Book - A textbook of histology, including microscopic technic (1910) Special Histology 8

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Böhm AA. and M. Von Davidoff. (translated Huber GC.) A textbook of histology, including microscopic technic. (1910) Second Edn. W. B. Saunders Company, Philadelphia and London.

A Textbook of Histology (1910): Introduction To Microscopic Technic | General Histology | I. The Cell | II. Tissues | Special Histology | I. Blood And Blood-Forming Organs, Heart, Blood-Vessels, And Lymph- Vessels | II. Circulatory System | III. Digestive Organs | IV. Organs Of Respiration | V. Genito-Urinary Organs | VI. The Skin and its Appendages | VII. The Central Nervous System | VIII. Eye | IX. Organ of Hearing | X. Organ of Smell | Illustrations - Online Histology
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Special Histology

VIII. The Eye

A. General Structure

THE organ of vision consists of the eyeball, or bulbus oculi, and the entering optic nerve.

In the eyeball we distinguish three tunics : (i) a dense external coat, the tunica fibrosa or externa, which may be regarded as a continuation of the dura mater, consisting of an anterior transparent structure, called the cornea, and the remaining portion, known as the tunica sclerotica, or, briefly, the sclera ; (2) within the tunica fibrosa a vascular tunic, the tunica vasculosa or media, subdivided into the choroid, ciliary body, and iris ; (3) an inner coat, the tunica interna, which consists of two layers, the inner being the retina ; the outer, the pigment membrane. The latter lines the internal surface of the tunica vasculosa throughout. Within the eyeball are the aqueous humor, the lens, and the vitreous body. The lens is attached to the ciliary body by a special accessory apparatus the zomda ciliaris. These two structures the lens and its fixation apparatus divide the cavity of the eyeball into two principal chambers, the one containing the aqueous humor and the other the vitreous. The former is further subdivided by the iris into an anterior and a posterior chamber. During life the latter is only a narrow capillary cleft.

B. Development of the Eye

In man the eyes begin to develop during the fourth week of embryonic life, and at first consist of a pair of ventrolateral diverticula, projecting from the anterior brain vesicle. These evaginations gradually push outward toward the ectoderm, and are then known as the primary optic vesicles. The slender commissural segments connecting the vesicles with the developing brain are termed the optic stalks.

Very soon a process of invagination takes place ; that portion of the vesicular wall nearest the ectoderm is pushed inward, thus forming a double-walled cup the secondary optic vesicle, or optic cup. An internal and an external wall may now be differentiated, continuous at the margin of the cup. At the same time a disc-like thickening of the adjacent ectoderm sinks inward toward the mouth of the cup-shaped optic vesicle, forming the first trace of the lens. During the development of the secondary optic vesicle a groove

Fig. 35 2 . Schematic diagram of the eye (after Leber and Flemming) : a, Vena vorticosa ; b, choroid ; /, lens.

is formed on its ventral side, extending from the marginal ring into the optic stalk. This is the embryonic optic fissure, or the choroidal fissure. At the edges of the groove the two layers of the optic cup are continuous. This groove serves for the penetration of mesoblastic tissue and blood-vessels into the interior of the optic cup, and in its wall the fibers of the optic nerve develop.

The outer layer of the secondary optic vesicle becomes the pigment membrane ; the inner, the retina. The optic nerve-fibers consist not only of the centripetal neuraxes of certain ganglion cells in the retina, but also of centrifugal neuraxes, which pass out from the brain (Froriep).

The invaginating ectoderm which later constitutes the lens is constricted off from the remaining ectoderm in the shape of a vesicle, the mesial half of which forms the lens fibers by a longitudinal growth of its cells, while the lateral portion forms the thin anterior epithelial capsule of the lens. The epithelium of the ectoderm external to the lens differentiates later into the external epithelium of the cornea and conjunctiva, neither of which structures is at this stage sharply defined from the remaining ectoderm. It is only during the development of the eyelids that a distinct demarcation is established. All the remaining portions of the eye, as the vitreous body, the vascular tunic with the iris, the sclera with the substantia propria of the cornea and the cells of Descemet's layer, are products of the mesoderm.

