SCLEROTICA-CORNEA-CHOROID.

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portion it is less transparent, and contains a few straggling blood-vessels, which are seen as red streaks on the white of the

eye.

'The sclerotic, one of the most complete of the tunics of the eye, and that on which the maintenance of the form of the organ chiefly depends, is a strong, opaque, unyielding, fibrous structure, composed of bundles of strong white fibres, which interlace with one another in all directions. The membrane covers about four-fifths of the eyeball, leaving a large opening in front, which is occupied by the transparent cornea, and a smaller aperture behind for the entrance of the optic nerve. The sclerotic is thickest at the back part of the eye, and thinnest in front.'

The cornea is a transparent structure, occupying the aperture left in the fore part of the sclerotic, and forming about one-fifth of the surface of the globe of the eye.' The two together complete the encasement of the eye, and no other portion is employed for the mere purpose of maintaining the form and rigidity of the ball.

Spread over the inner surface of the sclerotic lie two other membranous expansions, likewise termed coats or tunics, but of totally different nature and properties. Next the sclerotic is the choroid coat, which is a membrane of a black or deepbrown colour, lining the whole of the chamber up to the union of the sclerotic with the cornea, and then extending inwards as a ring stretching across the eye. It also is pierced behind by the optic nerve.

The choroid coat is an extremely vascular structure-that is to say, it is composed of a dense mass of blood vessels, which lie in two layers, the outermost of the two being the veins, and the other the arteries. Inside of these two vascular expansions is the layer containing the black pigment which gives to the coat its colour, and which it is the object of the numerous blood vessels to keep supplied. The pigment is enclosed in the cells of a membrane, and these cells are packed very closely together, and are about the thousandth part of an inch in diameter. Each cell has a transparent point in its centre, surrounded by a dark margin.

Within the choroid, and lining its surface, is the retina, which is a nervous expansion branching out from the optic nerve, and covering the interior chamber of the eye with a fine transparent network as far as the angle where the choroid bends inward to form the circular ring above-mentioned. The retina is a very delicate membrane, of almost a pulpy nature, and but loosely attached, although lying close, to the blackened surface of the choroid. In the middle of the back of the eye, and in a line with the axis of the eyeball, is a round yellow spot, about a line or a line and a half in diameter (or of an inch), and in the centre of this spot is what appears like a minute hole, called the foramen of Sommering. About a fifth of an inch from the inner or nasal side of the yellow spot is a flattened circular papilla corresponding with the place where the optic nerve pierces the choroid coat.

The retina is a compound membrane, consisting of three distinct layers, only one of three, the middle layer, being made up of purely nervous matter. The outer layer, in contact with the choroid, termed the membrane of Jacob, is made up of small columns or rods standing perpendicular to the surface, and each sharpened to a point at their outer extremities. The columns are of two different kinds, the one being smaller and more numerous than the other. The small rods are solid, six-sided bodies, grouped round the larger, these last being also the shorter of the two, and cleft at the point where they touch the choroid. Each one of the pigment cells of the choroid (of which there are about a million to the inch) receives as many as six or eight of the larger cleft twin cones, with the smaller single rods grouped about them; consequently the diameter of the smaller bodies must be only a fraction of the thousandth part of an inch.

The middle layer of the retina is the proper nervous portion. This is made up of radiating nervous fibres proceeding from the optic nerve, and spreading over the inner chamber of the eye. The fibres become more slender and spread more apart as they approach towards their termination in front. On both sides of the nervous expansion there are layers of nerve cells or vesicles, being the kind of substance com

posing the grey matter of the brain, and occurring also at the terminations of the nerves of special sense. The inner ends of the rods that make up the membrane of Jacob are therefore in contact, not with nervous fibres, but with nerve cells.

The innermost of the three fibres of the retina is the vascular layer, or the network of arteries and veins for supplying blood to the nervous portion.*

Before pointing out the different bodies that make up the bulk of the eye, and enable it to act as an optic lens, I must call attention to several other substances of a membranous or fibrous character lying under the cornea and near the junction with the sclerotic coat. The first of these is a narrow circular band, of a greyish-white colour, close behind the junction above-named. The foremost margin, the thicker of the two, gives attachment to the circular curtain called the iris. The thinner and posterior margin is blended with the choroid coat, which here prolongs itself inwards in a series of radiated folds called the ciliary processes. The band, or ligament, thus giving the two-fold attachment to the iris and the choroid is called the ciliary ligament. The ciliary processes lie behind the iris, and make a black, wrinkled, narrow rim, concealed from external view.

