From without inward the three tunics are: (1) A fibrous tunic, (Fig. 90869) consisting of the sclera behind and the cornea in front; (2) a vascular pigmented tunic, comprising, from behind forward, the choroid, ciliary body, and iris; and (3) a nervous tunic, the retina.
The Fibrous Tunic (tunica fibrosa oculi).The sclera and cornea (Fig. 90869) form the fibrous tunic of the bulb of the eye; the sclera is opaque, and constitutes the posterior five-sixths of the tunic; the cornea is transparent, and forms the anterior sixth.
The Sclera.The sclera has received its name from its extreme density and hardness; it is a firm, unyielding membrane, serving to maintain the form of the bulb. It is much thicker behind than in front; the thickness of its posterior part is 1 mm. Its external surface is of white color, and is in contact with the inner surface of the fascia of the bulb; it is quite smooth, except at the points where the Recti and Obliqui are inserted into it; its anterior part is covered by the conjunctival membrane. Its inner surface is brown in color and marked by grooves, in which the ciliary nerves and vessels are lodged; it is separated from the outer surface of the choroid by an extensive lymph space (spatium perichorioideale) which is traversed by an exceedingly fine cellular tissue, the lamina suprachorioidea. Behind it is pierced by the optic nerve, and is continuous through the fibrous sheath of this nerve with the dura mater. Where the optic nerve passes through the sclera, the latter forms a thin cribriform lamina, the lamina cribrosa scleræ; the minute orifices in this lamina serve for the transmission of the nervous filaments, and the fibrous septa dividing them from one another are continuous with the membranous processes which separate the bundles of nerve fibers. One of these openings, larger than the rest, occupies the center of the lamina; it transmits the central artery and vein of the retina. Around the entrance of the optic nerve are numerous small apertures for the transmission of the ciliary vessels and nerves, and about midway between this entrance and the sclerocorneal junction are four or five large apertures for the transmission of veins (venæ vorticosæ). In front, the sclera is directly continuous with the cornea, the line of union being termed the sclero-corneal junction. In the inner part of the sclera close to this junction is a circular canal, the sinus venosus scleræ (canal of Schlemm). In a meridional section of this region this sinus presents the appearance of a cleft, the outer wall of which consists of the firm tissue of the sclera, while its inner wall is formed by a triangular mass of trabecular tissue (Fig. 90870); the apex of the mass is directed forward and is continuous with the posterior elastic lamina of the cornea. The sinus is lined by endothelium and communicates externally with the anterior ciliary veins.
The aqueous humor drains into the scleral sinuses by passage through the pectinate villi which are analogous in structure and function to the arachnoid villi of the cerebral meninges.144
Fig. 90869 Horizontal section of the eyeball.
Structure.The sclera is formed of white fibrous tissue intermixed with fine elastic fibers; flattened connective-tissue corpuscles, some of which are pigmented, are contained in cell spaces between the fibers. The fibers are aggregated into bundles, which are arranged chiefly in a longitudinal direction. Its vessels are not numerous, the capillaries being of small size, uniting at long and wide intervals. Its nerves are derived from the ciliary nerves, but their exact mode of ending is not known.
The Cornea.The cornea is the projecting transparent part of the external tunic, and forms the anterior sixth of the surface of the bulb. It is almost circular in outline, occasionally a little broader in the transverse than in the vertical direction. It is convex anteriorly and projects like a dome in front of the sclera. Its degree of curvature varies in different individuals, and in the same individual at different periods of life, being more pronounced in youth than in advanced life. The cornea is dense and of uniform thickness throughout; its posterior surface is perfectly circular in outline, and exceeds the anterior surface slightly in diameter. Immediately in front of the sclero-corneal junction the cornea bulges inward as a thickened rim, and behind this there is a distinct furrow between the attachment of the iris and the sclero-corneal junction. This furrow has been named by Arthur Thomson145 the sulcus circularis corneæ it is bounded externally by the trabecular tissue already described as forming the inner wall of the sinus venosus scleræ. Between this tissue and the anterior surface of the attached margin of the iris is an angular recess, named the iridial angle or filtration angle of the eye (Fig. 90870). Immediately outside the filtration angle is a projecting rim of scleral tissue which appears in a meridional section as a small triangular area, termed the scleral spur. Its base is continuous with the inner surface of the sclera immediately to the outer side of the filtration angle and its apex is directed forward and inward. To the anterior sloping margin of this spur are attached the bundles of trabecular tissue just referred to; from its posterior margin the meridional fibers of the Ciliaris muscle arise.
