3. Layer of Nuclei.-The layer of nuclei is so called because its most characteristic elements have, in the main, the aspect of nuclei; although by some observers (Kölliker, Schultze), they are regarded as having rather the signification of nucleated cells, in which the enveloping cellsubstance is in small quantity as compared with the size of the nucleus. The nuclei themselves, sometimes called "grains" or "granules," are oval bodies, placed with their long axes perpendicular to the surface of the retina. There are two varieties of them mingled together, which differ mainly in size; the larger being from 9 to 13 mmm. in length, the smaller one-half or two-thirds as long. They are all contained in the interior of varicose enlargements of slender fibres, which are also directed perpendicularly to the surface of the retina, and extend uninterruptedly through the whole thickness of the layer. These fibres are presumed to be of the nature of modified nerve fibres, and to represent. either directly or indirectly, the continuations of those derived from the expansion of the optic nerve. At their outer extremities they are immediately continuous with the elements of the following layer.

4. Layer of Rods and Cones.-This is undoubtedly the most remarkable of the retinal layers, since it consists of elements which are more peculiarly constituted than those found elsewhere, and which are most

Fig. 194.

1

2

DIAGRAMMATIO SECTION, from the posterior portion of the human retina.-1. Layer of rods and cones. 2. Layer of nuclei. (Schultze.)

immediately connected with the physiology of luminous impressions. As the name indicates, these elements are of two kinds: distinguished, according to their shape, by the name of "rods" and "cones." There is reason to believe that their offices are essentially similar, and that they are to be regarded as modifications of each other.

The rods (Fig. 194) are straight, elongated, cylindrical bodies, composed of a transparent, homogeneous substance, remarkable for its highly refractive power. They are about 50 mmm. in length by a little less than 2 mmm. in diameter. They are all placed parallel with each other, closely packed side by side, standing perpendicularly to the surface of the retina, and extending through the whole thickness of the layer. At its outer extremity each rod terminates by a plane perpendicular to its axis; at its inner extremity it tapers suddenly to a point and is continuous. with a fibre of the preceding layer, and thus with one of its nucleated enlargements or grains. According to Schultze, the internal half of each rod is slightly thicker, and exhibits rather less refractive power than its external half.

The cones differ from the rods mainly in their tapering form and the greater diameter of their internal portion, which, as a general rule, is from two to three times as thick as that of the rods. They have the same transparent, highly refractive appearance, and are intercalated among the rods in the same position, that is, perpendicularly to the surface of the retina. Their outer extremities, in some regions, stop short of the external surface of the retina, while in others, particularly in that of most perfect vision, they reach the same level with the ends of the rods. Each cone is connected at its inner extremity with a nucleated fibre belonging to the preceding layer, the only difference in this respect being that both the fibres and the nuclei connected with the cones are larger than those connected with the rods.

Over the greater part of the retina the rods are more abundant than the cones. When viewed from the external surface (Fig. 195, A), their closely packed extremities present the appearance of a fine mosaic pattern, while the cones are interspersed among them in smaller numbers. At the borders of the macula lutea (p. 623), on the other hand, the cones are more abundant, being only separated from each other by single ranges of rods (B); and at its central portion (C) there are only cones, the rods being entirely absent. The cones at this point are also longer and more slender than elsewhere. The following figure indicates the appearance of the rods and cones, as shown in an external view of different parts of the retina. The smaller circles represent the rods, the larger circles the cones. In the interior of each cone is seen the section of its conical extremity.

Fig. 195.

B

OUTER SURFACE OF THE RETINA, showing the ends of the rods and cones.-4. From the lateral portion of the eyeball. B. From the posterior portion, at the edge of the macula lutea. C. From the macula lutea. (Helmholtz.)

Beside the distinctly marked layers above described, there are various others of less certain signification and less uniformity of extent, which are found in different parts of the retina. Throughout the membrane there also exists a certain proportion of delicate connective tissue, which serves for the support and attachment of its remaining anatomical elements.

Perception of Luminous Impressions by the Retina.-It appears, from the description given above, that the retina is not simply an expansion of the fibres of the optic nerve. It is a membrane of special structure, connected with the extremities of the optic nerve fibres, but containing also many additional anatomical elements. It is accordingly. a peculiar nervous apparatus, adapted to receive the impression of luminous rays, and connected, by means of the optic nerve, with the central

gray matter of the brain. An examination of the manner in which the impressions of light are perceived brings into view the following facts.

The optic nerve and its fibres are insensible to light. Notwithstanding that this nerve is capable of transmitting luminous impressions from the retina to the brain, yet in order to do this, it must first receive its own stimulus from the retina. The optic nerve fibres themselves, though sensitive to mechanical or galvanic irritation, cannot be called into activity by the direct influence of luminous rays. This is shown by the experiment of Donders, in which, by aid of the ophthalmoscope, a light of a certain degree of intensity is concentrated upon the optic nerve, without being allowed to reach the tissue of the retina. When the bottom of the eye is illuminated by the ophthalmoscope, the ob server sees the general surface of the retina of a red or brownish color, while the papilla, which corresponds to the entrance of the optic nerve, presents itself as a white circular spot. This spot is occupied entirely by optic nerve fibres, while the elements of the retina commence only beyond its borders. If the minute image of a candle flame at some distance be thrown by reflection upon the retina, its light is perceived by the person under observation, as well as its image by the observer. If the eye however be turned in such a direction as to bring the image of the flame upon the white circle of the optic nerve, this circle, and the nerve fibres of which it is composed, are visibly illuminated to a certain depth, owing to the translucency of their substance; but the light of the candle flame is no longer perceived by the person under examination. The moment, on the other hand, the image of the flame is allowed to pass beyond the limits of the white spot, and to touch the retina, its light becomes perceptible.

