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and is formed by an expansion of the optic nerve (Plate XI., fig. 3, n), commencing at the entrance of the optic nerve into the eyeball, and extending as far as 1, where it terminates by a finely-jagged border. The retina is an exceedingly delicate structure, and is admirably supported, as if on a water-bed, by a globular mass of almost fluid consistence. It is itself semi-transparent, and of a greyish-white colour; and is traversed by many branches of a blood-vessel (whence its derivation from rete), which is seen at o, in Plate XI., fig. 3, passing through the midst of the optic nerve to supply the retina with blood. The retina consists of two layers: Jacob's membrane, and the retina proper; Jacob's membrane (so named, because first described by Dr. Jacob) is the outer one of the two, and is seen as a thin film, by a careful dissection made in water: it is formed of a number of rod or club-shaped particles, lying side by side, and with the long axis vertical, the broad end of the particles being in contact with the choroid; the b other end, of course, in opposition with the retina proper. The appearance of the membrane seen from without is represented & in fig. 12, b; that of its vertical section in fig. 13, a; and that of the particles when detached at fig. 12, a.

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The retina proper may be described, in a general way, as made up of three parts: a fibrous, a vesicular, and a granular layer (fig. 13, b, c, d). The fibrous layer, d, is derived from the optic nerve; the fibres of which radiate from the end of the nerve, where it perforates the sclerotic and choroid, and forms a thin sheet of membrane: the vesicular layer, c, is composed of a number of delicate bodies called vesicles (vesicula, a little bladder or blister), seen separately in Plate XI., fig. 6, a. In the fibrous and vesicular layers the blood-vessels of the retina are distributed. The granular layer, b, is that which lies immediately beneath Jacob's membrane, a, and consists of particles, such as those in Plate XI., fig. 6, b.

I have said that the retina is supported and kept extended on a semi-fluid material; this is called the vitreous humour (vitrum, glass), and occupies the whole of the space marked m in Plate XI., fig. 3. It is of the consistence of soft jelly, and is formed of an exceedingly fine and delicate web of fibrous tissue, which is at the same time transparent, and which contains within its meshes a watery fluid. It is bounded, at its circumference, where it comes in contact with the retina, by a thin and transparent membrane (the line next to the scarlet

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FIG. 12.

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FIG. 13.

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retina, Plate XI., fig. 3), called the hyaloid membrane (hyalos, crystal); when this membrane reaches the ciliary processes of the choroid (at h, fig. 3), it forms a number of plaits, similar in number, disposition, and size to those of the ciliary processes, and which are received into the folds of those processes. This plaited part of the hyaloid is called the zonula ciliaris (zonula, a little girdle). If, in Plate XI., fig. 3, we trace the hyaloid membrane to its termination, we see that it ends on the front surface of the lens, i, and immediately behind it, at this point, is a space, k, called the canal of Petit: this little cavity has an important function, and is bounded behind by the vitreous body, in front by the hyaloid membrane, and internally by

the lens.

The crystalline lens (Plate XI., fig. 3, i, and fig. 14) is when seen from the front circular, when from the side doubly convex, in form; the posterior curve being the more convex of the two. The lens is surrounded by a membrane or capsule, that invests it completely; the capsule is of the same structure as the elastic lamina of the cornea. The front part of the capsule is three or four times thicker than the part behind; which latter part is adherent to the tissue of the vitreous body, and therefore is better supported than the capsule in front, which is in contact with a thin fluid, presently to be noticed.

The lens itself is of considerable consistence, such as the white of an egg when gently boiled; its density in the centre being greater than that of the outer part. Looking at it in front, we see, when the lens is beginning to lose its transparency, three lines, which commence at the centre and radiate outwards, dividing the lens into three equal parts; the same is observable on the posterior surface, but the lines behind are placed intermediately with respect to those in front. By hardening it in spirit we may separate the surfaces of the lens into three parts, and we then see that it is composed of layers or lamellæ (fig. 15); and these again consist of fibres, which, in the ox, have finely-jagged edges (fig. 16).* Notwithstanding all this complicated structure, the lens is perfectly transparent.

The parts remaining to be noticed are the anterior and posterior chambers of the aqueous humour, the iris and the ciliary muscles.

The anterior chamber (d, Plate XI., fig. 3) is bounded, in

* From Todd & Bowman.

FIG. 14.

FIG. 15.

FIG. 16.

front, by the posterior surface of the cornea, and a small portion of the sclerotic; behind, by the front of the iris; e being the pupil of the eye through which the two chambers. communicate. The posterior chamber is formed by the posterior surface of the iris, by the front of the ciliary processes, and lens, and a small part of the hyaloid membrane. These chambers contain a fluid nearly as thin as water; and therefore called aqueous humour (aqua, water).

The iris (Plate XI., fig. 2, b, 3, f, 8, 9, b,) is a circular diaphragm, with a central aperture called the pupil (fig. 3, e, 8, 9, a); it is attached, at its circumference, by means of the ciliary ligament (which is seen with the ciliary muscle at c, c, Plate XI., fig. 2) to the sclerotic. The iris consists of muscular fibre, of bloodvessels, and of colouring matter. The direction of the muscular fibres is mostly radiating; but at the inner margin of the iris are a few fibres which have a circular arrangement around the pupil. The vessels are numerous, but slender and delicate. The colouring matter is situated in irregularly-shaped cells, which very much obscure the microscopic structure of the iris. When we speak of a blue eye or a hazel eye, we always refer to the colour of the iris ; which is so named from its varying colours in different individuals (iris, a rainbow). The pupil is so named from pupilla, a young girl or child, as in the situation of the pupil, a diminutive image of the person looking at it is formed by reflection from the crystalline lens: amongst the Greeks, in the same way, the pupil was called kore, the daughter of the eve.

