Did This the cornea is penetrated by a zone of capillary loops derived | of connective tissue, in the intervals between which nume from the arteries of the conjunctiva; these loops, according rous stellate pigment cells are situated, which give to the to Lightbody, are invested by perivascular lymph spaces. entire thickness of the choroid its black appearance. The venous canal of Schlemm runs round the circumference stroma connects the outer surface of the choroid with the of the cornea, at the junction of its deeper layers with the inner surface of the sclerotic, and forms the lamina fusca. sclerotic. Leber states that it is not a simple canal, but Ramifying in the stroma are the blood-vessels and nerves. a plexiform arrangement of veins. The nerves of the The vessels of the choroid are arranged in two layers. Next cornea first seen by Schlemm have been carefully examined the lamina vitrea is a plexiform capillary layer, the meshes by recent observers. They arise from the ciliary nerves, of which are so minute, and the vessels so compacted toand enter the margin of the cornea in from twenty to forty gether, as to give the appearance of a vascular membrane, long fasciculi, which run from the circumference to the centre known as the membrana Ruyschiana. The capillaries radiand to the anterior surface of the cornea, and give off ate like minute stars from the terminal twigs of the choroidal numerous branches. The nerve fibres soon lose their arteries and veins. The choroidal arteries and veins form medullary sheath, and branch; adjacent branches then a layer external to the capillaries, i.e., next the lamina fusca. communicate, and form plexuses which possess nuclei at The arteries are the short posterior ciliary branches of the the points of intersection of the nerves. From these ophthalmic artery, which pierce the sclerotic close to the plexuses delicate branches again arise, some of which entrance of the optic nerve, and, running forwards in a tor penetrate between the cells of the anterior epithelium, tuous manner, divide dichotomously before ending in the whilst others end in the proper tissue of the cornea. capillaries. The veins of the choroid are arranged in a Kühne stated that the terminal fibres ended in the cornea series of remarkable whorls, named the venæ vorticosa, corpuscles, but this statement has not been confirmed. 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 Brücke. 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 fasciculi, 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 Müller. Iwanoff has shown that in long-sighted persons (hypermetropic) the annular muscle is strongly developed; whilst in short-sighted (myopic) eyes its fasciculi are very 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 also present, but according to H. Müller is no longer smooth but reticulated. The stroma does not contain 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 muscular 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 venæ vorticosæ. 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 Choroid coat forms the largest portion of the middle 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 structures of the iris. 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 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 cornea. The structure of the several divisions of the middle coat will now be considered. The Choroid 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 inany 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 contain double refracting crystals. Next the inner pigmentary layer is the lumina vitrea, the elastic layer of Kölliker. It forms a translucent membrane, described by some as structureless, but by Kölliker as faintly fibrous, which is intimately connected with the stroma of the choroid. The stroma consists of a plexiform arrangement of bundles The iris has its anterior surface covered by a layer of cells continuous with the endothelium 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 continuous 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 na stellate cells: in dark-coloured eyes, both are filled with dark pigment granules; whilst in light-coloured eyes the stellate cells of the stroma are either devoid of pigment or only faintly coloured. The iris contains numerous fasciculi of involuntary or non-striped muscular fibre arranged in two directions. Circularly arranged fibres surround the aperture of the pupil, and form the sphincter muscle, by the contraction of which the size of the pupil is diminished. Smooth muscular fibres also radiate from the pupillary to the ciliary border of the iris and form the dilatator muscle. The muscular nature of these fibres in the human iris was long disputed, but was satisfactorily demonstrated in 1852 by Lister. Jeropheef has also described circular fasciculi surrounding the ciliary border. In birds and reptiles the muscular tissue of the iris consists of transversely striped fibres. The arteries of the iris arise from the circulus arteriosus, and run radially forwards towards the pupil, where they anastomose and form the circulus iridis minor. They possess relatively thick external and muscular coats. The capillaries form a plexus not so compact as that of the choroid coat. The veins of the iris end in the venæ vorticosa. In the foetus the pupil is closed in by a delicate membrane, membrana pupillaris, into which the bloodvessels of the iris are prolonged. This membrane disappears by absorption during the later months of embryo life. The nerves of the middle coat of the eyeball are the long ciliary branches of the ophthalmic division of the 5th and the short ciliary branches of the ciliary ganglion (Pl. XIX. fig. 7,5). They pierce the sclerotic near the optic nerve, and run forward in the lamina fusca of the choroid. They give off branches to the choroid which form in it a plexus in which H. Müller found nerve cells. From this plexus delicate branches pass to the muscular coat of the choroidal arteries. The ciliary nerves then enter the ciliary muscle, and form plexuses with interspersed nerve cells, from which branches pass to the muscular fibres. Other branches of the ciliary nerves enter the iris, and form plexuses, from which branches proceed to the muscular tissue. The Retina is the delicate nervous coat of the eyeball which lies unmediately internal to the choroid, and extends from the place of entrance of the optic nerve as far forward as the ciliary processes, where it forms a jagged