C. Tunica Fibrosa Ocull

1. The Sclera

The Template:Sclera is the dense fibrous tissue covering of the eyeball, and is directly continuous with the transparent cornea. At the posterior mesial portion of the eyeball, the sclera is perforated for the entrance of the optic nerve, this region being known as the lamina cribrosa. The sclera consists of bundles of connective-tissue fibers arranged in equatorial and meridional layers. At the external scleral sulcus, in the vicinity of the cornea, the arrangement of the fibers is principally equatorial. The tendons of the ocular muscles are continuous with the scleral fibers in such a manner that those of the straight muscles fuse with the meridional fibers, while those of the oblique muscles are continuous with the equatorial fibers. In the sclera are many lymph-channels communicating with those of the cornea. They are much coarser and more irregularly arranged than those of the cornea, and in this respect simulate the lymphchannels found in aponeuroses. Pigmentation is constantly present at the corneal margin, in the vicinity of the optic nerve entrance, and also on the surface next the choroid. The innermost pigment layer of the sclera is lined by a layer of flattened endothelial cells, and is regarded by some as a separate membrane, known as the lamina fusca ; generally, however, it is regarded as forming a part of the outermost layer of the choroid (lamina suprachoroidea). The external surface of the sclera also presents a layer of flattened endothelial cells, belonging to the capsule of Tenon. Anteriorly, the mobile scleral conjunctiva is attached to the sclera by a loose connective tissue containing elastic fibers.

The cornea is inserted into the sclera very much as a watchcrystal is fitted into its frame. At the sclerocorneal junction is found an annular venous sinus, the canal of Schlemm, which may appear as a single canal or as several canals separated by incomplete fibrous septa. Anteriorly and externally this canal is bounded by the cornea and sclera ; internally, it is partly bounded by the origin of the ciliary muscle. The sclera comprises, therefore, one half of the canal-wall, and presents a corresponding circular sulcus, the so-called inner scleral sulcus.

The blood-vessels of the sclera are derived from the anterior and posterior ciliary vessels. The capillaries enter either into the ciliary veins or into the venae vorticosae. The numerous remaining vessels traverse the sclera, extending to the choroid, iris, or scleral margin. At the corneal margin the capillaries form loops.

2. The Cornea

The cornea is made up of the following layers : (i) the anterior or corneal epithelium ; (2) the anterior elastic membrane, or Bowman's membrane ; (3) the ground-substance of the cornea, or substantia propria ; (4) Descemet's membrane; (5) the endothelium of Descemet's membrane.

At the center of the human cornea the epithelium consists of from six to eight layers of cells, being somewhat thicker near the corneal margin. Its basilar surface is smooth and there are no connective-tissue papillae. The basal epithelial layer is composed of cylindric cells of irregular height ; the following layers contain irregular polygonal cells, superficial layers consist of flattened cells. The cells of the corneal epithelium are all provided with short prickles, which are, however, very difficult to demonstrate, and between are found lymph-canaliculi. The lower surfaces of the basal cells also possess short processes which penetrate into the anterior basement membrane.

Fig. 353. Section through the anterior portion while the two or three most of human cornea > X 50.

In man the anterior elastic or Bowman's membrane is quite thick, measuring from 6 to 8 tj. in thickness and is apparently homogeneous, but may be separated into fibrils by means of certain reagents. In structure it belongs neither to the elastic nor to the white fibrous type of connective tissue, and may be regarded as a basement membrane. Numerous nerve-fibers penetrate its pores to enter the epithelium. The thickness of this membrane decreases toward the sclera, and it finally disappears about I mm. from the latter.

The substantia propria consists of connective-tissue fibrils grouped into bundles and lamellae. Chemically they do not differ from true connective-tissue fibers (Morochowetz), but are doubly refracting, although the cornea as a whole yields chondrin and not glutin on boiling. There are about sixty lamellae in the human cornea. The fibrils composing each lamella are cemented together and run parallel to one another as well as to the surface of the cornea, but they are so arranged that the fibrils of each lamella cross those of the immediately preceding one at an angle of about twelve degrees. The lamellae themselves are likewise closely cemented to one another. The most superficial lamella, lying immediately beneath the anterior elastic membrane, is composed of finer fibers, the course of which is oblique to the surface of the cornea. Between the anterior and posterior elastic membranes are bundles of fibers, which perforate the various lamellae of the cornea and are consequently known as the perforating or arcuate fibers. Between the lamellae are peculiar, flattened cells, possessing irregular or lamella-like processes, the fixed corneal corpuscles ; these lie in special cavities in the ground substance of the substantia propria, which are known as corneal spaces. In these spaces there are also found a varying number of leucocytes. By means of various methods (silver nitrate and gold chlorid treatment), these corneal spaces may be shown to be part of a complicated lymphatic system, comparable to the lymph-canalicular system of fibrous connective tissue. This system of canals is also in communication with the lymph-channels at the corneal margin.

Fig. 354. Corneal spaces of a dog ; X 640.

The posterior elastic or Descemet's membrane is not so intimately connected with the substantia propria as Bowman's membrane. It is thinnest at the center of the cornea, and becomes thicker toward the margin. It may be separated into finer lamellae, is very elastic, resists acids and alkalies, but is digested by trypsin.

At the periphery that is, at the edge of the cornea Descemet's membrane goes over into the fibers of the ligamentum pectinatum.

