Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/947

Rh EYE.] the cornea is penetrated by a zone of capillary loops derived from the arteries of the conjunctiva ; these loops, according to Lightbody, are invested by perivascular lymph spaces. The venous canal of Schlemm runs round the circumference of the cornea, at the junction of its deeper layers with the sclerotic. Leber states that it is not a simple canal, but a plexiform arrangement of veins. The nerves of the cornea first seen by Schlemm have been carefully examined by recent observers. They arise from the ciliary nerves, and enter the margin of the cornea in from twenty to forty fasciculi, which run from the circumference to the centre and to the anterior surface of the cornea, and give off numerous branches. The nerve fibres soon lose their medullary sheath, and branch ; adjacent branches then communicate, and form plexuses which possess nuclei at the points of intersection of the nerves. From these plexuses delicate branches again arise, some of which penetrate between the cells of the anterior epithelium, whilst others end in the proper tissue of the cornea. Kiihne stated that the terminal fibres ended in the cornea corpuscles, but this statement has not been confirmed. Jhoroid The Choroid coat forms the largest portion of the middle at - coat of the eyeball. It lies immediately internal to the sclerotic, and extends as far forward as the corpus ciliare, or annulus albidus, where it forms the ciliary processes ; it is pierced posteriorly by the optic nerve. It has a deep black colour, from the numerous pigment cells it contains, and is abundantly provided with blood-vessels and nerves. The Corpus ciliare, or annulus albidus, is a greyish- white ring which surrounds the anterior border of the choroid close to the junction of the sclerotic and cornea. It consists of two portions an external, the ciliary muscle, which lies next the sclerotic, and an internal, the ciliary processes (Plate XIX. fig. 4). These processes, about 80 in number, are folds, separated from each other by furrows which extend forwards in the meridional direction as far as the iris, and form collectively a zone-Like plated frill around the circumference of the iris. On the one hand, they are continuous with the vasculo-pigmentary structures of the choroid ; on the other, with the vasculo-pigmentary struc tures of the iris. ris. The Iris is a circular, flattened disc-shaped diaphragm, situated behind the cornea, in front of the crystalline lens, and bathed by the aqueous humour. By its circumference or ciliary border the iris is not only continuous with the ciliary processes, but is connected by fibres, termed ligamentum pectinatum, with the posterior elastic lamina of the cornea. The iris is the structure which gives the characteristic colour to the eye blue, grey, brown, hazel, as the case may be. It is perforated at, or immediately to the inner side of, its centre by a circular aperture, the pupil, the size of which is regulated by the contraction or relaxation of the muscular tissue of the iris. The structure of the several divisions of the middle coat will now be considered. The CJioroid coat has its inner or anterior surface formed by a distinct pigmentary layer of hexagonal pigment cells (Fig. 43). In the eyes of Albinos, though the cells are present, they contain no pigment. In many mammals also, the pigment is absent from the inner surface, so that the choroid possesses a beautiful iridescent lustre, the tapetum lucidum. In ruminant animals and in the horse the iridescence is due to the reflection of the light by the bundles of the connective tissue stroma, but in cats from polygonal nucleated cells, which Schultze states con tain double refracting crystals. Next the inner pigmen tary layer is the lamina vitrea, the elastic layer of Kolliker. It forms a translucent membrane, described by some as structureless, but by Kolliker as faintly fibrous, which is intimately connected with the stroma of the choroid. The siroma consists of a plexiform arrangement of bundles 887 of connective tissue, in the intervals between which nume rous stellate pigment cells are situated, which give to the entire thickness of the choroid its black appearance. This stroma connects the outer surface of the choroid with the inner surface of the sclerotic, and forms the lamina fusca. Ramifying in the stroma are the blood-vessels and nerves. The vessels of the choroid are arranged in two layers. Next the lamina vitrea is a plexiform capillary layer, the meshes of which are so minute, and the vessels so compacted to gether, as to give the appearance of a vascular membrane, long known as the membrana Kuyschiana. The capillaries radi ate like minute stars from the terminal twigs of the choroidal arteries and veins. The choroidal arteries and veins form a layer external to the capillaries, i.e., next the lamina fusca. The arteries are the short posterior ciliary branches of the ophthalmic artery, which pierce the sclerotic close to the entrance of the optic nerve, and, running forwards in a tor* tuous manner, divide dichotomously before ending in the capillaries. The veins of the choroid are arranged in a series of remarkable whorls, named the venae vorticosce, which receive the blood not only from the capillaries of the choroid proper, but from those of the iris and ciliary body; they discharge their blood by means of from 4 to 6 veins into the ophthalmic vein. The ciliary muscle is the greyish white structure which forms the outer part of the ciliary body. It was at one time called the ciliary ligament, but its muscular nature was discovered almost simultaneously by Bowman and Briicke. It consists of smooth involuntary muscle, the fibres of which are arranged in two layers. The outer and thicker part of the muscle consists of fasci culi, which arise close to the canal of Schlemm, i.e., opposite the junction of the sclerotic and cornea, and radiate from before backwards in the meridian of the eyeball, between the ciliary processes and the sclerotic. The inner part of the muscle forms a ring-like arrangement of fasciculi close to the circumference of the iris, and is often called the annular muscle of Miiller. Iwanoff has shown that in long-sighted persons (hypermetropic) the annular muscle is strongly developed ; whilst in short-sighted (myopic) eyes its fasciculi are veiy feeble. The Ciliary Processes have on their inner surface a black pigmentary layer of cells continuous with that of the choroid. The vitreous layer is jilso present, but according to H. Mullsr is no longer smooth but reticulated. The stroma does not con tain so large a proportion of stellate pigment cells as in the choroid. The arteries have been carefully studied by Leber; they are the long posterior ciliary branches of the ophthalmic, and the anterior ciliary branches of the muscu lar branches of the ophthalmic. They pierce the sclerotic, run forwards, and at the anterior border of the ciliary muscle form by their anastomoses the circulus arteriosus, which gives origin to_ the arteries for the ciliary processes and the iris. The arteries for the ciliary processes are short, and divide into tortuous branches, which frequently anastomose, and form highly complex vascular plexuses, from which arise veins that join the venae vorticosse. Before the long ciliary arteries contribute to the formation of the arterial circle they send branches to the ciliary muscle, and recurrent branches to the anterior part of the proper choroid coat. The iris has its anterior surface covered by a layer of cells continuous with the eudothelium of the aqueous humour. This layer is continuous at the pupillary border with a thick layer of cells filled with black pigment granules, the uvea, which covers the posterior surface of the iris, and is continuoTis at its ciliary border with the pigmentary layer of the ciliary processes. The connective tissue stroma of the iris also contains stellate pigment cells. The variations in colour of the iris in different eyes depends upon the dis tribution and amount of the pigment in the uvea and the