The ciliary muscle (Plate XI., fig. 2, c, 3, h) is a circular band, attached on one side to the sclerotic, and by the other to the front of the ciliary processes. Its fibres radiate from the attachment to the sclerotic, some to the posterior part of the ciliary processes, some to the more prominent parts, where the processes approach the lens.

We will now consider the functions of the different parts of the eye during the passage of the light rays. We see an object, because rays of light are reflected from

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the surface of the object, and meet the eye, the impression subsequently passing to the brain. These reflected rays diverge from each other, more or less, according as the object seen is at a less or greater distance from the eye. At a distance of eighteen

b

FIG. 17.

feet, however, the rays of light are already so little divergent, that they are accounted parallel; whilst at a less distance than eighteen feet the rays are called divergent. The course of parallel rays is as follows (fig. 17): reflected from a, the rays pass to and through the cornea, where they are bent or reflected towards the axis of the eye (b, c); they then pass through the aqueous humour to the crystalline lens, where they are still more refracted, so as to cross the axis of the eye, and, traversing the vitreous body, terminate on the retina, where an inverted image of the object is formed.

If any divergent rays from a near object were to pass through the eye, whilst the eye was in the same state as that in which the parallel or distant rays come to a focus, the effect would be that the picture of the object would form behind the retina, and thus perfect and well-defined sight would be impossible for near objects. To meet this there is a little mechanical arrangement in the interior of the eye. The ciliary muscle (Plate XI., fig. 3, h), attached in front to the sclerotic, and behind to the ciliary processes, when in action (that is, when the eye is adjusted for near objects), pulls forward the ciliary processes, and therefore the plaitings of the hyaloid membrane; and by means of the attachment of the hyaloid to the circumference of the lens, this body is also brought forward; the canal of Petit prevents the muscle acting on the vitreous body, by leaving a space between. By this means a greater distance is produced between the lens and the retina, and thus the image of the object is brought to a focus directly upon the retina.

There is nothing that man can construct to equal, in beauty of mechanism, the iris. It is a curtain, circular in shape, and with an aperture in the middle, which can either contract or dilate without showing the least puckering or plaiting of the surface of the iris. The contraction of the pupil takes place in a strong light, and is intended to exclude an excess that would act injuriously on the retina; the dilatation takes place in dull light or darkness, for the purpose of allowing every possible ray to pass.

During adjustment of the eye for a near object, it is turned inwards, and the same nerve by which this movement is effected causes contraction of the pupil. This contraction probably merely excludes excess of light, and does not aid adjustment of the eye; as adjustment has been observed to be perfect even when the iris was entirely absent.*

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* For much interesting and valuable information respecting the eye, see the second volume of Todd & Bowman's "Physiological Anatomy; Griffiths & Henfrey's "Micrographical Dictionary" (Van Voorst), article "Eye"), and the books of reference there named.

EXPLANATION OF PLATE XI. AND WOODCUTS.

Fig. 1. The eye of the ox, seen laterally, the muscles having been removed. a, the pupil of the eye; b to b, the cornea; c, c, and d, ends of muscles; e, blood-vessels entering the eye: c, d, and e, are on the sclerotic; f, the optic nerve.

a, the

Fig. 2. The same; the cornea and sclerotic have been removed. crystalline lens, seen through the pupil; the band, c to c, is the ciliary ligament and muscle; b, the iris; e, the choroid, showing its vessels; ƒ, the optic nerve.

Fig. 3. A diagrammatic section of the eye, enlarged. a, the cornea, consisting of anterior elastic lamina, cornea proper, and posterior elastic lamina; b to b, by a, shows extent of cornea; c, c, c, the sclerotic; d, the anterior chamber of the eye; e, the pupil; f,f, the iris; g, g, the ciliary processes; h, h, ciliary ligament and muscle; i, the crystalline lens, surrounded by its capsule, j; k, k, the canal of Petit; shows the termination of the retina (the scarlet line); the black line between the retina and the sclerotic is the choroid; the light line beyond the retina is the hyaloid membrane, which passes on to the lens; m, the vitreous body; n, the optic nerve; o, the central artery of the retina.

Fig. 4. Fibres of the sclerotica.

Fig. 5. Epithelium of the choroid. a, from the dark part in front; b, over

the tapetum.

Fig. 6. a, detached vesicles; b, detached granules, from the retina.

Fig. 7. Fibrous layer of retina; a, a blood-vessel.

Fig. 8. The iris, seen in front; a, the pupil; b, the iris.

Fig. 9. The iris and ciliary processes, seen from behind; a, the pupil; b, the iris, partly hidden by c, the ciliary processes.

Fig. 10. Transverse section of the cornea proper.

Fig. 11. Section of anterior elastic lamina. a, cornea proper; b, epithelium; c, elastic lamina.

Fig. 12. Jacob's membrane. b, when seen from above; a, appearance of detached particles.

Fig. 13. Section of retina. a, Jacob's membrane; b, granular; c, vesicular; d, fibrous layer: along the latter is seen passing a blood-vessel. Fig. 14. The crystalline lens, seen in front.

Fig. 15. The lens, separating into three divisions, and showing its laminated

structure.

Fig. 16. Fibre of the crystalline lens (Todd and Bowman).

Fig. 17. Diagram to show the inverted image produced on the retina; b, c, the optic axis.

Figs. 4, 5, 6, 7, 10, 11, 12, and 13 are reduced from the microscopic appearances at 250 diameters, drawn by the camera lucida.

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