border, the ora serrata. In the hiving eye it is translucent and colourless, but shortly after death it becomes grey it is soft and 80 easily torn that it is difficult to display it in a dissection without injury. Its inner or anterior surface, concave forwards, is nioulded on the vitreous body, and presents the following appearances :-Almost exactly in the anteroposterior axis of the eyeball is a transversely oval yellow spot, about th inch in its long diameter, which amongst mammals is found only in man and apes, though, as Knox and Hulke have shown, it exists in reptiles, in the centre of this spot is a depression, the fovea centralis, about th inch to the inner side of the yellow spot is a slight elevation, the papilla optica, which marks the disc-like entrance of the optic nerve into the retina; here the fibres of the nerve radiate outwards and forwards to the ora serrata, and branches of the arteria centralis retina accompany then. The retina is highly complex in structure, and consists of nerve fibres and cells, of peripheral end-organs, of con 1 nective tissue, and of blood-vessels, arranged in several layers. Max Schultze, who is the chief authority on the subject, recognises ten layers, but includes among these the layer of hexagonal pigment cells just described as the inner pigmentary layer of the choroid. If this layer be omitted, nine layers may then be recognised, and, following Schultze, be named from before backwards as follows:-1. Membrana limitans interna; 2. Layer of optic nerve fibres; 3. Layer of ganglion cells; 4. Internal granulated (molecular) layer; 5. Internal granule layer; 6. External granulated Layer; 7. External granule layer; 8. Membrana limitans externa; 9. Bacillary layer (Fig. 78). These fibrils are inter The nervous elements of the retina will first be considered. The optic nerve fibres (2), where they pierce the sclerotic, as a rule lose the medullary sheath, and radiate outwards as non-medullated fibres from the optic disc to the ora serrata immediately behind and parallel to the membrana limitans interna. These fibres vary greatly in size, and are frequently varico86. When any of the optic nerve fibres retain the medullary sheath the retina is there rendered opaque. Immediately behind the nerve fibres is the layer of ganglionic nerve cells (3). These cells are either bipolar or multipolar. In the living eye the cell substance is hyaline and the nucleus transparent, but after death the substance both of the body of the cell and the processes assumes a fibrillated appearance, like the axial cylinder of an optic nerve fibre. One process, the central process, extends into the layer of optic nerve fibres; and another, internal granulated layer (4) contains the branching prothe peripheral, into the internal granulated layer. The continuous with an arrangement of excessively fine fibrils, cesses of the nerve cells, some of which apparently become probably nervous in their nature. mingled with a delicate plexus of connective tissue. The internal granule layer (5) contains numerous fusiform nucleated enlargements, the so-called internal granules, arranged in superimposed strata; from each fusiform enlargement a fibre proceeds in two directions, one centrally into the internal granulated layer, and one peripherally into the external granulated layer. These fibres possess vancosities, and resemble the optic nerve fibres. The external granulated layer (6) is very thin, and consists of an erpanded network of minute fibres, with nuclei situated at the points of intersection of the fibres. Krause has called it the membrana fenestrata. The external granule layer (7) contains numerous fusiform nucleated enlargements, the so-called external granules, arranged in superimposed strata from each enlargement a fibre proceeds in two directions, one centrally into the external granulated layer, and one peripherally through the membrana limitans externa to the bacillary layer, where it becomes continuous with the anterior end of either a rod or a cone, as the case may be. Hence these fibres of the external granule layer are called by Schultze rod and cone fibres, and the external granules are nucleated enlargements of these fibres. These fibres possess varicosities like those of the internal granule layer. The bacillary layer (9) or membrane of Jacob consists of multitudes of elongated bodies arranged side by side like rows of palisades, and vertically to the surfaces of the retina. Some of these bodies are cylindrical, and are named the rods of the retina; others flask-shaped, and named the cones of the retina: the rods equal in length the entire thickness of the bacillary layer; the cones are shorter than the rods, and are interspersed at regular intervals between them; the apex of each cone is directed towards, but does not reach, the plane of the posterior or choroidal surface of the retina. The posterior or outer end of each rod rests against the pigmentary layer of the choroid The anterior or inner ends of both rods and cones are con- | tinuous with the rod and cone fibres of the external granule layer, as already described. Each rod and cone is subdivided into an outer strongly refractile and an inner feebly refractile segment. By the action of various reagents the outer segments both of the rods and cones exhibit a transverse striation, and ultimately break up into discs. Hensen has described a longitudinal striation in the outer segments, and Ritter has stated that both in the outer and inner segments of the rods an axial fibre exists. Max Schultze has also seen the inner segments of both rods and cones longitudinally striped on the surface. Modifications in the relative numbers and appearances of the rods and cones have been seen in the eyes of various vertebrata. In birds, for example, the cones are much more numerous than the rods, whilst the reverse is the case in mammals generally. In the cartilaginous fishes the cones are entirely absent ; so also, as Schultze has shown, in the bat, hedge-hog, and mole; whilst in reptiles the bacillary layer is exclusively composed of cones. In all the vertebrata, except the mammalia, the twin or double cones described by Hannover probably exist. In the amphibia, lens-shaped bodies have been described in the inner segments of the cones. The rods and cones are the peripheral end-organs in connection with the fibres of the optic nerve, and their apparent relation to these fibres is as follows:-The optic nerve fibres are continuous with the central