Fig. 90870 Enlarged general view of the iridial angle. (Arthur Thomson.)
Structure (Fig. 90871).The cornea consists from before backward of four layers, viz.: (1) the corneal epithelium, continuous with that of the conjunctiva; (2) the substantia propria (3) the posterior elastic lamina; and (4) the endothelium of the anterior chamber.
The corneal epithelium (epithelium corneæ anterior layer) covers the front of the cornea and consists of several layers of cells. The cells of the deepest layer are columnar; then follow two or three layers of polyhedral cells, the majority of which are prickle cells similar to those found in the stratum mucosum of the cuticle. Lastly, there are three or four layers of squamous cells, with flattened nuclei.
The substantia propria is fibrous, tough, unyielding, and perfectly transparent. It is composed of about sixty flattened lamellæ, superimposed one on another. These lamellæ are made up of bundles of modified connective tissue, the fibers of which are directly continuous with those of the sclera. The fibers of each lamella are for the most part parallel with one another, but at right angles to those of adjacent lamellæ. Fibers, however, frequently pass from one lamella to the next.
The lamellæ are connected with each other by an interstitial cement substance, in which are spaces, the corneal spaces. These are stellate in shape and communicate with one another by numerous offsets. Each contains a cell, the corneal corpuscle, resembling in form the space in which it is lodged, but not entirely filling it.
Fig. 90871 Vertical section of human cornea from near the margin. (Waldeyer.) Magnified. 1. Epithelium. 2. Anterior elastic lamina. 3. substantia propria. 4. Posterior elastic lamina. 5. Endothelium of the anterior chamber. a. Oblique fibers in the anterior layer of the substantia propria. b. Lamellæ the fibers of which are cut across, producing a dotted appearance. c. Corneal corpuscles appearing fusiform in section. d. Lamellæ the fibers of which are cut longitudinally. e. Transition to the sclera, with more distinct fibrillation, and surmounted by a thicker epithelium. f. Small bloodvessels cut across near the margin of the cornea.
The layer immediately beneath the corneal epithelium presents certain characteristics which have led some anatomists to regard it as a distinct membrane, and it has been named the anterior elastic lamina (lamina elastica anterior; anterior limiting layer; Bowmans membrane). It differs, however, from the posterior elastic lamina, in presenting evidence of fibrillar structure, and in not having the same tendency to curl inward, or to undergo fracture, when detached from the other layers of the cornea. It consists of extremely closely interwoven fibrils, similar to those found in the substantia propria, but contains no corneal corpuscles. It may be regarded as a condensed part of the substantia propria.
The posterior elastic lamina (lamina elastica posterior; membrane of Descemet; membrane of Demours) covers the posterior surface of the substantia propria, and is an elastic, transparent homogeneous membrane, of extreme thinness, which is not rendered opaque by either water, alcohol, or acids. When stripped from the substantia propria it curls up, or rolls upon itself with the attached surface innermost.
At the margin of the cornea the posterior elastic lamina breaks up into fibers which form the trabecular tissue already described; the spaces between the trabeculæ are termed the spaces of the angle of the iris (spaces of Fontana); they communicate with the sinus venosus scleræ and with the anterior chamber at the filtration angle. Some of the fibers of this trabecular tissue are continued into the substance of the iris, forming the pectinate ligament of the iris; while others are connected with the forepart of the sclera and choroid.
The endothelium of the anterior chamber (endothelium cameræ anterioris; posterior layer; corneal endothelium) covers the posterior surface of the elastic lamina, is reflected on to the front of the iris, and also lines the spaces of the angle of the iris; it consists of a single stratum of polygonal, flattened, nucleated cells.
