows, that if any two luminous impressions are separated by a less interval, they are not distinguished from one another.
For this reason a “Catherine-wheel,” or a lighted stick turned round very rapidly by the hand, appears as a circle of fire; and the spokes of a coach wheel at speed are not separately visible, but only appear as a sort of opacity, or film, within the tire of the wheel.
7. The excitability of the retina is readily exhausted. Thus, looking at a bright light rapidly renders the part of the retina on which the light falls, insensible ; and on looking from the bright light towards a moderately-lighted surface, a dark spot, arising from a temporary blindness