Translations: Remarks on the Structure and function of the Retina

On the one hand, the difficulty, and almost the impossibility, of maintaining that the expansion of the optic nerve is the perceptive element for objective light becomes continually more and more obvious ; and, on the other, the mosaic-like contrivance for the reception of impressions separated by spaces, which was formerly generally assumed as a postulate, although in vain sought for, in consequence of the altered view respecting the structure of the retina, appears now to be admitted ; particularly since it has been shown that the radial fibres are continuous externally in “cones” and “rods,” but are internally in the closest contact with the expansion of the optic nerve, and probably in part connected with it. The circumstance, that such a radial arrangement of the retinal elements occurs in all the vertebrate classes (yid. Zeitsch. f. wiss. Zoologie, III., p. 234), notwithstanding all the divers variations which otherwise exist in the condition of the individual layers, confers an important significance upon this arrangement. But in favour of the notion, that it is the radially disposed elements, and not the horizontal fibres of the optic nerve, which first perceive the objective light, I find, besides the points adduced by Professor Kölliker, a weighty argument in the peculiar structure of the retina in the Cephalopoda, whose much developed eyes, of all the Invertebrata, come nearest to those of the vertebrate class.

In the Cephalopoda the innermost layer of the retina consists of elongated, slender, transparent cylinders, which are, in many respects, similar to the “rods” of the vertebrata; and like them densely crowded together, are disposed in the direction of the radius of the whole eye. Behind these is a layer of pigment, which is penetrated by fusiform filamentary prolongations of each cylinder. Whence is effected the connexion with the outer layers of the retina, of which the last or outermost is the horizontal expansion of the fibres of the optic nerve. Thus the arrangement of the elements is one, pretty nearly the opposite of that which obtains in the Vertebra ta.

In this case, at all events, it is evident that the light must penetrate the innermost bacillar layer, in order to reach the other elements of the retina.

At the same time it is hardly conceivable that the light should act directly upon the fibres of the optic nerve, which lie far behind the pigment, since it is certain that no image can be there formed.

The perception of the latter must rather, in the first instance, proceed only from the radially-disposed elements, which alone are opposed to the light.

This function must be assigned either to the continuations of the innermost “rod-like” cylinders, corresponding in some degree to the “cones” of the Vertebrata, and which project into the pigmentary layer, whilst the cylinders themselves would serve for the isolated conduction of the impression, or, it may be, the cylinders themselves are destined for the perception, and all that lies behind them, merely for the conduction.

Thus the arrangement agrees very much with the notion, which, a priori, appears to be the most plausible ; that most internally there is a mosaic layer for the reception of light, behind that a pigment for the absorption of the light which has been admitted, traversed by radial filaments, which communicate the impression to the horizontal fibres of the optic nerve.

Now, since in these eyes there can hardly be any doubt that the radial elements serve for the perception of the objective light, whilst the horizontal are simply subservient to the conduction of the impression, a similar condition becomes the more probable, also, in the Vertebrata.

The notion, that the radial elements serve for the reception of light, involves a change in the bases upon which are founded the considerations respecting the relations of the smallest distinguishable retinal images to the elementary constituents of that organ.

The difficulty presented in the supposition that very small portions of the same fibre, taken longitudinally, were necessarily to be regarded as possessing different perceptive properties, is removed, and, what must now be admitted, that a fibril of the optic nerve merely conducts different impressions, appears at all events less objectionable. The comparison of the mosaic portion of the retina, with the calculated proportions of the smallest images, would at the same time afford an indirect argument for or against the above view, on which account I will add a few statements to those already adduced by Professor Kölliker.

It is easily shown by experiments, as well as from the comparison of various statements (vid. Volkmann, ‘Handw.’ d. Phys. Art. ‘Sehen.,’ p. 331), that for a single impression, the calculated image may be almost infinitely small, if only the illumination be sufficiently powerful ; for instance, a small hole in a black lamp-shade, or an object glittering in the sun. The dimensions, thus obtained by calculation, are so many times less than the transverse diameter of the elementary parts of the retina in question, that unless a very incomplete union of the luminous rays in the eye be assumed, it must be concluded that it is only requisite that one of these elementary parts should be acted upon with sufficient intensity, only in a small point, in order to communicate the impression of light.