The Blind Spot.-The region, accordingly, occupied by the entrance of the optic nerve, and in which the elements of the retina proper are absent, is a blind spot, where luminous rays make no perceptible impression. The real diameter of this spot, according to the average measurements obtained by Listing, Hannover, and Helmholtz, is 1.65 millimetre, and it covers in the field of vision a space equivalent to about 6 degrees. Notwithstanding the existence of this insensible part at the bottom of the eye, no dark point is usually observed in the field of vision, for the following reasons. The blind spot is not situated in the visual axis of the eye, but is placed, corresponding with the entrance of the optic nerve, nearer the median line (Fig. 189). Consequently the image of an object which is directly examined in the normal line of vision cannot fall upon this spot, but is always outside of it, at the end of the visual axis. Even an object which is perceived in the field of vision ontside the direct line of sight, can never reach the blind spot of both eyes at the same time. If it happen to be so placed that its image falls upon the blind spot of one eye, it will necessarily reach the retina of the other eye at a different point, and is accordingly perceived. If, on the other hand, one eye alone be employed, there is always a small portion of the field of vision which is imperceptible. This deficiency is not generally

noticeable, because it is located in a part of the field to which our attention is not directed, and where the distinction of various objects, under moderate illumination, is so imperfect, that the momentary absence of one of them is not regarded. It may, however, be readily made apparent by using for the test a single strongly defined object, like a white spot on a black ground, the presence or absence of which may be noticed without difficulty, even in indirect vision.

If the left eye be covered and the right eye directed steadily at the white cross in figure 196, the circular spot will also be visible, though

DIAGRAM, for observing the situation of the blind spot. (Helmholtz.)

less distinctly, since it will be out of the direct line of sight. Let the page be held vertically at the height of the eyes, and at a convenient distance for seeing both objects in the above manner. If it be now moved slowly backward and forward, a point will be found where the circular spot disappears from sight, because its image has fallen upon the blind spot; while both within and beyond this distance it again becomes visible. It may also be made to reappear, even at the same distance, by inclining the page laterally to the right or left; since this brings the white circle either above or below the level of the blind spot.

The experiment may be varied by fixing two cards, at the height of the eyes, upon a dark wall, two feet apart from each other. If the left eye be covered, and the right eye fixed upon the left-hand card, the other one will disappear from view at a distance of about eight feet from the wall.

It is evident, furthermore, that the optic nerve fibres are not directly sensitive to light, even outside the blind spot, and where they form part of the retina. These fibres radiate from the point of entrance of the optic nerve, forming a continuous sheet on the inner surface of the retina; some of them terminating at successive points in the retinal membrane, others extending to its extreme border at the ora serrata. A luminous ray striking the retina near the fundus of the eye must, therefore, traverse a considerable number of nerve fibres, which are connected at their peripheral extremities with different parts of the retina; and such a ray, coming from a single point, would necessarily cause the sensation of multiplied luminous points or even of a more or less con

tinuous bright line. As distinct points are actually perceived as such by the retina, although the luminous ray emanating from each one has passed through the whole layer of nerve fibres on its internal surface, it follows that the sensibility of these fibres is not affected by the direct action of light.

The sensitive elements of the retina are in its posterior or external layers. This fact is deduced partly from the phenomena manifested when the retinal blood vessels are made visible in the interior of the eye. These blood vessels and their branches radiate from the central trunk which enters with the optic nerve. Their ramifications, down to a certain size, are all situated in the nerve fibre layer of the retina, and it is only the finest subdivisions which pass into the next layer of ganglionic nerve cells. The two outer layers, namely, the layer of nuclei, and that of the rods and cones, are completely destitute of blood vessels. Owing to this anatomical arrangement, the posterior or external layers of the retina, situated behind the main branches of the retinal blood vessels, must lie in the shadow of these branches, the light coming directly from the front through the pupil. The shadows thus thrown are not habitu. ally perceived by any diminution of the light, because the portions of the retina covered by them are always in shadow at the same points, and its sensibility to light is greater, in proportion as the quantity of light reaching it is less. But the shadows may be rendered perceptible by a lateral or oblique illumination, thus causing them to be thrown upon points of the retina unaccustomed to their presence.

Let a lighted candle be held, in a dark room, about three inches distant from the external angle of either eye, and about 45 degrees in advance of the plane of the iris. On moving the candle alternately upward and downward, the field of vision becomes filled with an abundant and elegant tracery of aborescent blood vessels, the exact counterpart of those of the retina. The form of the vessels is distinctly marked in purple-black, upon a finely granular grayish-red ground. The point of entrance of the vascular trunks may even be seen, with their division into two principal branches passing respectively upward and downward, and then breaking up into ramifications of various curvilinear form. If the candle be held immovable, the appearances rapidly fade, since the shadows in reality are quite faint, and are only made visible from the sudden contrast produced by throwing them successively upon different parts of the retina.

As the blood vessels which throw these shadows are at or near the anterior surface of the retina, the extent of their apparent movement on varying the position of the light, gives a means of ascertaining how far behind the anterior surface of the retina its sensitive elements are situated. According to the measurements of Müller,' this distance must be, in various cases, from 0.17 to 0.36 millimetre; and the same observer finds the posterior layers of the retina to be separated from its

'Cited in Helmholtz, Optique Physiologique. Paris, 1867, p. 289.