The posterior surface of the iris is co vered by pigmentum nigrum, and is called the uvea.
Under the choroid coat is found a third membrane of the eye-ball, called the reti na, which is formed by the expansion of the medullary substance of th e optic nerve, and forms the immediate organ of vision. It is of a yellowish grey colour, and so ex tremely soft as almost to be lacerated by the slightest touch. Its outer surface is entirely unconnected with the choroid coat; and the inner surface is expanded on the vitreous humour; but not connected to it. It terminates in front by a distinctly defined edge, where the ciliary processes begin to adhere to the vitreous humour. On the inside of the retina are seen the branches of an artery and vein, which en ter through the centre of the optic nerve, (arteria et vena centralis oculi). The part at which it enters the eye is termed the porus opticus, and is of course insensible ; and hence physiologists have explained the reason why the optic nerve is inserted out of the axis of the eye ; as otherwise the axis of vision would have fallen on an insensible part of the retina.
On the outer, or temporal side of the retina, there is a fold of the membrane of a bright yellow colour, in the recent state, and there is also said to be an aperture. These circumstances were first pointed out by Soemmerring, and have been named after him.
The vitreous humouroccupies the great est share of the globe of the eye. It con sists of a clear water contained in a cellular substance, which is so perfectly transpa rent DA to resemble pure glass, whence its name is derived. The cellular substance is condensed on the surface into a smooth membrane, called the membrana hyaloi. ties. This is marked in front by a circular series of black radiated lines, caused by the adhesion of the ciliary processes. which, like other parts of the chOroid, are covered with pigmentum nigrum. Under these a circular canal runs, named the ca nal of Petit.
The crystalline humour or lens is im• bedded in the front of the vitreous hu mour. Its size is about that of a pea, hut
it is much more flattened in form. It is of a waxy consistence, softer externally, and growing gradually firmer towards the cell tre. The lens is contained in its proper capsule, the posterior surface of wluch aclheres firmly to the tunica liyaloidea ; but its separation can sometimes be effect ed without rupturing it. It has no appa rent connection to this capsule. It is an opaque state of this body that constitutes the disease called cataract.
The aqueous humour is a small quantity of transparent water, placed immediately behind the cornea, and occupying the space between that membrane and the crystalline lens : it is easily reproduced when let out.
In the midst of the space occupied by this humour the iris is found, and it di vides the space into two portions, called the anterior and posterior chambers of the eye ; which communicate by. means of the pupil. The anterior is much the largest of these.
The choroid coat, ciliary processes, and iris, are very vascular, and derive their supply- from the ciliary branches of the ophthalmic artery.
The iris is very largely supplied with nerves from a small ganglion, named len ticular, formed on a branch of the ner vus motor, or nerve of the third pair. These are called the ciliary nerves.
Of the eye-lids and lacrymal apparatus.— The ey-e-ball is covered by- two inoveable curtains, formed by a folding of the com mon integument, and called the eye-lids. In order to keep these uniformly expa,nd ed, and to prevent them from forming wrinkles, each of them contains a thin por tion of cartilage, adapted in figure to the convexity- of the globe, and calledthe tar sus. In order to provide still further for the greatest possible facility of motion, the eye-lids are lined by a smooth and polished membrane, and the globe of the eye is covered by the same membrane, on its an terior part : this is called conjunctiva, as it serves to connect the front of the eye ball to the eye-lids.