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rior side of the retina—the famous bacillary layer (layer of rods—Stabchenschicht [membrana Jacobi]) (Fig. 85, #). This layer, which belongs to the most easily injured parts of the eye, and for this reason in many instances escaped the notice of earlier observers, consists, when viewed in
profile, of a very large quantity of closely packed little rods, arranged in a radiated form, and between which at certain intervals appear broader, conical bodies. When the retina is viewed from behind, i. e., from the choroid coat, we see regularly arranged between these cones fine points which correspond to the ends of the little rods.
Now that which intervenes between this bacillar layer and the proper expansion of the optic nerve, is likewise a very complex affair, in which a series of layers following one another in regular succession can be distinguished.
Fig. 85. A. Vertical section through the whole thickness of the retina, after it had been hardened in chromic acid. 1. Membrane limitans, with the ascending, supporting fibres, f. Fibrous layer of the optic nerve, g. Layer of ganglion-cells, n. Grey, finely granular layer, with the radiating fibres passing through it. k. Internal (anterior) granular layer, i. Intermediate, or intergranular, layer, P. External (posterior) granular layer, s. Layer of rods and cones. 300 diameters. B, C (after H. Midler). Isolated radiating fibres.
Immediately in front of the bacillary layer comes a comparatively thick stratum, which appears to be nearly entirely made up of coarse granules, the so-called external granular layer (Fig. 85, k'). Then comes a thinner layer which generally presents a tolerably amorphous appearance, the inter-granular layer (Fig. 85, i). Then we again have coarsish granules (the internal granular layer): these bodies in both layers having much the appearance of nuclei (Fig. S5 k). Next follows a second layer of a more uniform, finely granular or finely striated appearance, and of a more greyish hue (Fig. 85, n), and then only the tolerably thick stratum of the optic nerve, which in its turn is bounded by a membrane, the membrana limitans (Fig. 85, J), which is in close apposition to the vitreous body. Within this last layer we see, besides the fibres of the optic nerve, and situated behind them, a number of largish cells, which have the appearance of nerve-cells (Fig. 85, y).
This extremely complex structure in a membrane which at first sight is so simple and so delicate, readily accounts for its being extremely difficult to ascertain with certainty all the relations of its individual parts. It was one of the most important advances towards the knowledge of these relations which was made by the discovery of Heinrich Miiller, that namely from behind, from the bacillar layer into the most anterior layers, a series of rows of fine fibres could be traced (radiating fibres, also called Miillerian fibres), which both receive the granules, and support the cones and rods (Fig. 85, B, C). This very complicated apparatus is placed as nearly as possible perpendicularly to the course of the fibres of the optic nerve. The greatest difficulty which exists with regard to the anatomical connection of the parts, is to determine whether the radiating fibres, either by bending directly round, or by a lateral anastomosis, become continuous with the optic or ganglionic fibres, and are thus themselves nervous, or whether only an intimate apposition takes place, and so the nerves RETINA. 251
bear no other relations to the radiating fibres than those of proximity. A tactile body may also, you know, be either regarded as a body formed by a swelling of the nerve itself, or as a special structure up to which the nerve only proceeds or into which it enters. This question (of the connections of the radiating fibres) has not yet been definitively settled. At one time the probability became rather stronger that direct communications existed, at another that nothing more than a mere apposition took place. It can, however, even now no longer be doubted, that this apparatus is essential to the perception of light, and that the optic nerve might exist with all its parts without in any way possessing the power of receiving impressions of light, if it were not connected with this apparatus. It is well known that just- that point in the background of the eye, where there are only optic fibres and no such apparatus, is the only one which does not receive impressions of light (the blind spot). In order therefore that the light may be rendered at all capable of acting upon the optic nerve, it unquestionably requires to be collected by means of this apparatus of fibres, and it is therefore an extremely interesting question for delicate physical researches, whether the nerve itself receives at its extreme ends the vibrations of the waves of light, or whether another part exists, the oscillations of which act upon the optic nerve and produce a peculiar excitation in it. At all events there do ascend from the membrana limitans slightly curved fibres (Fig. 85, I), probably connective tissue with its corpuscles, which afford a kind of stay or support to the whole apparatus (supporting fibres (Stiitzfasern) ), and are not, I should suppose, freely connected with the rest of it.
We have, gentlemen, by the consideration of these relations brought out the fact, that the specific energy of individual nerves does not so much depend upon the peculiarity of the internal structure of their fibres as such, but that a great deal must be attributed to the special terminal ar
rangemcnt, with which the nerve is connected, either directly or by contact, and from which the different nerves of sense derive their peculiar powers. If for example we examine a transverse section of the optic nerve external to the eye, it offers no peculiarities as compared with other nerves, which could at all account for this particular nerve's being better able to conduct light than other nerves, whilst on the other hand the peculiar manner in which its extreme ends are distributed sufficiently explains the unusually great sensitiveness of the retina to light.
With regard to the terminations of nerves, there is still one mode to be mentioned; the plexiform distribution. This is a point to which more recent researches have been principally directed by Rudolph Wagner, inasmuch as this inquirer instituted investigations into the distribution of the nerves in the electrical organ of fishes, and in so doing gave the chief impulse to the doctrine of the ramification of nervefibres. Up to that time nerves had been regarded as continuous, single tubes, which remained single throughout the whole of their course from a nervous centre to their termination. At present we know that nerves are distributed like vessels. Now seeing that nerve-fibres directly divide, usually dichotomously, and their branches again divide and subdivide, extremely abundant ramifications may in this way in time arise, the import of which is extremely different, according as the nerve is motor or sensitive, and either collects impressions from.
Fig. SO. Division of a primitive nerve-fibre at t, where we Cud a constriction; b', b" branches. a. Another fibre, crossing the former one. 30O diameters.
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or diffuses motor impulses to, a considerable extent of surface. A truly marvellous instance has lately come to our knowledge in the electrical nerve of the electrical silurus (malapterurus), which has become so celebrated by the interesting experiments of Dubois. Here Bilharz has shown that the nerve which supplies the electrical organ is in the first instance, only a single microscopical primitive fibre, which keeps continually dividing until it finally resolves itself into an enormously great number of ramifications which spread themselves out upon the electrical organ. Here therefore the nervous influence must all at once diffuse itself from one point over the whole extent of the electrical plates.
In man we are still in want of distinct evidence with regard to this question, because the immense distances, over which individual nerves extend, render it almost impossible to follow any single given primitive fibres from their central origin to their extreme peripheral termination. But it is not at all improbable that in man too analogous arrangements exist in some organs, although perhaps not such striking ones. If we compare the size of the nervous trunks in certain parts with the total number of operations which are effected in an organ, for example, in a gland, it can scarcely appear doubtful that at least analogous arrangements exist there also. This mode of distribution offers peculiar interest in this respect, that many parts which are separated by intervals of space are thereby connected with one another. The electrical organ is composed of a number of plates, but not every plate is supplied with nerves proceeding from the centre and intended only for it. The silurus does not set one or other of its plates in motion, but is obliged to set the whole of them in motion; it is quite unable to divide the action. It can increase or diminish the intensity, but must always call the whole into operation. If in like manner we con