The iris may rightly be regarded as a process of the choroid; it is continuous with it, although of a modified structure. It forms a vertical curtain, stretched in the aqueous humour before the lens, and perforated for the transmission of light. It is attached all round at the junction of the sclerotic and the cornea, so near indeed to the latter that its anterior surface becomes continuous with the posterior elastic lamina.' 'The anterior surface of the iris has a brilliant lustre, and is marked by lines accurately described by Dr. Jacob, taking a more or less direct course towards the pupil. These lines are important as being indicative of a fibrous structure.' When the pupil is contracted, these converging fibres are stretched;

*For a minute description of the retina, see the 'Eye,' in TODD and BOWMAN'S Physiology.

when it is dilated, they are thrown more or less into zigzags. The pupil is nearly circular, and is situated rather to the inner side of the centre of the iris. By the movements of the iris it is dilated or contracted, so as to admit more or less light to the interior; and its diameter under these circumstances may vary from about to of an inch.'-TODD and BOWMAN, Vol. II. p. 25.

The iris is thus to be considered as a muscular structure, its fibres being of the unstriped variety, or of the kind that prevail among the involuntary muscles, as the muscular fibres of the intestine. It is abundantly supplied with nerves. While the radiating fibres above described serve to dilate the pupil, a second class of fibres, arranged in circles round the opening and best seen at the inner margin and behind, operate in contracting it. The action is purely reflex, and is regulated by the intensity of the light. In the dark, or in a very faint light, the dilating fibres are tense and contracted to the full, making the pupil very wide. The stimulus of light brings the circular or contracting fibres into play by a reflex or unconscious action, and contracts the opening. The changes thus effected are useful in adapting the eye to different lights, admitting a larger quantity with a feeble light, and a smaller quantity with one that is too strong. When this reflex power of adaptation reaches its limit, and the brilliancy is still too great, we then put forth the voluntary efforts of closing the eye, or turning the head away from the object.

Behind the ciliary ligament, and covering the outside of the ciliary processes is a greyish, semi-transparent structure, known as the ciliary muscle. It belongs to the unstriped variety of muscle, and its fibres appear to radiate backwards from the junction of the sclerotic and cornea, and to lose themselves on the outer surface of the ciliary body. The ciliary muscle must have the effect of advancing the ciliary processes, and with them the lens, towards the cornea. The muscular nature of this structure is confirmed by its anatomy in birds, where it is largely developed, as noticed by Sir P. Crampton.'-TODD and BOWMAN, II. 27.

A peculiar interest has come to attach to this muscle from

HUMOURS OF THE EYE.

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its supposed action in the ill-understood operation of adapting the eye to objects at different distances.

Passing now from the coats of the eye to the substance, we find three humours, or transparent masses occupying it in the following order: in the front is the aqueous humour; next, the crystalline lens; and backmost the vitreous humour.

The aqueous or watery humour is a clear, watery liquid lying under the cornea in front, and bounded behind by the crystalline lens and the folds of the ciliary processes. This humour is very nearly pure water, containing in solution a small quantity of common salt and albumen; and is enclosed in a membrane which is in contact with the inner surface of the cornea in front and the ciliary processes and lens behind. The liquid is partly before and partly behind the iris.

The vitreous or glassy humour lies behind the crystalline lens, and occupies the entire posterior chamber of the eye, being about two-thirds of the whole. It consists of a clear, thin fluid enclosed in a membrane, which membrane not merely surrounds it, but radiates inwards into its substance like the partitions of an orange, so as to make up a half-solid gelatinous body-the vitreous body, or posterior lens of the eye. These partitions are very numerous, and point to the axis of the eye, but do not reach to it; and consequently there is a central cylinder passing from front to back, composed only of the fluid of the body. The form of the vitreous body is convex behind, while before there is a deep cup-shaped depression for receiving the crystalline lens. The membrane that surrounds it on all sides, as well as entering into the interior, has a twofold connexion in front; it doubles so as to receive the crystalline lens between its folds, and it unites with the ciliary processes, which surround the lens without reaching its border. Thus the partition between the aqueous humour in front and the vitreous humour behind is made up of three successive portions enclosing one another; the wrinkled black ring of the ciliary processes outermost; within this a ring of the doubled membrane of the vitreous humour; and inmost of all the crystalline lens, enclosed between the two folds of the membrane,

The crystalline lens is a transparent solid lens, double