The Eyeball is a globular chamber. Its walls consist of several layers. The outermost layer is called the sclerotic, is a tough fibrous coat formed for protection and maintaining the shape of the ball, and is thicker behind than in front. This coat is white in appearance, and is the part easily visible to which the phrase "white of the eye" is applied. In the very front of the globe the sclerotic is abruptly trans formed into a transparent portion which is cir cular and which forms a window through which one can see into the interior. This is the cornea. The sclerotic is supplied with vessels and nerves, but the cornea, though containing nerves, has no blood-vessels. It is composed of layers of fibres with numerous minute spaces between them, in which little masses of proto plasm lie. The masses send off numerous pro cesses which communicate with one another, so that the substance of the cornea is traversed by fine threads of protoplasm connected with masses. No doubt by this living material, in lieu of vessels, the nourishment of the cornea is maintained without its transparency being interfered with. The visible part of the white of the eye is covered, as already noted, by the delicate membrane, the conjunctiva, reflected from the inner surface of the lids. This mem brane has the structure of mucous membrane, fibrous tissue covered by layers of epithelial cells. :But when the conjunctiva reaches the cornea, only its epithelial layers are continued over the cornea. In inflammation of the cornea blood-vessels rapidly shoot into its substance from the conjunctiva around.
- Lining the inner surface of the sclerotic is the second coat of the eyeball—the choroid. This is essentially the blood-vessel coat of the eyeball.
It contains a multitude of small arteries and veins whose connecting capillaries form a very close net-work. Connective tissue surrounds the vessels, and in the tissue are branched cells so loaded with colouring matter as to be quite black. Their appearance is shown in Fig. 173. The front part of the choroid terminates about the place where the sclerotic passes into the cornea in a series of ridges, the ciliary processes. The circular space thus left in front by the termina tion of the choroid is occupied by the iris, a round curtain, the structure seen through the cor nea, differently coloured in dif ferent individuals. In its centre is a round hole, the pupil, which appears as if it were a black spot. The margin of the iris is con nected firmly with the eyeball all round, at the line of junction of the sclerotic and cornea.
The iris forms a sort of trans verse partition dividing the ca vity of the eyeball into two cham bers, a small anterior chamber, extending from the front part of the iris to the back part of the cornea, filled with the aqueous humour, a fluid consisting al most entirely of water with a very small quantity of saline material in solu tion, and a large posterior chamber, filled with vitreous humour, a kind of fine transparent, colourless jelly. The iris consists of a frame
work of connective tissue, and its posterior sur face is lined by cells containing pigment which gives the colour to the eye. In its snbstance are bundles of involuntary muscular fibres, one set being arranged in a ring round the margin of the pupil, the other set radiating from the pupil like the spokes of a wheel. When the cir cular fibres contract the pupil is made smaller, but if these fibres relax the radiating fibres cause the pupil to dilate more or less widely. The object of this will be seen hereafter.
Just behind the pupil is the crystalline lens, resembling a small, very strongly magnifying glass, convex on each side, though more so be hind. It is perfectly transparent in the healthy state. The front face of the lens is quite close up to the curtain of the eye, and the vitreous humour, occupying the posterior chamber, is closely in contact with its back face. But the lens is not loosely placed in the eyeball; it is inclosed in a capsule, the suspensory liga ment, which not only retains it in position, but is capable of altering its shape. For the lens is elastic, its capsule is connected with the ciliary processes, and is kept usually tense, so that the lens is flattened somewhat by the pres sure exerted on it. But all the edge where the cornea, sclerotic, and choroid meet is a ring of involuntary muscular fibres, forming the ciliary muscle. When this muscle con tracts it pulls forwards the attachment of the suspensory ligament of the lens, whose pressure on the lens is consequently diminished, and the elasticity of the lens causes it at once to bulge wards, and become thereby more convex. The Value of this movement will be understood immediately.
Reference to Fig. 174 will rein kr the position of the parts already described more intelligible. It represents the eye lying in its socket, partly covered by the eyelids, and completely opened up by a cut from front to back. In the figure muscles (r, N) of the eyeball are shown, and a meibomian gland opened up (Q, a) is repre sented in each lid. A is the cornea which, at the place across which the lines from F and o pass, joins the white sclerotic (n). The cornea closes the front of the anterior chamber (n), which is filled with aqueous humour, and the back wall of which is formed by the curtain of the iris (n). In the middle of the back wall is the opening of the pupil (c), through which is seen the lens (E). F and o point to the re gion of the ciliary muscle and ciliary processes, the forward termination of the choroid coat (r). Behind the lens is the posterior chamber (L), filled with vitreous humour.