Discoveries and Inventions of the Nineteenth Century

lens in the same position, but having some flatter curvature. It is plain that the same object would be gained if our lens could be made of some elastic material, which, on being pulled out radially at its edges, could be made to assume the required degree of flatness without losing its lenticular form. Now, it is precisely with an automatic adjustment of this kind that the crystalline lens of the eye is provided, for the lens is suspended by an elastic ligament, G, by the tension of which its surfaces 456are more flattened than they would otherwise be; but when the tension of this ligament is relaxed, by the action of certain delicate muscles which draw it down, the elasticity of the lens causes it to assume a more convex form. Fig. 235. Section of Retina. These optical adjustments give, on the inner surface of the coats of the eye, a more or less perfect real image of the objects to which the eye is directed, and it is on the back part of this inner surface that the network of nerves, called the retina, H, is spread out. The sclerotic coat, already spoken of, is lined internally with another, named the choroid, which is composed of delicate blood-vessels, intermingled with a tissue of cells filled with a substance of an intensely black colour. It is upon this last layer that the delicate membrane of the retina is spread out between the choroid and the vitreous humour. The retina is, in part, an expansion of the fibres of the optic nerve over the back part of the eyeball. If we suppose the globe of this cut vertically into two portions, and so divide the front from the back part of the eye, the retina would be seen spread out on the concave surface of the back part, and in the middle of this part, opposite the crystalline lens, would be seen a spot in which the retina assumes a yellowish colour, and in the centre of this, a little round pit or depression. The spot is called the macula lutea, or yellow spot, and the little central pit, which is of the highest importance in vision, is termed the fovea centralis. A little way from the yellow spot, and nearer the nose, is a point from which a number of fibres are seen to radiate, and this is, in fact, the part at which the optic nerve enters the eyeball, and from which it sends out its ramifications over the retina. This part, for a reason which will shortly appear, is called the blind spot. When the minute structure of the retina is examined by the microscope, its physiological elements are found to undergo very remarkable modifications at the yellow spot. In the retina, although the total thickness does not exceed the 180th part of an inch, no fewer than eight or ten different essential or nervous layers have been distinguished. Fig. 235 rudely represents a section. The lowest stratum, A, which is next the choroid, and forms about a quarter of the total thickness, is formed of a multitude of little rod-shaped bodies, a, ranged side by side, and among these are the conical or bottle-shaped bodies, b. This lowest stratum of the retina is called the layer of rods and cones. At their front extremities the rods and cones pass into very delicate fibres, which, going through an extremely fine layer of fibres, B, are connected with a series of small rounded bodies, which form 457the layer of nuclei, C, separated by a layer of nervous fibres, D, from a granular layer, E, in front of which is a stratum of still finer granules, F, underlying a layer of ganglionic nerve-cells, G, of a larger size than any of the other elements, and these ganglionic cells send out numerous branching nerve-fibres, forming the layer H. Finally, on the front surface of the retina there is a thin stratum formed of fibres, which issue from the optic nerve, K, Fig. 234, and in fact constitute the expansion of this nerve on the inner surface of the eyeball. The terminations of some, at least, of these nerve-fibres have been traced, and have been found to form junctions with those branching from the ganglionic cells. Fig. 236. Of the part played by each of these delicate structures in exciting visual impressions little is yet known. How light, or the pulsations of ether, if such there be, is ultimately converted into sensation will probably for ever remain a mystery, although it is quite likely that the kind of visual impression which is conveyed by each part of the elaborate structure of the retina may ultimately be distinguished. One curious result of modern investigation is that light falling directly upon fibres of the optic nerve is quite incapable of exciting any sensation whatever. Light has no more effect on this nerve and its fibres than it would have on any other nerve of the body if exposed to its action. The apparatus of rods, cones, and other structures are absolutely essential to enable light to give that stimulus to the optic nerve which, conveyed to the brain, is converted into visual sensations. So if this apparatus were absent in our organs of vision, in vain would the optic nerve proper be spread out over the interior of the eyeball: we should be no more able to see with such eyes than we are able to see with our hands. We now invite the reader’s careful consideration to the diagram, Fig. 236, which is a section of the retina through the yellow spot. The upper part of the figure is the front, and the deep depression is the little pit already spoken of—the fovea centralis. The lowest dark line represents the basement membrane of the retina, and immediately above is seen the layer of rods and cones, and the various strata already spoken of are represented