Vessels and Nerves.The cornea is a non-vascular structure; the capillary vessels ending in loops at its circumference are derived from the anterior ciliary arteries. Lymphatic vessels have not yet been demonstrated in it, but are represented by the channels in which the bundles of nerves run; these channels are lined by an endothelium. The nerves are numerous and are derived from the ciliary nerves. Around the periphery of the cornea they form an annular plexus, from which fibers enter the substantia propria. They lose their medullary sheaths and ramify throughout its substance in a delicate net-work, and their terminal filaments form a firm and closer plexus on the surface of the cornea proper, beneath the epithelium. This is termed the subepithelial plexus, and from it fibrils are given off which ramify between the epithelial cells, forming an intraepithelial plexus.
The Vascular Tunic (tunica vasculosa oculi) (Figs. 90872, 90873, 90874).The vascular tunic of the eye is formed from behind forward by the choroid, the ciliary body, and the iris.
The choroid invests the posterior five-sixths of the bulb, and extends as far forward as the ora serrata of the retina. The ciliary body connects the choroid to the circumference of the iris. The iris is a circular diaphragm behind the cornea, and presents near its center a rounded aperture, the pupil.
Fig. 90872 The choroid and iris. (Enlarged.)
Fig. 90873 The arteries of the choroid and iris. The greater part of the sclera has been removed. (Enlarged.)
The Choroid (chorioidea).The choroid is a thin, highly vascular membrane, of a dark brown or chocolate color, investing the posterior five-sixths of the globe; it is pierced behind by the optic nerve, and in this situation is firmly adherent to the sclera. It is thicker behind than in front. Its outer surface is loosely connected by the lamina suprachorioidea with the sclera; its inner surface is attached to the pigmented layer of the retina.
Structure.The choroid consists mainly of a dense capillary plexus, and of small arteries and veins carrying blood to and returning it from this plexus. On its external surface is a thin membrane, the lamina suprachorioidea, composed of delicate non-vascular lamellæeach lamella consisting of a net-work of fine elastic fibers among which are branched pigment cells. The spaces between the lamellæ are lined by endothelium, and open freely into the perichoroidal lymph space, which, in its turn, communicates with the periscleral space by the perforations in the sclera through which the vessels and nerves are transmitted.
Internal to this lamina is the choroid proper, consisting of two layers: an outer, composed of small arteries and veins, with pigment cells interspersed between them; and an inner, consisting of a capillary plexus. The outer layer (lamina vasculosa) consists, in part, of the larger branches of the short ciliary arteries which run forward between the veins, before they bend inward to end in the capillaries, but is formed principally of veins, named, from their arrangement, the venæ vorticosæ. They converge to four or five equidistant trunks, which pierce the sclera about midway between the sclero-corneal junction and the entrance of the optic nerve. Interspersed between the vessels are dark star-shaped pigment cells, the processes of which, communicating with those of neighboring cells, form a delicate net-work or stroma, which toward the inner surface of the choroid loses its pigmentary character. The inner layer (lamina choriocapillaris) consists of an exceedingly fine capillary plexus, formed by the short ciliary vessels; the net-work is closer and finer in the posterior than in the anterior part of the choroid. About 1.25 cm. behind the cornea its meshes become larger, and are continuous with those of the ciliary processes. These two laminæ are connected by a stratum intermedium consisting of fine elastic fibers. On the inner surface of the lamina choriocapillaris is a very thin, structureless, or faintly fibrous membrane, called the lamina basalis; it is closely connected with the stroma of the choroid, and separates it from the pigmentary layer of the retina.
Fig. 90874 The veins of the choroid. (Enlarged.)
One of the functions of the choroid is to provide nutrition for the retina, and to convey vessels and nerves to the ciliary body and iris.
Tapetum.This name is applied to the outer and posterior part of the choroid, which in many animals presents an iridescent appearance.
The Ciliary Body (corpus ciliare).The ciliary body comprises the orbiculus ciliaris, the ciliary processes, and the Ciliaris muscle.