The endothelium of Descemet's membrane consists of low, quite regular hexagonal cells, which in certain vertebrates (dove, duck, rabbit) are peculiar in that a fibrillar structure may be seen in that portion of each cell nearest the posterior elastic membrane. By means of these fibers, not only adjacent cells, but also those further apart, are joined together. Thus we have here to a marked degree the formation of fibers which penetrate the cells and connect them with one another, conditions already met with in the prickle-cells of the epidermis.

The cornea is nonvascular. In fetal life, however, the capillaries from the anterior ciliary arteries form a precorneal vascular network immediately beneath the epithelium, a structure which is obliterated shortly before birth and only rarely seen in the newborn. Its remains are found at the corneal limbus either as an episcleral or conjunctival network of marginal capillary loops. Fine branches of the anterior ciliary arteries extend superficially along the sclera to the corneal margin, and form here a network of capillaries also ending in loops, from which numerous veins arise, constituting a corresponding network emptying into the anterior ciliary veins. The conjunctival vessels likewise form a network of marginal loops at the corneal limbus, and are connected with the episcleral vessels (Leber). Under pathologic conditions the cornea may become vascularized from the marginal episcleral network.

The nerves of the cornea are derived from the sensory fibers of the ciliary nerves, which form a plexus at the corneal margin ; from this, nonmedullated fibers penetrate the cornea itself and form two plexuses, a superficial and a ground plexus ; the latter is distributed throughout the whole substantia propria with the exception of its inner third (Ranvier, 81). The two plexuses are connected by numerous anastomoses. At one time it was supposed that direct communication existed between the corneal corpuscles and the nervefibers of both plexuses. This view, however, contradicts the generally accepted neurone theory.

Nerve-fibers from the superficial plexus pass through the anterior, elastic membrane and form a plexus over the posterior surface of the epithelium, known as the subcpithelial plexus. From the latter nerve-fibers extend between the epithelial cells, terminating in telodendria with long slender nerve-fibrils, which end in small nodules. Many of the fibrils reach almost to the surface of the epithelium (Rollet, 71 ; Ranvier, 81 ; Dogiel, 90).

Smirnow (1900) has described a rich nerve -supply for the sclera, consisting of both medullated sensory fibers and nonmedullated sympathetic fibers, derived mainly from the ciliary nerves. The sympathetic fibers supply the blood-vessels; the sensory fibers terminate in free endings between the connective-tissue lamellae.

D. The Vascular Tunic of the Eye. the Choroid, the Ciliary Body, and the Iris

From without inward the following layers may be differentiated in the choroid : (i) the lamina suprachoroidea ; (2) the lamina vasculosa Halleri ; (3) the lamina choriocapillaris ; and (4) the glassy layer, or vitreous membrane.

The lamina suprachoroidea consists of a number of loosely arranged, branching and anastomosing bundles and lamellae of fibrous tissue, joined directly to the sclera. These bundles and lamellae consist of white fibrous connective tissue containing numerous elastic fibers, among which a few connective-tissue cells are distributed. Pigment cells are also present in varying numbers. The bundles and lamellae are covered by endothelial cells, and the spaces and clefts between them, and between the lamina suprachoroidea and the lamina fusca, constitute a system of lymph-channels the peric /toroidal lymph-spaces In the inner portion of this layer is found a narrow zone, in the human eye only about 10 fj. in thickness, consisting largely of elastic fibers and free from pigment cells, known as the boundary zone. This zone is somewhat thicker in many mammals, and in some of these presents a characteristic structure. In the eyes of ruminants and horses this zone consists of several layers of connective-tissue bundles, and is known as the tapetwn fibrositm. It gives the peculiar luster often seen in the eyes of these animals. In the eyes of carnivora this zone consists of several layers of endothelioid cells, containing in their protoplasm numerous small crystals and forming the iridescent layer known as the tapetum cellulosinn.

Fig. 355. Section through the human choroid ; X I 3 The lamina vasculosa of the choroid is also composed of similar lamellae, which, however, are more closely arranged. The bloodvessels constitute the principal portion of this layer, the vessels being of considerable caliber, not capillaries. They are so distributed that the larger vessels, the veins, occupy the outer layer of the lamina vasculosa. The venous vessels converge toward four points of the eyeball, forming at the center of each quadrant one of the four vence vorticosa. The arteries, on the other hand, describe a more meridional course.

The lamina choriocapillaris contains no pigment and consists principally of capillary vessels, which form an especially dense network in the neighborhood of the macula lutea. As the venous capillaries become confluent and form smaller veins, the latter arrange themselves in long, radially directed networks, and form in this way the more or less pronounced stellulce vasculosce (Winslowii).

The vitreous or glassy membrane is a very thin (2 /*) homogeneous membrane which shows on its outer surface the impressions of the vessels composing the lamina choriocapillaris, and on its inner surface those of the pigment epithelium of the retina.

