This membrane is perforated by the rod-fibers and cone-fibers. The end-plates of the fibers give off externally short, inflexible fibrils, which form the fiber-baskets containing the basilar portions of the inner segments of the rods and cones. (Vid. Fig. 338.) 7. THE RELATIONS OF THE ELEMENTS OF THE RETINA TO ONE ANOTHER. We shall now take up the relationships existing between the various elements of the retinal strata, giving the theories now generally accepted and based on observations made with the Golgi and methylene-blue methods, and more particularly on the investigations of Ramôn y Cajal (see diagram, Fig. 338): I. The inner processes of the rod-visual cells end, as a rule, in small expansions within the outer molecular layer, in which also the processes of the cone-visual cells terminate in broader branched pedicles. In this layer also are situated the terminal arborizations of the dendrites and neuraxes of certain cells belonging to the inner nuclear layer. 2. The inner nuclear layer consists, as we have seen, (a) of bipolar cells, which constitute the principal portion of this layer, (b) of horizontally placed cells lying immediately beneath the outer molecular layer, and (c) of the layer of spongioblasts situated at the junction of the inner nuclear with the inner molecular layer. The bipolar cells comprise the following: (a) Bipolar cells of the rod-visual cells the dendrites of which intertwine around the basilar portions of the rodvisual cells, and the neuraxes of which end in telodendria in the neighborhood of the cell-bodies of the nerve-cells of the ganglion-cell layer. (3) Bipolar cells of the cone-visual cells. The dendrites of these cells, which also end in the outer molecular layer, are there in relation to the basilar processes of the cone-fibers. Their neuraxes come in contact, by means of terminal arborizations, with the dendrites of the ganglion cells of the ganglion-cell layer at varying depths of the inner molecular layer. (7) Besides these, there are also bipolar cells which, as in the case of a and B, form contact with the rod- and cone-visual cells, but end on the cell-bodies of the ganglion cells of the ganglion-cell layer. The horizontal cells send their dendrites into the outer molecular layer, while their neuraxes extend horizontally and give off numerous collaterals to the same layer, ending there in telodendria. These cells are of two varieties: the smaller, indirectly connecting the cone-visual cells with one another by means of their dendrites and neuraxes; and the larger, more deeply situated cells, connecting in a similar manner the basilar ends of the rodvisual cells. A few cells of the second variety give off one or two dendrites each, which penetrate through the inner nuclear layer into the inner molecular layer. 3. The inner molecular layer. This is composed of five strata. The majority of the spongioblasts in the inner nuclear layer send Diffuse spongioblast. their processes upward into the inner molecular layer, in which some end in fine arborizations in the first, others in the second, and still others in the third interstice, separating the strata of the inner molecular layer from one another. Besides these so-called stratum spongioblasts, there are also others in the inner nuclear layer, the diffuse spongioblasts, whose ramifications end simultaneously in sev Fig. 338.-Schematic diagram of the retina according to Ramón y Cajal: The line a, after passing eral or in all of the strata of the inner molecular layer. Besides the ramifications of the spongioblasts just mentioned, autochthonous cells are also present. These lie in one of the interstices of the molecular layer, their ramifications spreading out in a horizontal direction. Besides all these structures the dendrites of the cells Rod. in the ganglion-cell layer also ramify throughout the inner molecular layer. 4. The ganglion cell layer. The cell-bodies are irregularly oval; their dendrites extend into the inner molecular layer, and their neuraxes into the nerve-fiber layer. According to the manner of their dendritic termination, the ganglion cells may be divided into three groups: (1) those the dendrites of which extend into but one stratum of the molecular layer; (2) those the dendrites of which extend into several strata of the molecular layer; and (3) those the dendrites of which are distributed throughout the entire thickness of the molecular layer. Thus, these three groups are made up of the so-called mono-stratified, poly-stratified, and diffuse cells; by means of their dendrites they come in contact with one or several of the neuraxes of the bipolar cells of the inner nuclear layer. 5. The nerve-fiber layer of the retina. This layer consists of centripetal neuraxes from the ganglion cells of the ganglion-cell layer, and of centrifugal nerve-fibers ending in various layers of the retina, including the outer molecular layer. 