The orbiculus ciliaris is a zone of about 4 mm. in width, directly continuous with the anterior part of the choroid; it presents numerous ridges arranged in a radial manner (Fig. 90875).
The ciliary processes (processus ciliares) are formed by the inward folding of the various layers of the choroid, i.e., the choroid proper and the lamina basalis, and are received between corresponding foldings of the suspensory ligament of the lens. They are arranged in a circle, and form a sort of frill behind the iris, around the margin of the lens (Fig. 90875). They vary from sixty to eighty in number, lie side by side, and may be divided into large and small; the former are about 2.5 mm. in length, and the latter, consisting of about one-third of the entire number, are situated in spaces between them, but without regular arrangement. They are attached by their periphery to three or four of the ridges of the orbiculus ciliaris, and are continuous with the layers of the choroid: their opposite extremities are free and rounded, and are directed toward the posterior chamber of the eyeball and circumference of the lens. In front, they are continuous with the periphery of the iris. Their posterior surfaces are connected with the suspensory ligament of the lens.
Fig. 90875 Interior of anterior half of bulb of eye.
Structure.The ciliary processes (Figs. 90876, 90877) are similar in structure to the choroid, but the vessels are larger, and have chiefly a longitudinal direction. Their posterior surfaces are covered by a bilaminar layer of black pigment cells, which is continued forward from the retina, and is named the pars ciliaris retinæ. In the stroma of the ciliary processes there are also stellate pigment cells, but these are not so numerous as in the choroid itself.
According to Henderson the aqueous humor is a secretion formed by the active intervention of the epithelial cells lining the apices of the ciliary processes.146
The Ciliaris muscle (m. ciliaris; Bowmans muscle) consists of unstriped fibers: it forms a grayish, semitransparent, circular band, about 3 mm. broad, on the outer surface of the fore-part of the choroid. It is thickest in front, and consists of two sets of fibers, meridional and circular. The meridional fibers, much the more numerous, arise from the posterior margin of the scleral spur (page 1007); they run backward, and are attached to the ciliary processes and orbiculus ciliaris. One bundle, according to Waldeyer, is inserted into the sclera. The circular fibers are internal to the meridional ones, and in a meridional section appear as a triangular zone behind the filtration angle and close to the circumference of the iris. They are well-developed in hypermetropic, but are rudimentary or absent in myopic eyes. The Ciliaris muscle is the chief agent in accommodation, i. e., in adjusting the eye to the vision of near objects. When it contracts it draws forward the ciliary processes, relaxes the suspensory ligament of the lens, and thus allows the lens to become more convex.
Fig. 90876 Vessels of the choroid, ciliary processes, adn iris of a child. (Arnold.) Magnified 10 times. a. Capillary net-work of the posterior part of the choroid, ending at b, the ora serrata. c. Arteries of the corona ciliaris, supplying the ciliary processes, d, and passing into the iris e. f. The capillary net-work close to the pupillary margin of the iris.
Fig. 90877 Diagram of the blood vessels of the eye, as seen in a horizontal section. (Leber, after Stöhr.). Course of vasa centralia retinæ: a. Arteria. a.147 Vena centralis retinæ. B. Anastomosis with vessels of outer coats. C. Anastomosis with branches of short posterior ciliary arteries. D. Anastomosis with chorioideal vessels. Course of vasa ciliar. postic. brev.: I. Arteriæ, and I1. Venæ ciliar. postic. brev. II. Episcleral artery. II1. Episcleral vein. III. Capillaries of lamina choriocapillaris. Course of vasa ciliar. postic. long.: 1. a. ciliar. post. longa. 2. Circulus iridis major cut across. 3. Branches to ciliary body. 4. Branches to iris. Course of vasa ciliar. ant.: a. Arteria. a1. Vena ciliar. ant. b. Junction with the circulus iridis major. c. Junction with lamina choriocapill. d. Arterial, and d1. Venous episcleral branches. e. Arterial, and e1. Venous branches to conjunctiva scleræ. f. Arterial, and f1. Venous branches to corneal border. V. Vena vorticosa. S. Transverse section of sinus venosus scleræ.