At the ora serrata the choroid changes in character ; from this region forward, the choroidal tissue assumes more the appearance of ordinary connective tissue, and the choriocapillary layer is wanting.

The region of the vascular coat extending from the ora serrata to the base of the iris is known as the ciliary body. Its posterior portion, about 4 mm. broad, the orbiculus ciliaris, is slightly thicker than the choroid, and presents on its inner surface numerous small folds, meridionally placed, consisting of connective tissue and bloodvessels. Anterior to the orbiculus ciliaris the ciliary body is thickened by a development of nonstriated muscle the ciliary muscle (see below) ; and on the inner surface of this annular thickening are placed about seventy triangular folds, meridionally arranged the ciliary processes. The attached border of these processes measures from 2 to 3 mm. The anterior border attains a height of about I mm. On and between these folds are found numerous small secondary folds or processes of irregular shape. The ciliary processes consist of fibrous connective tissue and numerous smaller and larger vessels, which have in the main a meridional arrangement. The vitreous membrane extends over the ciliary body, attaining in the region of the ciliary processes a thickness of 3 /J. or 4 fj.. Internal to the vitreous membrane, the ciliary body is covered by a double layer of epithelial cells, the continuation forward of the retina {pars ciliaris retince). Of these, the outer layer is composed of cells, which are deeply pigmented, and are of cubic or short columnar shape, and derived from the outer layer of the secondary optic vesicle, while the cells of the inner layer are nonpigmented and of columnar shape, and are developed from the inner layer of the secondary optic vesicle. In the region of the ciliary processes their epithelial lining presents here and there evaginations of glandular appearance, lined by the unpigmented cells. These evaginations are known as ciliary glands, and to them is attributed in part, at least the secretion of the fluid found in the anterior chamber of the eye ; it is, however, still a question as to whether these structures are to be regarded as true glands or simply as depressions or crypts in the epithelium.

The ciliary muscle is bounded anteriorly (toward the anterior chamber) by the ligamentum pectinatum iridis, externally by the cornea and sclera, posteriorly by the orbiculus ciliaris, and internally by the ciliary processes. It consists of nonstriated musclefibers in the majority of vertebrates. This muscle is divided into three portions. The outer or meridional division extends from the posterior elastic lamina of the cornea and its continuation, forming the inner wall of the sinus venosus sclerae, to the posterior portion of the ciliary ring. The origin of the middle division is identical with that of the outer, but its fibers (assuming that we have before us a meridional section) spread out like a fan, and occupy a large area at their insertion into the ciliary ring and ciliary processes. The radial course of these fibers is mterrupted by circular bundles. The third or inner division {fibrce circulares, fibers of Mutter} is situated between the ligamentum pectinatum, the ciliary processes, and the middle portion of the muscle just mentioned, and is thus near the base of the iris.

Fig. 356. Meridional section of the human ciliary body ; X 2O

Between the ciliary muscle and the posterior elastic membrane of the cornea is an intermediate, richly cellular tissue, which maybe regarded as a continuation of this elastic membrane, and which forms a part of the wall of the sinus venosus. Another structure internal to the foregoing and directed posteriorly is foe.' ligamentum pectinatum iridis, which encircles the anterior chamber and is a continuation of Descemet's membrane to the base of the iris. It consists of fibers and lamellae lined by endothelial cells, and bounds certain intercommunicating spaces lying in the ligament, known as the spaces of Fontana. The latter communicate on the one side with the perivascular spaces of the sinus venosus sclerae (canal of Schlemm), and on the other with the anterior chamber.

The iris must be looked upon as a continuation of the choroid, and is connected at its anterior peripheral portion with the ligamentum pectinatum. The iris possesses the following layers, beginning anteriorly : (i) the anterior endothelium; (2) the ground layer, or stroma of iris, together with the sphincter muscle of the pupil ; and (3) the two-layered, pigmented epithelium the pars iridica retinae, of which the anterior is in part replaced by a peculiar muscle tissue, developed from the ectoderm and forming the dilator of the pupil.

The anterior endothelium is a single layer of irregularly polygonal, nonpigmented cells, and is directly continuous with the endothelium of the pectinate ligament.

The ground-layer or stroma of iris consists anteriorly of a fine reticulate tissue rich in cellular elements (reticulate layer). The remaining strata which form the bulk of the ground-layer constitute its vascular layer. The vessels are here peculiar in that they are covered by coarse, circular, connective -tissue fibers forming vascular sheaths. There is also an entire absence of muscular tissue in the vessel walls. The nerves, too, are enveloped by a dense connective tissue. In all eyes (except the albinotic) pigment is found in the connective tissue.

