THE EYE OF THE OX.

BY E. BECKIT TRUMAN.

HOMER, the sublimest, because the most natural, of the classic poets, expresses his idea of the majesty and beauty of the countenance of Juno, by the epithet boopis, ox-eyed. The simile may, perhaps, appear a very humble one; but, by a little study of the formation of the bullock's eye, we may see that not only is the object worthy of the Homeric application, but that it contains numberless beauties of which the ancient Grecian never dreamed, disclosed to us by the aid of the dissecting-knife and the microscope.

The delicacy of the internal organization of the eye requires it to have an abode where it will be protected from external violence. Such a dwelling-place we find in the orbit or socket of the eye which is formed of bone on all sides, and lined by a cushion of fat, in which the eyeball is embedded; so that in the event of a blow being received on the only attainable aspect, the front of the eye, its force is transmitted to, and broken by, this soft layer of fat, and thereby prevented from rebounding injuriously on the eye from the unyielding wall surrounding it.

Having removed the eyeball from its socket, and divested it of the fat and the muscles, let us examine the contour of the eye (Plate XI., fig. 1). We see it to consist of two portions, of differing curvatures, the part in front (a) having the greater convexity; the remaining and greater portion having the form of a somewhat-flattened globe. The projecting body (f) is the optic nerve, the nerve that conveys the visual impressions to the brain; at c and d are the attachments of muscles cut short; and at e some blood-vessels are seen.

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We will now examine, one by one, the different structures of which the eyeball is composed; and which consist (1) of tunics or protective coverings, (2) of a series of refracting bodies to bring the rays of light to a focus internally, and (3) a nerve to receive the impression of light (with certain auxiliaries).

The outermost covering of the eye consists of two very dissimilar parts; namely, the cornea and the sclerotic. The cornea,

FIG. 10.

so named from its similarity to horn (corneus, horny), is compared in shape to a watch-glass; it is perfectly transparent (in Plate XI., figs. 1 and 3, extending from b to b). Although perfectly transparent, even as glass (that is, to the naked eye), it has a fibrous structure (fig. 10), the fibres being very even and regular, and running parallel to each other; between the fibres are a number of tubes, pursuing also a regular, parallel course. In front of the cornea is a thin elastic layer, covered by a transparent cuticle; and behind the cornea is another elastic layer, which passes on and terminates in the iris. These elastic membranes preserve the proper convexity of the cornea (in fig. 3 the situation of these laminæ may be seen; in fig. 11, their structure is shown).

b

FIG. 11.

The sclerotic (sklēros, hard) is an exceedingly strong structure directly continuous with the cornea, and investing the eyeball completely, except where the cornea comes in (extending from b round by c, c, c, to b again, in Plate XI., fig. 3). It is about 1 of an inch in thickness, and its fibres (Plate XI., fig. 4) are not, as those of the cornea, parallel, but run in contrary directions, mostly crossing each other at right angles; hence the dense and strong nature of this covering. What is popularly known as the white of the eye, is a part of the sclerotic, covered by a thin mucous membrane. To the sclerotic are attached the various muscles that move the eyeball, and behind, it is perforated for the passage of the optic nerve (Plate XI., fig. 3, n).

Looking at Plate XI., fig. 3, it will be observed that at h is a part coloured yellow this consists of two structures, one behind the other; the first is called the ciliary ligament, the second, the ciliary muscle. By means of the ciliary ligament (a ligament being something that binds and connects parts together: ligo, to tie or bind), the iris (Plate XI., fig. 3, ƒ, ƒ) is attached to, and suspended from, the sclerotic. On removing the cornea and sclerotic, during which we must separate the latter from the ciliary ligament, we obtain a body similar to Plate XI., fig. 2. We notice that the eye has lost its higharched form: this is due to the fact that the semi-fluid material

forming the bulk of the eye has now but very weak structures surrounding it, and therefore tends to flatten downwards.

The choroid coat of the eye (like the chorion, which is a vascular membrane of peculiar formation, chorion, eidos), is that next beneath the sclerotic; it extends from the entrance

of the optic nerve, which perforates it, to the ciliary ligament (Plate XI., fig. 2, c, e, c), where it forms the iris and ciliary processes. The choroid consists almost entirely of bloodvessels, and colouring matter, or pigment, contained in irregularly-shaped cells in the interval between the blood-vessels. By washing gently with water, the pigment on the surface may be removed, and we then see the veins running in great numbers, and in a beautifully-arranged manner, from behind forwards. The irregularly parallel lines seen in Plate XI., fig. 2, are the veins. The innermost layer of the choroid consists of a series of little bodies, called epithelial cells, arranged in the manner of a pavement; in the eye of man, and in the front of the eye of the ox, these are full of dark colouring matter (Plate XI., fig. 5, a). The back of the eye of the ox, however, instead of presenting a dull black appearance (as in man), is of a mingled blue and green colour, and very brilliant, having quite a metallic lustre. This is due to the presence of certain fibrous elements in the choroid, so arranged as to give the play of colours named: for there is, on examination, no colouring matter to be seen; the epithelial cells of this part having the microscopical appearance of b, Plate XI., fig. 5. This bright and coloured surface is of considerable extent, and is called the tapetum (tapēte, a carpet).

At c, Plate XI., fig. 2, is the ciliary ligament and muscle. At this point the choroid bends inwards to form the ciliary processes and iris. The ciliary processes (cilium, an eyelid or eyelash), (Plate XI., fig. 3, g, g, and the outer circle in Plate XI., fig. 9, where they are seen from behind), consist microscopically of the same structures as the choroid; they are formed by the choroid bending inwards and backwards from the ciliary ligament towards the lens (Plate XI., fig. 3, i); together they constitute a circular band, with a large central aperture at its circumference continuous with the choroid, and also attached to the iris, but free at its margin and front surface. These processes consist of folds or plaits which are arranged around the lens, "like the many petals of a flower;" the folds are both long and short: the long ones, about sixty in number, running concentrically along the entire breadth of the processes; the short ones are placed alternately with the long ones, commencing at the circumference, but terminating about halfway from the margin of the large central aperture.

The consideration of the iris and ciliary muscle we will reserve for the present, and will examine the layer immediately beneath the choroid, namely, the retina (rete, a net) (fig. 3, the scarlet line running around the globe). This is the nervous structure which receives the impression of light,