The Iris.The iris has received its name from its various colors in different individuals. It is a thin, circular, contractile disk, suspended in the aqueous humor between the cornea and lens, and perforated a little to the nasal side of its center by a circular aperture, the pupil. By its periphery it is continuous with the ciliary body, and is also connected with the posterior elastic lamina of the cornea by means of the pectinate ligament; its surfaces are flattened, and look forward and backward, the anterior toward the cornea, the posterior toward the ciliary processes and lens. The iris divides the space between the lens and the cornea into an anterior and a posterior chamber. The anterior chamber of the eye is bounded in front by the posterior surface of the cornea; behind by the front of the iris and the central part of the lens. The posterior chamber is a narrow chink behind the peripheral part of the iris, and in front of the suspensory ligament of the lens and the ciliary processes. In the adult the two chambers communicate through the pupil, but in the fetus up to the seventh month they are separated by the membrana pupillaris.
Structure.The iris is composed of the following structures:
1. In front is a layer of flattened endothelial cells placed on a delicate hyaline basement membrane. This layer is continuous with the endothelium covering the posterior elastic lamina of the cornea, and in individuals with dark-colored irides the cells contain pigment granules.
2. The stroma (stroma iridis) of the iris consists of fibers and cells. The former are made up of delicate bundles of fibrous tissue; a few fibers at the circumference of the iris have a circular direction; but the majority radiate toward the pupil, forming by their interlacement, delicate meshes, in which the vessels and nerves are contained. Interspersed between the bundles of connective tissue are numerous branched cells with fine processes. In dark eyes many of them contain pigment granules, but in blue eyes and the eyes of albinos they are unpigmented.
3. The muscular fibers are involuntary, and consist of circular and radiating fibers. The circular fibers form the Sphincter pupillæ; they are arranged in a narrow band about 1 mm. in width which surrounds the margin of the pupil toward the posterior surface of the iris; those near the free margin are closely aggregated; those near the periphery of the band are somewhat separated and form incomplete circles. The radiating fibers form the Dilatator pupillæ; they converge from the circumference toward the center, and blend with the circular fibers near the margin of the pupil.
Fig. 90878 Iris, front view.
4. The posterior surface of the iris is of a deep purple tint, being covered by two layers of pigmented columnar epithelium, continuous at the periphery of the iris with the pars ciliaris retinæ. This pigmented epithelium is named the pars iridica retinæ, or, from the resemblance of its color to that of a ripe grape, the uvea.
The color of the iris is produced by the reflection of light from dark pigment cells underlying a translucent tissue, and is therefore determined by the amount of the pigment and its distribution throughout the texture of the iris. The number and the situation of the pigment cells differ in different irides. In the albino pigment is absent; in the various shades of blue eyes the pigment cells are confined to the posterior surface of the iris, whereas in gray, brown, and black eyes pigment is found also in the cells of the stroma and in those of the endothelium on the front of the iris.
The iris may be absent, either in part or altogether as a congenital condition, and in some instances the pupillary membrane may remain persistent, though it is rarely complete. Again, the iris may be the seat of a malformation, termed coloboma, which consists in a deficiency or cleft, clearly due in a great number of cases to an arrest in development. In these cases the cleft is found at the lower aspect, extending directly downward from the pupil, and the gap frequently extends through the choroid to the porus opticus. In some rarer cases the gap is found in other parts of the iris, and is not then associated with any deficiency of the choroid.
Vessels and Nerves.The arteries of the iris are derived from the long and anterior ciliary arteries, and from the vessèls of the ciliary processes (see p. 571). Each of the two long ciliary arteries, having reached the attached margin of the iris, divides into an upper and lower branch; these anastomose with corresponding branches from the opposite side and thus encircle the iris; into this vascular circle (circulus arteriosus major) the anterior ciliary arteries pour their blood, and from it vessels converge to the free margin of the iris, and there communicate and form a second circle (circulus arteriosus minor) (Figs. 90877 and 90878).