On the posterior inner surface of the ground-layer is a band of smooth muscle-fibers encircling the pupil the sphincter muscle of the pupil. Posterior to this and in intimate relation with the layer of pigmented epithelium covering the posterior surface of the iris is a layer of spindle-shaped cells having a radial arrangement and containing pigment. Closer microscopic inspection reveals the fact that in all probability these elements represent muscular tissue. Here, therefore, we have to deal with a dilator muscle of the pupil. There has been much discussion as to the existence and structure of this muscle. Recent investigations (Szili) indicate that it is developed from the outer layer of the secondary optic vesicle.

The posterior epithelium is the direct continuation of the two epithelial layers of the ciliary body, and represents the anterior portion of the secondary optic vesicle, the two layers being continuous at the margin of the pupil. In the iris both layers of cells, so far as they exist, are pigmented.

The arteries of the choroid are derived from the short posterior ciliary, the long ciliary, and the anterior ciliary arteries. The short posterior ciliary arteries penetrate the sclera in the vicinity of the optic nerve, where they anastomose with branches from the retinal vessels, and spread through the choroid, where they form the choriocapillary layer. The long posterior ciliary arteries (a mesial and a lateral) penetrate the sclera and course forward between choroid and sclera to the ciliary body, forming there the circulus arteriosus iridis major; they also supply the ciliary muscle, the ciliary processes, and the iris, and anastomose in the ciliary ring with the branches of the short posterior and anterior ciliary arteries. The latter lie beside and partly within the straight ocular muscles, penetrating the latter at the anterior margin of the sclera ; they give off branches to the circulus arteriosus iridis major and to the ciliary muscles, anastomosing at the same time with the posterior ciliary arteries. (Compare Figs. 352 and 357.) Within the iris the blood-vessels generally take a radial direction, but also anastomose with one another, forming capillaries, and subsequently the circulus arteriosus iridis minor at the inner pupillary margin. From the region supplied by the posterior ciliary arteries most of the blood is carried toward the vorticose veins. The anterior ciliary veins convey the blood coming from the arteries of the same name. Into these veins is also poured the blood from the veins lying in the canal of Schlemm, the canal itself being in reality an open venous sinus. Besides this, these veins convey also venous blood from the conjunctiva (Leber). The nonstriated muscle of the ciliary body and iris receives its innervation through sympathetic nerve-fibers, neuraxes of sympathetic neurones, the cell- bodies of which are situated either in the ciliary ganglia or in the superior cervical ganglia. The neuraxes of the sympathetic cells forming the ciliary ganglia form the short

ciliary nerves, which pierce the sclera in the neighborhood of the optic nerve and pass forward, to terminate in the muscle of the ciliary body and the sphincter muscle of the pupil. Stimulation of these nerves causes a contraction of the ciliary muscle and a closure of the pupil'. The cell-bodies of the sympathetic neurones forming the ciliary ganglia are surrounded by pericellular plexuses, the terminations of small medullated nerve -fibers (white rami fibers) which reach the ciliary ganglia through the oculomotor nerves. Neuraxes of sympathetic neurones, the cell-bodies of which are situated in the superior cervical ganglia, reach the eye through the cavernous plexuses, to terminate, it is thought, in part, at least, in the dilator of the iris, since stimulation of these nerves causes a dilatation of the pupils. The cell-bodies of these sympathetic neurones are surrounded by pericellular plexuses, the terminations of white rami fibers which leave the spinal cord through the first, second, and third thoracic nerves (Langley), and which reach the superior cervical ganglia through the cervical sympathetic.

Fig. 357. Injected blood-vessels of the human choroid and iris ; X 7

Melkirch and Agababow have shown that numerous sensory nerves terminate in free sensory endings in the connective tissue of the ciliary body and iris. The sensory nerve-supply of the iris is especially rich.

E. The Internal or Nervous Tunic of the Eye

This tunic is composed of two layers : the outer, or stratum pigmenti ; and the inner, or retina.

1. The Pigment Layer

The pigment layer develops, as we have seen, from the outer layer of the secondary optic vesicle. It consists of regular hexagonal cells, 12 fj. to 1 8 ii in length and 9 // in breadth, which contain black pigment in the form of granules. The inner surfaces of these cells possess long, thread-like and fringe-like processes, between which project the external segments of the rods and cones of the retina, yet to be described. The nuclei of the pigment cells lie in the outer ends of the cells, the so-called basal plates, and are not pigmented. The distribution of the pigment varies according to the illumination of the retina. If the latter be darkened, the pigment collects at the outer portion of each cell ; if illuminated, the pigment is evenly distributed throughout the whole cell. The pigment granules are therefore mobile (Kiihne, 79).

2. The Retina

The retina has not the same structure throughout. In certain areas peculiarities are noticeable which must be described in detail ; such areas are : (i) the macula lutea ; (2) the region of the papilla (papilla nervi optici) ; (3) the ora serrata ; (4) the pars ciliaris retinae ; and (5) the pars iridica retinae.

We shall begin with the consideration of that portion of the retina lying between the ora serrata and the optic papilla (exclusive of the macula lutea).

