The cornea proper is closely and intimately connected to the sclerotic at its circumference. There does not appear to be any mechanical adaptation resembling the fitting of a watch-glass into the bezel, as stated in books ; but a ming ling of texture, as in many other instances in the body. The two structures cannot be separated without anatomical artifice and much vio lence. If the eye be macerated in water for a month, and then plunged into boiling water, the cornea may be torn from the sclerotic ; but these destructive processes prove little with re gard to animal organization. The conjunctival covering of the cornea is, as has been already stated,con tinuous with the rest of the conju nctiva, and the elastic cornea is continued for a short distance beneath the sclerotic, as if slipped in between it and the ciliary ligament.
The cornea, thus composed of three different structures, varies in appearance at different periods of life. In the fcetus at birth it is slightly cloudy, and even of a pinkish tint, as if it contained some red particles in its blood ; this is, however, more apparent on examination after death than during life ; it is also thicker in its centre. In old age it is harder, tougher, and less transparent than in youth, and fre quently becomes completely opaque at its cir cumference, presenting the appearance denomi nated in the books areas maths. How far the alteration iu the power of adaptation to distance, which occurs in advanced life, is to be attri buted to change in curvature of the cornea, is not settled.
If the foregoing account be correct, the ap parently simple transparent body which fills the aperture in the anterior part of the sclerotic, is composed of three distinct varieties of organic structure, liable to changes from disease equally distinct and varied. When the aqueous hu mour becomes the subject of description, I will endeavour to shew that there is good rea son for believing that a fourth may be added to these three, the membrane which lines the chamber in which this fluid is lodged, and by which it is secreted. Let it not be supposed that this division of an apparently simple piece of organization into so many distinct parts, is merely an exhibition of minute anatomical re finement. The distinction is essentially neces sary to enable the surgeon to account for the appearances produced by disease in this part, and to guide him in the diagnosis and treat ment.
Of the choroid cont.—This membrane has been so called from its supposed resem blance to the chorion of the gravid uterus; it has also sometimes been called urea from its resemblance to a grape, a term, however, which is now more frequently applied to the iris. It has already been stated that the
spherical external case of the eye called the sclerotic embraces another spherically disposed membrane, called the choroid coat, accurately fitted and adhering to it throughout. This spherically disposed membrane has also its cir cular aperture anteriorly, into which is fitted the screen or diaphragm called the iris. This choroid membrane cannot be considered essen tial to the perfection of the organ considered merely as a piece of optical mechanism, as a spherical camera °Noma, but is obviously an important part of its anatomical organization, and an essential provision for the perfection of its vital functions. It appears to be destined to secure the requisite mechanical connexion be tween the coarser and more rigid sclerotic case and the parts within, as well as to secure these delicate parts in their situation, and preserve their form, at the same time affording a me dium for the distribution and support of the vessels and nerves.
This membrane is of a deep brown or black colour, being stained with the colouring matter called the black pigment; but when this is removed, it exhibits a high degree of arterial and venous vascularity. Its external surface is comparatively rough, coarse, and flocculent, and obscured by the cellular membrane which connects it to the sclerotic. The inner surface, which is in contact with the retina, presents a very different appearance. It is soft and smooth, and when minutely injected, resembles the more delicate mucous membranes, and exhibits a remarkable degree of minute villous vascu larity. The external surface being composed of the larger branches of arteries, veins, and nerves, may be torn away from the soft, smooth, and more closely interwoven inner layer, or the inner layer may he partially dissected up from it, with some care, especially in the eyes of the larger quadrupeds. This manoeuvre• having been executed by Ruysch, and prepara tions so formed displayed by him, the inner layer has been denominated the tunica Ruys chiana. But this is a mere anatomical artifice. There is no natural division into two layers, the soft, smooth, and highly vascular inner surface being formed by the ultimate subdivi sion and distribution of the larger branches of vessels, which exhibit themselves separately on the outside. It is a condition somewhat analo gous to that of the skin, where the soft, smooth, vinous external surface presents so remarkable a contrast to the rough internal surface with its layer of cellular membrane uniting it to the subjacent parts.