Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/946

Rh 886 ANATOMY [ORGANS OP SENSE this coat possesses anteriorly numerous folds, the ciliary processes, which are continuous with the iris, a structure which lies behind the cornea. The third or internal or nervous coat is named the retina, and in it the optic nerve FltJ. 77. Diagrammatic section through t ha eyeball. &amp;lt;7, conjunctiva; co, cornea; Sc, sclerotic; ch, choroid; pc, ciliary processes; me, ciliary muscle; O, optic nerve; B, retina; I, iris; ag, &quot;anterior chamber of aqueous humour; L,lens; V, vitreous body ; Z, zonule of Zinn, the ciliary process being removed to show it; p, canal of Petit; m, yellow spot. The dotted line behind the cornea re presents its posterior epithelium. terminates. The enclosed refracting media occupy the axis of the globe, and are named from before backwards the aqueous humour, crystalline lens, and vitreous body. The Sclerotic coat, called from its white appearance the white of the eye, is a firm, unyielding fibrous membrane, which forms the posterior fths of the outer coat of the eyeball. It is thicker behind than in front, and where pierced by the optic nerve it has a cribriform structure, as the bundles of nerve fibres do not pass through one large, but several small openings. The sclerotic consists of the white fibrous form of connective tissue, intermingled with a small proportion of elastic fibres. The bundles of white fibres lie in two directions; some pass in the meridian of the globe from the optic nerve towards the cornea, others lie parallel to its equator. The sclerotic is joined by accessory fibres behind, derived from the perineurium of the optic nerve, where the nerve pierces it ; and in front from the tendons of the recti and obliqui muscles, which arc inserted into it. In the cetacea the sclerotic possesses extra ordinary thickness. In fish and amphibia it consists largely of cartilage, and in birds a ring of bone is developed around its anterior margin. It is the protecting coat of the eyeball. The Cornea forms the translucent anterior ^th of the outer coat of the eyeball. It is almost circular in form, and is blended at its circumference with the anterior border of the sclerotic. Its anterior surface is convex, and covered by the conjunctival epithelium. The forward projection of the cornea is always greater in young than in aged persons. Its posterior surface is concave, and bounds the chamber in which the aqueous humour is contained : if the chamber be punctured, and the humour evacuated, the cornea loses its translucency, its tension, and its forward con vexity, and becomes flaccid and opaque. It has considerable thickness, and can be readily split up into laminae. When antero-posterior sections are made through it and the epithelium on its anterior and posterior surfaces, four distinct series of structures may be seen, viz., the anterior epithelium, the proper tissue of the cornea, the posterior clastic lamina, and the posterior epi-(cndo)-thelium. The anterior epithelium of the cornea, often called the con junctival epithelium, is stratified. The deepest layer, which lies next the cornea, is formed of elongated cells, which lio vertically to the plane of the surface of the cornea. Tho more superficial layers are squamous cells, often with fluted surfaces and serrated or spinous edges. The intermediate layers are irregular in shape, and often possess, as Cleland pointed out, long digitate processes, which interlock with those of the adjacent cells. The proper tissue of the cornea is a modified form of connective tissue. When examined fresh it appears as if perfectly homogeneous, but after a time, and more espe cially if hardened in alcohol, chloride of gold, and other reagents, it is seen to consist of cells and an intercellular matrix. The cells consist of two kinds, those which belong to the cornea, and those which have migrated into it. The proper cornea cells or cornea corpuscles were first seen by Toynbee, and have been carefully studied by numerous subsequent observers. They are large stel late, flattened cells, and lie with their surfaces parallel to the surfaces of the cornea ; they possess many branching processes, and the processes of adjacent cells anastomose to form, a cell network. They consist of nucleated masses of protoplasm, which Kiihne showed to be contractile, and are apparently destitute of a cell ,wall. In vertical sections through the cornea the corpuscles seem as if shaped like elongated spindles. The migrating cells of the cornea were first seen by Von Recklinghausen. They resemble white blood corpuscles, and possess active amoeboid movements, so that they can wander through the corneal tissue. In a healthy cornea they have migrated out of the marginal blood-vessels; but in an inflamed cornea, where their num ber is greatly increased, they are in part white corpuscles derived from the blood, and in part produced by prolifera tion of the proper cornea corpuscles. The intercellular matrix of the cornea consists of a laminated substance, the lamellae being arranged parallel to the surfaces of the cornea. The lamellae consist of fasciculi of extremely delicate fila ments ; immediately under the anterior epithelium the fasciculi decussate with each other, and at the circumference of the cornea the fasciculi run into the connective tissue of the sclerotic. Bowman described a translucent structureless layer or anterior elastic lamina between the conjunctival epithelium and the cornea proper, but it is doubtful if this layer exists as a constant arrangement. Bowman and other observers have injected tubular spaces in the cornea which are apparently situated between the lamellae. The exact nature of these spaces is somewhat doubtful, but Thin believes them to be lymph-vessels traversing its sub stance, for he has seen an endothelial lining similar to the endothelial cells of the lymphatics. It is probable that these spaces serve as the channels for the migrating cor puscles to wander through. Thin also describes the proper cornea corpuscles as lying in lacxinos, which communicate with each other and with the lymph-vessels. The posterior elastic lamina forms a distinct translucent, structureless layer adherent to the back of the proper tissue of the cornea, from which it may be stripped off without much difficulty. When torn across, the edges curl inwards towards the corneal tissue. It is from ^-jV^r to -5-^-5 th inch thick, and resists the action of various reagents. This lamina thins off at its circumference and splits into fibres, which become continuous with the pectinate ligament of the iris. The posterior epithelium of the cornea, also called the endothelium of the aqueous humour, forms a single layer of polygonal cells on the back of the posterior elastic lamina. It is continuous with the endothelial covering of the pec tinate ligament and of the anterior surface of the iris. The cornea is not in the adult traversed by blood-vessels, though in the foetus a layer of capillaries lies near its anterior surface. In the adult, however, the margin of