processes of the ganglion cells of the retina, the peripheral branching processes of which pass into the internal granulated layer, where they may possibly become continuous with the central processes of the inner granular layer. The peripheral processes of the inner granular layer enter the external granulated layer, but it is difficult to say whether or not they become continuous with the central processes of that layer. There can, however, be no doubt that the peripheral processes of this layer are directly continuous with the rods and cones, of the bacillary layer. The entire arrangement is sometimes called the radial nervous fibres of the retina. In addition to the nervous structures just described, the retina contains a delicate supporting connective tissue like the neuroglia of the brain and spinal cord. Not only does it lie between the fibres, cells, and so-called granules in the several nervous layers, and form in them a radial arrangement of supporting fibres, but it constitutes the two limitary membranes of the retina. The membrana limitans externa (8) is excessively thin, and appears in vertical sections through the retina as a mere line between the, bacillary and external granular layers, continuous on the one hand with the connective tissue which passes for a short distance between the rods and cones, and on the other with the connective tissue framework of the external granule layer. The membrana limitans interna (1) covers the anterior surface of the retina, and lies next the vitreous body; its posterior surface blends with the radial arrangement of connective tissue between the optic nerve fibres, but its anterior or hyaloid surface, as J. C. Ewart has recently shown, possesses a mosaic appearance, like that of a layer of squamous endothelium. The yellow spot exhibits some structural differences from the rest of the retina. It owes its colour to the presence of yellow pigment deposited in the more anterior layers of the retina. Except at its central depression, the fovea centralis, it is thicker than the surrounding parts, of the retina; but it is much softer, a condition which is due to the almost complete absence of the layer of optic nerve fibres, and a diminution in the amount of the supporting connective tissue, the membrana limitans interna is, however, relatively stronger In the fovea centralis itself the rods of the bacillary layer have entirely disappeared, and are replaced by cones which are distinguished by their close arrangement, and the more slender form and increased length, especially of their outer segments. The external granule layer is well marked, and the central fibres belonging to it, instead of passing vertically forwards, incline very obliquely or almost horizontally outwards to the internal granule layer, which, together with the layers anterior to it, is so thin as almost to have disappeared. In the yellow spot surrounding the fovea the bacillary layer is also composed of cones which are not, however, so slender or so long as at the fovea itself. The layer of nerve cells and the inner part of the external granule layer are thicker than in the rest of the retina. The yellow spot is the part of the retina most sensitive to light. At the ora serrata or anterior border of the retina the nervous layers, including the rods and cones, cease to exist. The radial connective tissue and internal limiting membrana are present; from the radial tissue a layer of cells is prolonged forward in contact with the deep surface of the ciliary processes as the pars ciliaris retina. The retina is supplied with blood by the arteria centralis, which, traversing the axis of the optic nerve, reaches the retina at the optic disc. In the retina it branches dichotomously in the nerve fibre layer, avoiding however the yellow spot, and its terminal twigs reach the ora serrata. The capillaries form in the more anterior layers of the retina a distinct network, which does not enter the external granule and bacillary layers, but penetrates the yellow spot, though not the fovea centralis. The blood is conveyed from the retina by the central vein which accompanies the artery in the optic nerve, and opens either into the ophthalmic vein or directly into the cavernous sinus. The veins and capillaries of the retina have been described by His as completely invested by perivascular lymphatic sheaths, whilst the arteries only possess such sheaths for a limited part of their course. The Optic Nerve itself passes from the orbit through the Nerve of optic foramen into the cranial cavity, where it arises from sight. the optic commissure. This commissure is a flattened band formed by the junction of the two optic tracts. Each tract winds backwards around the tuber cinereum and crus cerebri to arise from the optic thalamus, corpora quadrige. mina, and geniculata; and some observers also state that it derives fibres from the tuber cinereum and lamina cinerea. In the commissure an interchange takes place between the fibres of opposite nerves and tracts, so that not only does an optic nerve contain fibres derived from the tract on its own side, but from the opposite tract, and it has even been stated that fibres pass across the commissure from one optic nerve to the other, and from one optic tract to the other. The Aqueous Humour is a limpid watery fluid, containing Refracti a little common salt in solution, which occupies the space media. between the cornea and the front of the crystalline lens. In this space the iris lies, and imperfectly divides it into two chambers, an anterior and a posterior, which communicate with each other through the pupil The anterior chamber, of some size, is situated between the iris and cornea; but as the iris is in contact with the front of the lens, the posterior chamber is reduced to a mere chink between the circumference of the iris and that of the lens. It The Crystalline Lens is situated behind the iris and pupil, and in front of the vitreous body. It is a transparent bi-convex lens, with its antero-posterior diameter d less than the transverse, its posterior surface more convex than the anterior, and with its circumference rounded. consists of a capsule and the body of the lens enclosed by the capsule. The lens capsule is a transparent, smooth, structureless, and very elastic membrane, about twice as thick on the anterior as on the posterior surface of the lens. It is non-vascular in the adult, though in the fœtus a branch of the central artery of the retina which traverses the |