From without inward, we differentiate: (i) the layer of visual cells, including the outer nuclear layer ; (2) the outer molecular (plexiform) layer ; (3) the inner nuclear or granular layer ; (4) the inner molecular (plexiform) layer ; (5) the ganglion-cell layer ; (6) the nerve-fiber layer. Besides these, we must also consider the supporting tissue of the retina and Miiller's fibers, together with the internal and external limiting membranes.

The visual cells are either rod-visual cells or cone-visual cells. The rod-visual cells consist of a rod and a rod-fiber with its nucleus. The rod (40 // to 50 // in length) consists of two segments, an outer and an inner, the former of which is doubly refractive and may be separated into numerous transverse discs by the action of certain reagents. The inner is less transparent than the outer segment, and its inner end shows a fine superficial longitudinal striation due to impressions from the fiber-baskets formed by Miiller's fibers. In the lower classes of vertebrates a rod-ellipsoid (a fibrillar structure) may easily be demonstrated in the outer region of each inner portion ; in many mammalia and in man the demonstration of this is more difficult. This structure is a planoconvex, longitudinally striated body, the plane surface of which is coincident with the external surface of the inner segment, its inner convex surface lying at the junction of the outer and middle thirds of the inner segment. The rod-fibers extend as far as the outer molecular layer of the retina, where they end in small spheric swellings. The nuclei of the rod-visual cells are found at varying points within the rodfibers, but rarely close to the inner segment. When treated with certain fixing agents and stains, the rod-nuclei of certain animals (cat and rabbit) are seen to show several zones, which stain alternately light and dark (striation of the rod-nuclei). This striation is not gen erally observed in the rod-nuclei of the human retina.

Fig. 358. Section of the human retina ; X 7

The cone-visual cells consist, similarly to the rod-visual cells, of a cone and a cone-fiber with its nucleus. The cone (15 fi to 25 fJL in length) is, as a whole, shorter than the rod, and its inner segment is considerably broader than that of the rod. The cone ellipsoid comprises the outer two-thirds of the inner segment, and the outer segment has a more conical shape. The cone-fiber likewise extends as far as the outer molecular layer, where it ends in a branched basal plate. Its somewhat larger nucleus is always found in the vicinity of the inner segment of the cone. The inner surfaces of the inner segments, not only of the cone-cells, but also of the rod-visual cells, lie in one plane, corresponding to the external limiting membrane, a structure composed of the sustentacular fibers of Miiller. The rod-fibers and cone-fibers, with the nuclei of the rod- and cone-visual cells, lie between the external limiting membrane and the outer molecular layer. It will be observed, therefore, that the visual cells include the layer of rods and cones and the outer nuclear layer.

The outer molecular layer consists : (i) of the ramifications of Miiller's fibers ; (2) of the knob and tuft-like endings of the visual cells ; and (3) of the dendritic processes of the bipolar cells of the inner nuclear layer. These structures will be considered more in detail in discussing the relations of the elements comprising the retina.

The inner nuclear layer contains: (i) the nucleated stratum of Miiller's sustentacular fibers ; (2) ganglion cells situated in the outer region of the layer and extending in a horizontal direction ; (3) bipolar ganglion cells with oval nuclei, densely placed at various depths of the layer and vertical to it ; (4) amacrine cells (neurones, apparently without neuraxes) lying close to the inner margin of the layer and forming with their larger nuclei a nearly continuous layer of so-called spongioblasts. The numerous processes of these spongioblasts lie in the inner molecular layer, the composition of which will be further discussed later.

The ganglion-cell layer of the optic nerve consists, aside from centrifugal neuraxes and the fibers of Miiller, which are here present, of multipolar ganglion cells, the dendrites of which extend outward and the neuraxes of which are directed toward the optic nerve-fiber layer. These cells vary in size, and their nuclei are typical, being relatively large, deficient in chromatin, and always provided with large, distinct nucleoli. In man the optic nervefibers of the retina are nonmedullated.

All these structures are typical of that portion of the retina lying behind the ora serrata. The retina in the vicinity of the optic papilla and macula lutea must be taken up separately.

3. Region Of The Optic Papilla

The optic papilla is the point of entrance of the optic nerve into the retina. At the center of the papilla, in the region where the nerve-fibers spread out radially in order to supply the various areas of the retina, is a small, funnel-shaped depression, the physiologic excavation. The fibers of the optic nerve lose their medullary sheaths during their passage through the sclera and choroid, and then continue to the inner surface of the retina, over which they spread in a layer which gradually becomes thinner toward the ora serrata. On account of the deflection of the nerve- fibers, and because, during their passage through the sclera, they lose their medullary sheaths at one and the same point, the optic nerve becomes suddenly thinner. The result is a deeply indented circular depression in this region. On this depression border the three ocular tunics. At this point the retina is interrupted, the outer layers extending to the bottom of the depression, while the inner cease at its margin. In many cases the outer layers of the retina are separated from the optic nerve by a thin lamina of supporting tissue (intermediate tissue).