in their due order in the marginal parts of the diagram. Now observe the remarkable modifications of the nervous structures in the neighbourhood of the fovea centralis, some of which are visible in the diagram. In the first place, the cones are there much longer, more slender, and more closely set, so that there is a far greater number of them on a given surface; but 458the rods are comparatively few, and are, in fact, not found at all under the floor of the little pit. The layer of nuclei, into which the cones extend, is thinner, and is found almost immediately below the anterior surface, for all the other layers thin out in the fovea in a very curious manner. It is, however on the margin of the fovea that the stratum of ganglionic cells, G, Fig. 235, attains its greatest thickness, for there it is formed by the superposition of eight or ten cells, being here thicker than any other layer, while it is so thinned off towards the margin of the retina that it no longer forms even a continuous stratum. This layer, however, becomes much thinner in the fovea, which contains, in fact, but few superposed cells. The tint of the yellow spot is said to be derived from a colouring matter, which affects all the layers except that of the cones. The centre of the yellow spot, where the fovea centralis is situated, is extremely transparent, and is so delicate that it is very easily ruptured, and has frequently been taken for an aperture. We should not have risked wearying the reader with these details concerning the little pit in the centre of the retina had it not possessed an extreme importance in the mechanism of the eye, a fact which he will at once appreciate when we say that of the whole surface of the retina, the only spot where the image of an object can produce distinct vision is the fovea centralis. Since this is undoubtedly true, it follows that the physiological elements which we there find are precisely those which are most essential for producing this effect. The case may be exemplified by recurring to the comparison of the eye with a photographer’s camera, by supposing his screen to be of such a nature that only on one very small spot near its centre could a distinct image be possibly obtained of just one point of an object. Such a defect in his camera would render the photographer’s art impossible, and this defect (if it may be so called) in the eye would render it almost equally useless, had not an adjustment, which more than compensates for it, been afforded in the extreme mobility of our organs of vision. This adjustment is so perfect that people in general do not even suspect that the image of each point of an object which they distinctly see must be formed on one particular spot on the retina—a spot about one-tenth of the diameter of an ordinary pin-head! We may venture, without any disrespect to the reader, to assume that the chances are that it is new to him to learn how each letter in the lines beneath his eye must successively, but momentarily, form its image in the very little pit in the centre of his retina; and the chances are at least a hundred to one that, even if aware of this, he has passively received the statement, and that he has not made the least attempt to realize the truth for himself. Yet nothing is easier. Let him request a friend to slowly peruse some printed page, while he meanwhile intently watches his friend’s eyes. He will then perceive that before a single word can be read there is a movement of the eyeballs, which are, quite unconsciously to the person reading, so directed that the image of each letter (for the area of distinct vision is incapable of receiving more than this at once) shall fall upon the only parts of the retinæ from which a distinct impression can be conveyed along the optic nerve. Thus it is that the eye, without any conscious effort of the observer, is directed in succession to the various points of an object, and it is only by an effort of will in fixing the eyes upon one spot that one becomes aware of the blurred and confused forms of all the rest of the visual picture. Yet so readily do the eyeballs turn to any part of the indistinct picture on which the attention is fixed, that it is not improbable a person unversed in such experiments, wishing to verify our conclusions by looking, 459say, at one spot on the opposite wall, will be very apt, in thinking of the features of the rest of the picture, to direct his eyes there, and then declare that he, at least, sees no such vague forms. If such be his experience, the correction is easy. He has only to ask some one to watch closely his eyes while he repeats the experiment, and after a few trials he will succeed in maintaining the requisite immobility of the eyeballs—a condition upon which the success of many such experiments depends. Fig. 237.—Muscles of Eyes.The muscles of the eyeballs viewed from above:—B, the internal rectus; E, the external rectus; S, the superior rectus; T, the superior oblique, passing through a loop of ligament at U, and turning outwards and downwards to its insertion at C. The inferior rectus and the inferior oblique are not visible in the figure: the superior rectus is removed from the right eyeball in order to show the optic nerve N. This extreme mobility of the eyeballs more than compensates for the loss of the clear and well-defined picture, for it calls into action one of the most sensitive of all the impressions of which we are capable, and one which possesses in so high a degree the power of uniting with our other sensations, that this sixth sense has been, as already stated, utterly overlooked, except by