tends to prove rather a part of the retina than of the choroid tunic.

The retina is much thinner in one part than in others; in other words, the passage of the nerve filaments from within to without is much shorter; they exhibit no enlargement, and end directly in their terminal organ. This point, which is

Fig. 120.-Elements and structure of the retina.*

tinged yellow, is known by the name of yellow spot (macula lutea), and is (Fig. 121) situated a little outside of the optic papilla, or precisely at the posterior extremity of the anteroposterior diameter of the globe of the eye. At this point, the terminal organs are all represented by cones, while in other parts the rods and cones are intermixed, the former becoming more rare as we examine the anterior part of the retina, that is, the part farthest from the yellow spot. At this part of the retina (region of the ora serrata; see p. 437, Fig. 118, 15), all elements of which partake of the nature of nerves gradually disappear, their place being occupied by connective tissue elements, which are also found,

*A, Vertical section of the substance of the retina, hardened by chromic acid., Membrane, called the membrana limitans, with the ascending supporting fibres (of Muller). f, Layer of filaments of the optic nerve. 9, Layer of the nerve cells. n, Gray layer, finally granular, crossed by radiating fibres. k, Interior (anterior) granular layer. , Intergranular layer. k', Exterior (posterior) granular layer. 8, Layer of the rods and cones. B and C, Detached filaments, enlarged.

though in very small quantities, in the other parts of the

retina.

Finally, the retina contains vessels, and terminal branches of the central artery of the optic nerve, which emerges in the centre of the papilla, and surrounds the yellow spot with its ramifications (Fig. 121).

Fig. 121.-Appearance of the posterior half of the retina of the left eye,
examined with the ophthalmoscope (Liebreich).

The retina forms essentially the sensitive membrane of the eye; by whatever cause its sensibility is excited, it always gives rise, as a subjective phenomenon, to what is known by the name of luminous sensation. If the retina be pricked (Magendie), compressed (phosphenes, phosphaina, studied by Serre of Uzès), twitched by any sudden movement of the eye, or, in short, excited in any way, an impression of light will be produced; the same effect follows the use of electricity. The special method, by means of which the luminous sensation is distinguished from all others, does not, therefore, reside in the qualities which are peculiar to external light; there is no exclusive connection between light and luminous

sensation. Light is only the usual normal and physiological excitant of this sensation; the retina, being situated in the depth of the eyeball, and protected by the cavity of the orbit, is almost entirely removed from the influence of all other agents than the rays of light; these are able to reach it unobstructed by passing through the transparent media of the eye. We have already seen that in cases where the refringent apparatus of the media of the eye is in working order, the images of external objects are painted (upsidedown) upon the retina; an impression is then made upon the membrane, and the excitation transmitted to the cerebral centres (corpora quadrigemina and cerebral lobes), by means of a peculiar mechanism, which we shall endeavor to describe.

The retina is not, however, in every part equally sensitive to light; there is a point which is quite insensitive to it, viz., where the optic nerve (papilla) begins, and is called, on that account, the punctum cæcum. This may be easily proved by the following experiment: if two small objects, one of which is white and the other red, be placed on the same plane, at a certain distance from each other, and we look at either of them with one eye only, we shall see the other also; but if the latter be moved so as to make its image pass over the whole retina, a moment will come when this image will be formed exactly on the optic papilla; at that moment the object will be quite invisible, being depicted on the punctum сӕсит. An experiment made by Mariotte consists in marking two black points upon the paper, at a distance of five centimetres from each other, and standing at a distance of fifteen centimetres from the paper, the left eye being closed, while the point on the left side (A) is observed with the right eye; in this position the point on the right side (B) will not be seen, but it will become visible in any other part, whether nearer or farther off. We find, by calculation, that,

B

in the position indicated, the image of the point on the right side falls upon the punctum cæcum, and, consequently, is invisible.