On the other hand, the possibility of distinguishing the smallest distance, shown by Volkmann to exist, might depend upon this, whether several luminous pencils are incident upon one or several elementary parts. It is necessary, however, previously to consider the site of the most distinct vision, because as we proceed towards the lateral portions of the retina, both the optical and the anatomical conditions become more complex.

Volkmann recognized the duplicity of the two filaments of a spider’s web, at a calculated distance of the retinal images of 0·0044‴, and for a friend who had the most acute vision, of 0·0025‴.

Valentin (Physiologie, II. 3, Abth. p. 259) distinguished the distance of two micrometer lines, with a distance of 0·0022‴ ; and in a second case, with one of 0·0014‴, on the retina.

As regards my own eyes, the results derived from the observation of a whole series of micrometer lines, or of the lines in a steel engraving, under favourable conditions of illumination, varied between 0·0025‴ and 0·003‴.

On account of the difference, which, moreover, occurs in the perception of lines and points, I thought it would be necessary to consider the latter also, but I found that the differences are not very important. The distance from the eye at which the lined and pointed spaces of a steel engraving can no longer be recognized in their separate elements, but appear uniform, was pretty nearly equal, with equal interspaces between the latter.

Two lines, made with a fine needle-point, with an interspace of 0·2‴, could be recognized as double, at a distance of about 3 feet, by transmitted light. If the number employed by Volckman (1. c. p. 289 and 331), viz. 6·23‴, be applied as the distance of the focus from the axial point of the retina, the distance between the retinal images would be 0·0022‴ ; in the instance of several holes, 1-7‴ apart, seen at a distance of 20 inches, the distance between the retinal images is 0·0037‴; in that of a wire-sieve, 44 of the openings in which, taken longitudinally, go to an inch, at a distance of about 3 feet, at which they were still very distinctly separable, the distance of the retinal image is : 0·0039‴ ; and at 4 feet : 0·0027‴.

If these calculations be based upon the proportions of the eye, given by Listing (Handworterb. der Phys. IV. p. 496), it is true that somewhat larger numbers are afforded ; for instance, instead of 0·0039‴ : 0·0042‴. This difference, however, particularly in greater distances, is of inconsiderable moment. On the other hand, in peculiarly acute eyes and under highly favourable conditions, somewhat smaller values are presented.

Now, if the above numbers are compared with the diameter of the larger elements in the bacillar layer, viz., the cones (bulbs), which Kolliker found to measure 0·0025 — 0·0045‴, but in the “yellow spot,” not more than 0·002 : 0·0024‴, only the one statement of Valentine is decidedly less, all the others equal, or a little (but within moderate limits) larger than the cones of the “yellow spot.” The diameter of the “rods,” on the other hand, is many times exceeded.

It would be impossible in any case to arrive at an absolute correspondence, and particularly do the greater values of the distances between the images admit of an easy explanation. In general the intervals of several perceptible points must be somewhat greater, because the arrangement of the points will not readily be exactly conformable to the arrangement of the parts of the retina, and consequently the image of a point sometimes falls between, or in other words, upon two retinal elements ; sometimes a single element will be touched by the images of two points.

In greater measurements this must take place, for the reason that the focus never represents an absolute point, but small circles of diffraction, and the larger these are in the eye, so much the less will it be in a condition, as Volkmann has stated, to appreciate the smallest distances.

By the circumstance that the image of a point touches several elements, the phenomena of irradiation may be explained, within certain limits. The distinct perception, also, of two points, the distance between the image of which is not quite equal to the diameter of one of the retinal elements, might, according to the above view, be possible in a very acute eye, because the image in a particular position might nevertheless touch two different elements.

Thus, the facts hitherto ascertained, appear, in general, not to be opposed to the notion that the perception of the smallest distances depends upon the circumstance that different elements of the bacillar layer are touched by the image, and this correspondence again, equally favours the view that, that layer is the apparatus for the reception of the luminous impressions.