The nerves of the choroid and iris are the long and short ciliary; the former being branches of the nasociliary nerve, the latter of the ciliary ganglion. They pierce the sclera around the entrance of the optic nerve, run forward in the perichoroidal space, and supply the bloodvessels of the choroid. After reaching the iris they form a plexus around its attached margin; from this are derived non-medullated fibers which end in the Sphincter and Dilatator pupillæ their exact mode of termination has not been ascertained. Other fibers from the plexus end in a net-work on the anterior surface of the iris. The fibers derived through the motor root of the ciliary ganglion from the oculomotor nerve, supply the Sphincter, while those derived from the sympathetic supply the Dilatator.
Membrana Pupillaris.In the fetus, the pupil is closed by a delicate vascular membrane, the membrana pupillaris, which divides the space in which the iris is suspended into two distinct chambers. The vessels of this membrane are partly derived from those of the margin of the iris and partly from those of the capsule of the lens; they have a looped arrangement, and converge toward each other without anastomosing. About the sixth month the membrane begins to disappear by absorption from the center toward the circumference, and at birth only a few fragments are present; in exceptional cases it persists.
Fig. 90879 Interior of posterior half of bulb of left eye. The veins are darker in appearance than the arteries.
The Retina (tunica interna).The retina is a delicate nervous membrane, upon which the images of external objects are received. Its outer surface is in contact with the choroid; its inner with the hyaloid membrane of the vitreous body. Behind, it is continuous with the optic nerve; it gradually diminishes in thickness from behind forward, and extends nearly as far as the ciliary body, where it appears to end in a jagged margin, the ora serrata. Here the nervous tissues of the retina end, but a thin prolongation of the membrane extends forward over the back of the ciliary processes and iris, forming the pars ciliaris retinæ and pars iridica retinæ already referred to. This forward prolongation consists of the pigmentary layer of the retina together with a stratum of columnar epithelium. The retina is soft, semitransparent, and of a purple tint in the fresh state, owing to the presence of a coloring material named rhodopsin or visual purple; but it soon becomes clouded, opaque, and bleached when exposed to sunlight. Exactly in the center of the posterior part of the retina, corresponding to the axis of the eye, and at a point in which the sense of vision is most perfect, is an oval yellowish area, the macula lutea; in the macula is a central depression, the fovea centralis (Fig. 90879). At the fovea centralis the retina is exceedingly thin, and the dark color of the choroid is distinctly seen through it. About 3 mm. to the nasal side of the macula lutæ is the entrance of the optic nerve (optic disk), the circumference of which is slightly raised to form an eminence (colliculus nervi optici) (Fig. 90880); the arteria centralis retinæ pierces the center of the disk. This is the only part of the surface of the retina which is insensitive to light, and it is termed the blind spot.
Fig. 90880 The terminal portion of the optic nerve and its entrance into the eyeball, in horizontal section. (Toldt.)
Structure (Figs. 90881, 90882).The retina consists of an outer pigmented layer and an inner nervous stratum or retina proper.
The pigmented layer consists of a single stratum of cells. When viewed from the outer surface these cells are smooth and hexagonal in shape; when seen in section each cell consists of an outer non-pigmented part containing a large oval nucleus and an inner pigmented portion which extends as a series of straight thread-like processes between the rods, this being especially the case when the eye is exposed to light. In the eyes of albinos the cells of this layer are destitute of pigment.
Retina Proper.The nervous structures of the retina proper are supported by a series of nonnervous or sustentacular fibers, and, when examined microscopically by means of sections made perpendicularly to the surface of the retina, are found to consist of seven layers, named from within outward as follows:
1. Stratum opticum.
2. Ganglionic layer.
3. Inner plexiform layer.
4. Inner nuclear layer, or layer of inner granules.
5. Outer plexiform layer.
6. Outer nuclear layer, or layer of outer granules.
7. Layer of rods and cones.
1. The stratum opticum or layer of nerve fibers is formed by the expansion of the fibers of the optic nerve; it is thickest near the porus opticus, gradually diminishing toward the ora serrata. As the nerve fibers pass through the lamina cribrosa scleræ they lose their medullary sheaths and are continued onward through the choroid and retina as simple axis-cylinders. When they reach the internal surface of the retina they radiate from their point of entrance over this surface grouped in bundles, and in many places arranged in plexuses. Most of the fibers are centripetal, and are the direct continuations of the axis-cylinder processes of the cells of the ganglionic layer, but a few of them are centrifugal and ramify in the inner plexiform and inner nuclear layers, where they end in enlarged extremities.