8. THE OPTIC NERVE. Within the orbit the optic nerve possesses an external sheath, which is an extension of the dura mater and is continuous with the scleral tissue, and an inner sheath, which is a prolongation of the pia mater. Between these two sheaths is a fissure, divided into two smaller clefts by a continuation of the arachnoid. Both these clefts are traversed by connective-tissue trabeculæ. The inner cleft communicates with the subarachnoid space; and the outer narrower cleft, with the subdural space. The fibers of the optic nerve are medullated, but there is no neurilemma (sheath of Schwann), the latter being represented by the neuroglia. In the region of the sclera and choroid the optic nervefibers lose their myelin, and the septa of the inner or pial sheath become better developed and relatively more numerous. Connective-tissue fibers from the sclera and choroid also traverse this region of the optic nerve, giving rise to what is known as the lamina cribrosa. At from 12 to 2 cm. from the eyeball there enter into the optic nerve laterally and ventrally (according to J. Deyl, mesially) the central artery and vein of the retina, which very soon come to lie within the axis of the nerve. Here they are surrounded by a common connective-tissue sheath which is in direct connection with the perineurium. The optic nerve-fibers extend through the lamina cribrosa into the retina, where they spread out as the nerve-fiber layer in the manner previously described. 9. BLOOD VESSELS OF THE OPTIC NERVE AND RETINA. Vein. The blood-vessels of the optic nerve are principally derived from the vessels of the pial sheath. In that portion of the nerve containing the central vessels of the retina the latter anastomose with the pial vessels, so that this portion of the optic nerve is also supplied by the central vessels. At their entrance through the sclera the short posterior ciliary arteries form a plexus around the optic nerve, the arterial circle of Zinn, which communicates, on the one hand, with the vessels of the pial sheath, and, on the other, with those of the optic nerve. At the level of the choroid the vessels of the latter communicate by means of capillaries with the central vessels of the optic nerve. Artery. Zone sur- Fig. 339.-Injected blood-vessels of the human retina; surface preparation; X 18. The central artery and vein of the retina enter and leave the retina at the optic papilla, dividing here, or even within the nerve itself, into the superior and inferior papillary artery and Vascular plexus of macula lutea with wide meshes. Fovea centralis, free from vessels. Fig. 340.-Injected blood-vessels of human macula lutea; surface preparation; X 28. vein. Both the latter again divide into two branches, the nasal and temporal arteriole and venule, known, according to their posi tions, as the superior and inferior nasal and temporal artery and vein. Besides these vessels, two small arteries also arise from the trunk of the central artery itself, and extend to the macula. Two similar vessels extend toward the nasal side as the superior and inferior median branches. Within the retina itself the larger vessels spread out in the nerve-fiber layer, forming there a coarsely meshed capillary network connected by numerous branches with a finer and more closely meshed network lying within the inner nuclear layer. The venous capillaries of this network return as small venous branches to the nerve-fiber layer, in which they form a venous plexus, side by side with the arterial plexus. The arteries of the retina are of smaller caliber than the veins. The larger arteries possess a muscular layer; the smaller, only an adventitia. All the vessels possess highly developed perivascular sheaths. The visual-cell layer is nonvascular, as are also the fovea centralis and the rudimentary retinal layers lying anterior to the ora serrata. The arteries of the retina anastomose with one another solely by means of capillaries (end-arteries), and it is only in the ora serrata that coarser venous anastomoses exist. F. THE VITREOUS BODY. The vitreous body consists of a semifluid tissue containing very few fixed cellular elements and only a small number of leucocytes. The latter are found only on the surface of the vitreous humor, between it and the retina. Thin structureless lamellæ and fibers occur throughout the entire vitreous body, with the exception of the hyaloid canal. These are particularly numerous at the periphery and especially in the region of the ciliary body. The outer or hyaloid membrane of the vitreous body, separating the latter from the retina, is somewhat thicker in the region of its close attachment around the physiologic excavation of the optic nerve and to the external limiting membrane of the retina in the ciliary region. In the latter region the hyaloid membrane is closely connected with the epithelium of the pars ciliaris retina. It does not, however, penetrate into and between the ciliary processes, but extends like a bridge over the furrows between them. This arrangement gives rise to spaces, the recessus camera posterioris, which form a division of the posterior chamber, and are inclosed between the hyaloid membrane, the ciliary processes, the suspensory ligament of the lens, and the lens itself; these spaces are filled with aqueous humor. In the region of the ciliary processes the hyaloid membrane splits up into numerous fibers, which diverge fan-like toward the lens and become blended with the outer lamella of the lens-capsule. Those coming from the free ends of the ciliary processes become attached |