"hyaloid membrane" (Fig. 189, 10). Its inner surface is in contact with the vitreous body, its outer surface with the retina. It extends uninterruptedly over the posterior and middle portions of the vitreous body until it reaches a point anteriorly corresponding with the ciliary body of the choroid. Here it becomes thicker and divides into two layers. The anterior layer, which is the stronger of the two, the zone of Zinn, extends forward and inward, remaining adherent to the folds of the ciliary body, and terminates in the capsule of the crystalline lens, just in front of its lateral border. The posterior layer of the hyaloid membrane, after separating from the anterior, passes inward and a little backward, and terminates also in the capsule of the lens, but a little behind its lateral border. The triangular canal left between the two separated layers of the hyaloid membrane and the lateral border of the lens is the canal of Petit (Fig. 189, 11), and is filled with a little transparent serosity. The lens is thus suspended on all sides by a double layer derived from the hyaloid membrane. The anterior portion of this double layer, or the zone of Zinn, being the stronger of the two, and presenting a distinctly fibrillated texture, is regarded as more especially fulfilling the part of a suspensory ligament of the crystalline lens.

Crystalline Lens.-The lens is a transparent, refractive body, of circular form, with convex anterior and posterior surfaces, placed directly behind the pupil, and retained in its position by the counterbalancing pressure of the aqueous humor and the vitreous body, and by the two layers of the hyaloid membrane attached to its capsule round its circular border. It is composed of flattened fibres, adherent to each other by their adjacent surfaces and edges, and so arranged as to pass in a curvilinear direction, parallel to the surface of the lens, from one of its two opposite poles to the other. Notwithstanding the fibrous structure of the lens, the ribbon-shaped elements of which it is composed being united by simple juxtaposition, without the intervention of any different material, the entire body is transparent, and allows the passage of the light without perceptible absorption or irregular dispersion.

As the refractive power of the substance of the crystalline is greater than that of the cornea or the aqueous humor, it acts, by virtue of its double-convex form, as a converging lens, to change the direction of the luminous rays passing through it, and bring them to a focus at some point situated behind its posterior surface. The amount of convergence thus effected by a refractive lens depends both upon the index of refraction of the substance of which it is composed and the greater or less curvature of its surfaces. The stronger the curvatures, for lenses composed of the same material, the greater the amount of convergence impressed upon luminous rays passing through them. In the case of the crystalline lens of the human eye, the two surfaces are different in curvature; the anterior surface being comparatively flat, the posterior much more convex. According to the estimates of Listing, based upon a variety of measurements, and adopted by Helmholtz, the

radius of curvature for the anterior surface is, on the average, 10 milli metres, that for the posterior surface 6 millimetres.

This makes the crystalline lens the most powerfully refracting body in the eyeball, and by its aid parallel or diverging luminous rays, after passing through the pupil, are brought to a focus at the situation of the retina. This effect is not due entirely to the lens, since the convex form of the cornea and the more or less spheroidal figure of the whole eyeball necessarily have in some degree a similar action upon rays entering from the front. According to Helmholtz, parallel rays would be brought to a focus by the cornea alone, if they were sufficiently prolonged, at a point situated 10 millimetres behind the retina. But on passing through the lens, their convergence is increased to such a degree that they are concentrated at the situation of the retina itself.

The function of the crystalline lens is to produce distinct perception of form and outline. If the eye consisted merely of a sensitive retina, covered with transparent integument, although the impressions of light would be received by such a retina, they could give no idea of the form of particular objects, but would only produce the sensation of a confused luminosity. This condition is illustrated in Fig. 190, where the arrow, a, b, represents the luminous object, and the vertical dotted line, at the right of the diagram, represents the retina. The rays, diverging from every point of the object in every direction, will thus reach every part of the retina. The different parts of the retina, consequently, 1, 2, 3, 4, will each receive rays coming both from the point of the arrow, a, and from its butt, b. There will, therefore, be no distinction, upon the retina, between the different parts of the object, and no definite perception of its figure. But if, between the object and the retina, there be inserted a double convex refracting lens, with the proper curvatures and density, as in Fig. 191, the effect will be different. All the rays emanating from Fig. 190.

Fig. 191.

a will then be concentrated at r, and all those emanating from b will be concentrated at y. Thus the retina will receive the impression of the point of the arrow separate from that of its butt; and all parts of the object, in like manner, will be distinctly and accurately perceived.

The action of a refractive body with convex surfaces, in thus focussing luminous rays at a particular point, may be readily illustrated in the following manner. If a sheet of white paper be held at a short distance from a candle flame, in a room where there is no other source of light,

the whole of the paper will be moderately and uniformly illuminated by the diverging rays. But if a double convex glass lens, with suitable curvatures, be interposed between the paper and the light, the outer portions of the paper will become darker and its central portion brighter, because a portion of the rays are diverted from their original course and bent inward toward each other. By varying the position of the lens and its distance from the paper, a point will at last be found, where none of the light reaches the external parts of the sheet, but all of it is concentrated upon a single spot; and at this spot will be seen a distinct inverted image of the end of the candle and its flame.

Distinct perception of the figure of external objects thus depends upon the action of the crystalline lens in converging all the rays of light, emanating from a given point, to an accurate focus at the retina. For this purpose, the density of the lens, the curvature of its surfaces, and its distance from the retina, must all be properly adapted to each other. If the lens were too convex, and its refractive power excessive, or if its distance from the retina were too great, the rays would converge to a focus too soon, and would not reach the retina until after they had crossed each other and become partially dispersed, as in Fig. 192. The visual impression, therefore, coming from any particular point in the object, would not be concentrated and distinct, but diffused and dim, from being dispersed more or less over the retina, and interfering with the impressions from other parts. On the other hand, if the lens were too flat, as in Fig. 193, or placed too near the retina, the rays Fig. 192.