2. The ganglionic layer consists of a single layer of large ganglion cells, except in the macula lutea, where there are several strata. The cells are somewhat flask-shaped; the rounded internal surface of each resting on the stratum opticum, and sending off an axon which is prolonged into it. From the opposite end numerous dendrites extend into the inner plexiform layer, where they branch and form flattened arborizations at different levels. The ganglion cells vary much in size, and the dendrites of the smaller ones as a rule arborize in the inner plexiform layer as soon as they enter it; while those of the larger cells ramify close to the inner nuclear layer.
Fig. 90881 Section of retina. (Magnified.)
3. The inner plexiform layer is made up of a dense reticulum of minute fibrils formed by the interlacement of the dendrites of the ganglion cells with those of the cells of the inner nuclear layer; within this reticulum a few branched spongioblasts are sometimes imbedded.
Fig. 90882 Plan of retinal neurons. (After Cajal.)
4. The inner nuclear layer or layer of inner granules is made up of a number of closely packed cells, of which there are three varieties, viz.: bipolar cells, horizontal cells, and amacrine cells.
The bipolar cells, by far the most numerous, are round or oval in shape, and each is prolonged into an inner and an outer process. They are divisible into rod bipolars and cone bipolars. The inner processes of the rod bipolars run through the inner plexiform layer and arborize around the bodies of the cells of the ganglionic layer; their outer processes end in the outer plexiform layer in tufts of fibrils around the button-like ends of the inner processes of the rod granules. The inner processes of the cone bipolars ramify in the inner plexiform layer in contact with the dendrites of the ganglionic cells.
The horizontal cells lie in the outer part of the inner nuclear layer and possess somewhat flattened cell bodies. Their dendrites divide into numerous branches in the outer plexiform layer, while their axons run horizontally for some distance and finally ramify in the same layer.
The amacrine cells are placed in the inner part of the inner nuclear layer, and are so named because they have not yet been shown to possess axis-cylinder processes. Their dendrites undergo extensive ramification in the inner plexiform layer.
5. The outer plexiform layer is much thinner than the inner; but, like it, consists of a dense net-work of minute fibrils derived from the processes of the horizontal cells of the preceding layer, and the outer processes of the rod and cone bipolar granules, which ramify in it, forming arborizations around the enlarged ends of the rod fibers and with the branched foot plates of the cone fibers.
6. The outer nuclear layer or layer of outer granules, like the inner nuclear layer, contains several strata of oval nuclear bodies; they are of two kinds, viz.: rod and cone granules, so named on account of their being respectively connected with the rods and cones of the next layer. The rod granules are much the more numerous, and are placed at different levels throughout the layer. Their nuclei present a peculiar cross-striped appearance, and prolonged from either extremity of each cell is a fine process; the outer process is continuous with a single rod of the layer of rods and cones; the inner ends in the outer plexiform layer in an enlarged extremity, and is imbedded in the tuft into which the outer processes of the rod bipolar cells break up. In its course it presents numerous varicosities. The cone granules, fewer in number than the rod granules, are placed close to the membrana limitans externa, through which they are continuous with the cones of the layer of rods and cones. They do not present any cross-striation, but contain a pyriform nucleus, which almost completely fills the cell. From the inner extremity of the granule a thick process passes into the outer plexiform layer, and there expands into a pyramidal enlargement or foot plate, from which are given off numerous fine fibrils, that come in contact with the outer processes of the cone bipolars.
