EYE.] ANATOMY 889 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 sub divided 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 trans verse striation, and ultimately break up into discs. Hensen has described a longitudinal striation in the outer segments, and Bitter 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 rela tion 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 gramilated 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 con tinuous 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 arrange ment 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 be tween the rods and cones, and on the other with the connec tive 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 yelloiv 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 con nective 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 belong ing 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 retinae. , 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 dicho- tomously 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 Ner optic foramen into the cranial cavity, where it arises from si s h 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 cms 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 Refn a little common salt in solution, which occupies the space 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 commu nicate 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. The Crystalline Lens is situated behind the iris and pupil, and in front of the vitreous body. It is a trans parent bi-convex lens, with its antero-posterior diameter Jd less than the transverse, its posterior surface more convex than the anterior, and with its circumference rounded. It consists of a capsule and the body of the lens enclosed by the capsule. The lens capsule is a transparent, smooth, struc tureless, 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 foetus a branch of the central arterv of the retina, which traverses the
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