Fig. 193.

would fail to come together at all, and would strike the retina separately, producing a confused image, as before. In both these cases, the immediate cause of the confusion of sight is the same, namely, that rays coming from the same point of the object strike different points of the retina; but in the first instance, this is because the rays have actually converged and crossed each other; in the second, it is because they have only approximated, but have never converged to a focus.

The proof that the rays emanating from luminous objects are actually thus concentrated, in the interior of the living eye, upon the retina, is furnished by the use of the ophthalmoscope. This instrument consists essentially of a mirror, so placed as to illuminate by reflected light, through the pupil, the bottom of the eye which is under observa

tion, and perforated at its centre by a small opening through which the observer looks. By this means the retina and its vessels, as well as the images delineated upon it, may be distinctly seen. According to the observations of Helmholtz, objects at a certain distance, which are perceived with distinctness, present to the eye of the observer, if sufficiently illuminated, perfectly well-defined inverted images upon the ret ina, like those which would be thrown upon a screen by a system of glass lenses properly arranged. If the eyeball furthermore be taken out from a recently killed animal, and a circular portion of the sclerotic and choroid removed from its posterior part, similar inverted images of illuminated objects in front of the cornea may be seen by transparency upon the exposed portion of the retina.

It is accordingly certain that luminous rays in passing through the eyeball are brought to a focus at the retina, principally by means of the crystalline lens. The formation of a visible image at this spot does not by itself explain all the phenomena of vision, since these images are not seen by the individual, and we should not even know of their existence except for the results of physiological experiment and observation. But the formation of such an image shows that all the light coming from each different part of the object is made to fall upon a separate and distinct point of the retina; and it thus becomes possible to perceive the figure and extension of an object, as well as its luminosity. Retina. The retina is the most essential part of the organ of vision, since it is the only one of its tissues directly sensitive to light. It forms a delicate, colorless, nearly transparent membrane, composed of nervous elements, situated between the inner surface of the choroid and the outer surface of the hyaloid membrane, and extending from the entrance of the optic nerve outward and forward to the commencement of the ciliary body. Here it terminates by an indented border, termed the ora serrata, which is situated nearly at the plane of the posterior surface of the crystalline lens. In front of this region it is replaced by an attenuated layer, which remains in contact with the surface of the ciliary body, but which contains no nervous elements. The retina proper has, accordingly, the form of a thin membrane moulded upon a nearly hemispherical surface, the concavity of which is directed forward, and which receives the luminous rays admitted through the pupil, and traversing the transparent and refracting media of the eyeball. Its greatest thickness is in the immediate vicinity of the entrance of the optic nerve, where it measures, according to Kölliker, 0.40 millimetre. At a short distance from this point it is reduced to 0.20, and thence becomes gradually thinner in its middle and anterior portions. At its terminal border, at the ora serrata, it is only 0.09 millimetre in thickness. The retina consists of a variety of superimposed layers, in which many different microscopic elements alternate with each other. In regard to its physiological properties, so far as these have been determined with a sufficient degree of certainty, four of these layers may be distinguished as representing the essential constituent parts of the

membrane. These layers, counting from the internal to the external surface of the retina, are as follows: 1. The layer of nerve fibres, derived from the expansion of the optic nerve; 2. The ganglionic layer of nerve cells; 3. The layer of nuclei; 4. The layer of rods and

cones.

1. Layer of Nerve Fibres.-The optic nerve joins the posterior part of the eyeball at a point about 2 millimetres inside its longitudinal axis, and slightly below the horizontal plane of this axis. The neurilemma of the nerve at once becomes continuous with the sclerotic coat of the eyeball, while the nerve fibres alone penetrate into its cavity. Up to this point the fibres of the optic nerve present the usual dark-bordered appearance of medullated nerve fibres, and have, according to Kölliker, a diameter of from 1 to 4.5 mmm. But at their entrance into the cavity of the eyeball the nerve fibres not only lose the prolongations of connective tissue which previously surrounded their different bundles, but also become much smaller in size, being reduced, on the average, to less than 2 mmm., and many of them to less than 1 mmm. in diameter. Owing to these changes, the nerve appears suddenly diminished in size at its passage through the sclerotic and choroid membranes. Internally it forms a slight prominence on the inner surface of the wall of the eyeball, the so-called papilla; and from a depression at its middle part, the central artery and vein of the retina send out their branches to supply the retinal capillary plexus. From the papilla as a centre the optic nerve fibres, which have thus reached the inner surface of the retina, diverge in every direction under the form of a closely set layer. This layer diminishes gradually in thickness from within outward, and from behind forward, owing to the fact that the nerve fibres of which it is composed terminate successively in the deeper parts of the membrane, thus establishing a connection between every point of the retina and the nervous centres in the brain. The longest fibres continue their course until they reach the ora serrata at the anterior limit of the retina, beyond which none are visible.

2. Ganglionic Layer of Nerve Cells.-This layer is situated immediately outside the former, and contains, as its special distinguishing element, multipolar nerve cells, similar to those of the gray matter of the brain. According to Kölliker, they vary in size from 9 to 36 mmm. in diameter, and are provided with a number of pale, ramified prolongations. Some of these prolongations are directed outward, penetrating into the more external portions of the retina; others pass in a horizontal direction, and, according to some observers (Kölliker, Müller, Corti), become connected with optic nerve fibres. For the most part, however, it is only the identity in appearance between some of the prolongations. of these nerve cells and the more slender optic nerve fibres, which leads to the presumption of their direct terminal continuity. It is, in any case, possible that some of the fibres of expansion of the optic nerve are connected with prolongations of the nerve cells, while others con tinue their course to the deeper layers of the retinal tissue.