The sensibility of the retina in other parts differs greatly; it reaches its highest point in the yellow spot (which corresponds exactly with the posterior pole of the eye) and de

creases in the anterior part; thus it is 150 times less at the equatorial plane of the eye than in the yellow spot or macula lutea; thus, if we place two wires very close together, but with sufficient space between them to enable us to distinguish one from the other, and then so direct the eye that their image shall fall, first upon the yellow spot, and then upon the great circle of the eye, we shall find, in the latter case, that the wires to seem distinct must be placed at a distance from each other 150 times greater than when they are painted upon the yellow spot. This experiment is exactly similar to that made in regard to the distance between the points of the dividers, by means of which we estimated the degree of sensibility of the skin (see p. 398).

The yellow spot is, therefore, the principal seat of distinct vision. We make use chiefly of this in order to see clearly, and all the movements of the eyeball are designed to bring the image of the objects observed to this extremely sensitive point in the eye. The entire surface of the retina is about 15 square centimetres, while the surface of the macula lutea is only 1 millimetre; we therefore make use of only part of the surface of the retina for the purpose of distinct vision. Thus, in reading, we see distinctly only two or three words at a time, their image would fall directly on the yellow spot; and the eye must pass over the whole line in order to read it; in other words, it must bring the image of every single word to the sensitive point. In order to decide exactly what is the number of letters, or the extent of surface, painted on the retina, the eyes are fixed, in a dark room, upon the page of a book; the number of letters which can be seen by a flash of lightning or by an electric spark, is then counted, and the dimensions calculated. Starting from this datum, the known dimensions of the yellow spot may be calculated.

Having observed the various degrees of sensibility of the different parts of the retina, we must now examine the substance of this membrane, and see whether, among its numerous layers, there is not one which is peculiarly sensitive, and containing an element which is essentially susceptible to the influence of light. A simple experiment will supply us with a sufficiently satisfactory answer to this inquiry: this experiment is known by the name of Purkinjé's vascular tree, and consists in the perception of the vessels, or, rather, of the shadow of the vessels of the retina itself. These vessels, which are situated in the anterior layers of the retina, always cast their shadow upon the posterior layers of this membrane,

and we can only suppose that it is the force of habit which prevents our being generally conscious of this shadow; the question was, whether it could not be rendered visible by being thrown, artificially, upon some other part of the eye. This was done in the following manner: the person making the experiment looks at a dark obscurity, while a lighted candle is placed either below, or at the side of his eye (Fig. 122); the rays proceeding from this light (B) will be concentrated by the crystalline lens upon a point very much to one side of the retina, the source of light (the candle) being very far beyond the visual centre. The image of the candle on the retina itself constitutes an interior source of light (B') which is sufficiently strong to carry a considerable quantity of light into the vitreous body. It is plain that, under the influence of this light, the vessels of the retina (C and D) will

cast their shadow upon the posterior layers of the retina, not, however, in the usual portions (that is, C' and D'). The shadow will be displaced, and thrown upon the side opposite to that of the source of light in the retina, which is on Dthe same side as the candle (the original source of light). The field of vision being then illuminated by a light of a Fig. 122. Experiment yellowish red, a network of dark-colored by Purkinjé.* vessels is seen to appear, exactly resembling the vessels of the retina, as sketched from an anatomical preparation. (Vascular tree of Purkinjé.)

B

The posterior layers of the retina are, therefore, sensitive to light; but this experiment shows us which of these layers is especially sensitive. By means of a mathematical process which we cannot now describe, and judging by the movements of the shadows of the vessels when the source of light is displaced, or, in other words, by the apparent magnitude of the movement produced in the field of vision by the vascular tree, Helmholtz has inferred that the layer which receives

1 See Helmholtz, "Optique Physiologique." Traduct. franç. par E. Javal et Th. Klein. Paris, 1867, p. 214.

*B, A candle placed at the side of the eye, that is, as much to the side of the centre of the cornea as possible. B', Interior luminous source, formed by the rays of light concentrated by the crystalline lens upon the extreme lateral portion of the eye. CD, Two vessels of the retina (the size of the retina is here greatly exaggerated). The shadow of these two vessels is seen as if projected at D' and C.