Fig. 359. Section through point of entrance of human optic nerve ; X 4

At the center of the macula lutea is a trough-like depression, the fovea centralis, the deepest part of which, \hzfnndits, lies very close to the visual axis. Here the layers of the retina are practically reduced to the cone-visual cells. The margin of this depression is somewhat thickened, owing to an increase in the thickness of the nerve-fiber and ganglion-cell layers. Toward the fundus of the fovea each of the four inner retinal layers becomes reduced in thickness, the inner layer first and the three others in their order : the inner molecular layer, however, seems to extend as far as the fundus. As we have seen, only the cone-visual cells are found in the fovea centralis, there being an entire absence of the rod-visual cells. Since the nuclei of the cone -visual cells are in the immediate neighborhood of the cones, and since the cone-fibers, in order to reach the outer molecular layer, must here describe a curve, there arises a peculiar 1 ayer, composed of obliquely directed fibers, known as the outer fiber-layer or Henle's fiber layer. In other words, the fibers of this region are more distinctly seen because they are not covered by the rod-nuclei and rod-fibers.

Fig. 360. Section through human macula lutea and fovea centralis ; X 1 S- As a result of treatment with certain reagents, the fovea centralis is deeper and the margin more precipitous than during life.

The yellowish color of the fovea centralis is due to pigment held in solution within the layers of' the retina. The cone-visual cells themselves contain no pigment.

5. Ora Serrata, Pars Ciliaris Retinae, And Pars Iridica Retinae

In the region of the ora serrata the retina suddenly becomes thinner. As seen from the inner surface of the retina, its decrease presents the appearance of an irregular curve rather than of the segment of a sphere. Shortly before the retina terminates, its layers become markedly reduced, certain ones disappearing entirely ; first the nerve-fiber layer, then the ganglion-cell layer and cone- and rodvisual cells, their place being taken by an indifferent epithelium. The inner molecular layer of the retina gradually loses the processes which penetrate inward. In the region of the ora serrata the sustentacular fibers are markedly developed. Relatively large hollow spaces are often found in the retina at the ora serrata ; they are thought to be due to edema.

The pars ciliaris retinae consists essentially of two simple layers of cells, of which the external represents the pigment layer and the internal the inner epithelium of the secondary optic vesicle. In the pars iridica retinae the arrangement is similar ; here both layers are pigmented.

6. Muller's Fibers of the Retina

Genetically, the sustentacular fibers, or fibers of Miiller, in the retina are, like the whole retina, of ectodermic origin, and represent a highly developed form of neurogliar tissue. They penetrate the retina from within and extend as far as the inner segments of the rods and cones. Each fiber represents a long, greatly modified epithelial cell, terminating in one or more broad basal plates, which come in contact with those of adjacent fibers, thus forming a sort of membrane the internal limiting membrane. Owing to its marked plasticity, each fiber presents certain peculiarities within the various layers of the retina through which it penetrates. Thus, within the molecular layers the fiber is provided with transversely directed processes and platelets. Within the nuclear layers, on .the other hand, are numerous lateral indentations, which correspond to the impressions produced by the cells of these layers. At the inner surface of the cones and rods the fibers terminate in endplates, which represent cuticular formations, and, blending with one another, form a single membrane the external limiting membrane. 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. (Via. Fig. 361.) Miiller's fibers do not appear as fibers in chrome-silver preparations, but as complicated cellular structures, as above depicted. In preparations of the retina, stained in a differential neuroglia stain (Benda's method), clearly defined fibers, stained after the manner of neuroglia fibers, may be differentiated. These fibers are in contact with or are imbedded in the protoplasm of the Miiller's fibers.

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 Ramon y Cajal (see diagram, Fig. 361) :

1. 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 (f) 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 neu raxes of which end in telodendria in the neighborhood of the cell-bodies of the nerve-cells of the ganglion-cell layer. (/9) 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, (j] Besides these, there are also bipolar cells which, as in the case of a and ft. form contact with the rod- and cone-visual cells, but end on the cell-bodies of the ganglion ceils 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 (amacrine or parareticular cells) in the inner nuclear layer send 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 several 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 layers, their ramifications spreading out in a horizontal direction. Besides all these structures the dendrites of the cells 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 : (i) 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 piamater. 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 trabeculae. 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 they possess no neurilemma. They are grouped into small bundles by septa and bands of fibrous tissue penetrating the optic nerve from the inner or pial sheath. Within these bundles the nerves are separated by neuroglia tissue, neuroglia cells and fibers, which further forms a thin sheath about each bundle. In the region of the sclera and choroid the optic nerve-fibers 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 I y 2 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 pial sheath. The optic nerve-fibers extend through the lamina cribrosa into the retina, where they spread out as the nervefiber layer in the manner previously described.