7. The Layer of Rods and Cones (Jacobs membrane).The elements composing this layer are of two kinds, rods and cones, the former being much more numerous than the latter except in the macula lutea. The rods are cylindrical, of nearly uniform thickness, and are arranged perpendicularly to the surface. Each rod consists of two segments, an outer and inner, of about equal lengths. The segments differ from each other as regards refraction and in their behavior toward coloring reagents; the inner segment is stained by carmine, iodine, etc.; the outer segment is not stained by these reagents, but is colored yellowish brown by osmic acid. The outer segment is marked by transverse striæ, and tends to break up into a number of thin disks superimposed on one another; it also exhibits faint longitudinal markings. The deeper part of the inner segment is indistinctly granular; its more superficial part presents a longitudinal striation, being composed of fine, bright, highly refracting fibrils. The visual purple or rhodopsin is found only in the outer segments.
The cones are conical or flask-shaped, their broad ends resting upon the membrana limitans externa, the narrow-pointed extremity being turned to the choroid. Like the rods, each is made up of two segments, outer and inner; the outer segment is a short conical process, which, like the outer segment of the rod, exhibits transverse striæ. The inner segment resembles the inner segment of the rods in structure, presenting a superficial striated and deep granular part, but differs from it in size and shape, being bulged out laterally and flask-shaped. The chemical and optical characters of the two portions are identical with those of the rods.
Supporting Frame-work of the Retina.The nervous layers of the retina are connected together by a supporting frame-work, formed by the sustentacular fibers of Müller; these fibers pass through all the nervous layers, except that of the rods and cones. Each begins on the inner surface of the retina by an expanded, often forked base, which sometimes contains a spheroidal body staining deeply with hematoxylin, the edges of the bases of adjoining fibers being united to form the membrana limitans interna. As the fibers pass through the nerve fiber and ganglionic layers they give off a few lateral branches; in the inner nuclear layer they give off numerous lateral processes for the support of the bipolar cells, while in the outer nuclear layer they form a network around the rod- and cone-fibrils, and unite to form the membrana limitans externa at the bases of the rods and cones. At the level of the inner nuclear layer each sustentacular fiber contains a clear oval nucleus.
Macula Lutea and Fovea Centralis.In the macula lutea the nerve fibers are wanting as a continuous layer, the ganglionic layer consists of several strata of cells, there are no rods, but only cones, which are longer and narrower than in other parts, and in the outer nuclear layer there are only cone-granules, the processes of which are very long and arranged in curved lines. In the fovea centralis the only parts present are (1) the cones; (2) the outer nuclear layer, the cone-fibers of which are almost horizontal in direction; (3) an exceedingly thin inner plexiform layer. The pigmented layer is thicker and its pigment more pronounced than elsewhere. The color of the macula seems to imbue all the layers except that of the rods and cones; it is of a rich yellow, deepest toward the center of the macula, and does not appear to be due to pigment cells, but simply to a staining of the constituent parts.
At the ora serrata the nervous layers of the retina end abruptly, and the retina is continued onward as a single layer of columnar cells covered by the pigmented layer. This double layer is known as the pars ciliaris retinæ, and can be traced forward from the ciliary processes on to the back of the iris, where it is termed the pars iridica retinæ or uvea.
The arteria centralis retinæ (Fig. 90879) and its accompanying vein pierce the optic nerve, and enter the bulb of the eye through the porus opticus. The artery immediately bifurcates into an upper and a lower branch, and each of these again divides into a medial or nasal and a lateral or temporal branch, which at first run between the hyaloid membrane and the nervous layer; but they soon enter the latter, and pass forward, dividing dichotomously. From these branches a minute capillary plexus is given off, which does not extend beyond the inner nuclear layer. The macula receives two small branches (superior and inferior macular arteries) from the temporal branches and small twigs directly from the central artery; these do not, however, reach as far as the fovea centralis, which has no bloodvessels. The branches of the arteria centralis retinæ do not anastomose with each otherin other words they are terminal arteries. In the fetus, a small vessel, the arteria hyaloidea, passes forward as a continuation of the arteria centralis retinæ through the vitreous humor to the posterior surface of the capsule of the lens.