9. BLOOD-VESSELS OF THE OPTIC NERVE AND RETINA. 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. 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 vein. Both the latter again divide into two branches, the nasal and temporal arteriole and venule, known, according to their positions, as the superior and inferior nasal and temporal artery and vein.

Fig. 362. Injected blood-vessels of human retina ; surface preparation ;

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.

Fig. 363. Injected blood-vessels of human macula lutea ; surface preparation ; X 28.

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 is a tissue which consists almost entirely of fluid, containing very few fixed cellular elements and only a small number of leucocytes, which are found more particularly in its outermost portion. Thin structureless lamellae and fibers occur throughout the entire vitreous body, with the exception of the hyaloid canal. These fibrils form an interlacing network with wide meshes. They differ chemically from both the white fibrous tissue and yellow elastic fibers, resembling in some respects cuticular formations (von Ebner). These are particularly numerous at the periphery and especially in the region of the ciliary body. Toward the surface the fibrils are more densely arranged, forming the hyaloid membrane of the vitreous body, separating the latter from the retina. This membrane is somewhat thicker in the region of its close attachment around the physiologic excavation of the optic nerve and to the internal limiting membrane of the retina in the ciliary region. In the latter region the hyaloid membrane is in close relation with the epithelium of the pars ciliaris retinas. 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 is closely associated with numerous fibers, which diverge fan-like toward the lens and become blended with the outer lamella of the lens-capsule. These fibers appear to arise from the epithelium of the pars ciliaris retinas, and may be regarded as cuticular formations. Those coming from the free ends of the ciliary processes become attached along the equator of the lens and to the adjacent posterior portion of the lens-capsule. On the other hand, the fibers originating between the ciliary processes attach themselves to the anterior surface of the lens-capsule in the immediate vicinity of the equator. Together these fibers constitute the zonula ciliaris, zonulc of Zinn, or the suspensory ligament of the lens. Between these fibers of the zonula and the lens itself there is, consequently, a circular canal divided by septa, the canal of Petit, which communicates by openings with the anterior chamber.

G. The Crystalline Lens

As we have already, seen, the crystalline lens originates as an ectodermic invagination, which then frees itself from the remaining ectoderm in the shape of a vesicle and becomes transformed into the finished lens. In this process the cells of the inner wall of the vesicle become the lens-fibers, while those of the outer portion remain as the anterior epithelium of the lens. The lens is surrounded on all sides by the lens-capsule.

The lens capsule is a homogeneous membrane, nearly twice as thick on the anterior surface of the lens as on the posterior. Its chemic reactions differ from those of connective tissue, and in this respect it may be compared with the membranae propriae of glands. In sections the lens capsule appears to possess a tangential striation ; under the influence of certain reagents, and under proper preliminary treatment, lamellae may be detached from its surface which are found to be directly connected with the fibers of the suspensory ligament.

The anterior epithelium consists, in the fetus, of columnar cells , in children, of cells approaching the cubic type ; and in the adult, of decidedly flattened cells. Toward the equator of the lens, in the so-called transitional zone, the cells increase in height and gradually pass over into the lens fibers.

The lens fibers are also derivatives of epithelial cells ; they are long, flattened, hexagonal prisms, which extend through the entire thickness of the lens. In the adult the lens may be differentiated into a resistant peripheral and a softer axial substance. The surfaces of the fibers present irregularities, and it is with the help of these serrations and a cement substance that the fibers are bound together. Each fiber possesses one or more nuclei, which, although they have no constant position, are usually found in the middle of the fibers situated near the lens-axis, and in the anterior third of those at some distance from the axis. The course of the fibers in the lens is extremely complicated.

H. The Fetal Blood-Vessels of the Eye

In the eye of the embryo the vitreous body and the capsule of the lens contain blood-vessels. The vessel which later becomes the central artery of the retina passes through the space subsequently occupied by the vitreous body as far as the posterior surface of the lens (anterior hyaloid artery) and branches in the region of the posterior and anterior lens-capsule. The anterior vascular membrane of the lens capsule of the embryo is known as the membrana capsulopupillaris, and that portion corresponding to the pupil, as the membrana pupillaris. In the embryo numerous other vessels arise at the papilla and extend over the surface of the vitreous body close to the hyaloid membrane ; these are the posterior Jiyaloid arteries. These vessels later disappear. In place of the anterior hyaloid artery there remains in the vitreous humor a transparent cylindric cord containing no fibers nor lamellae, as is the case in the remaining portion of the vitreous body, and consisting of a more fluid substance ; this is the hyaloid canal, or the canal of Cloquet.

With regard to the posterior hyaloid vessels, the generally accepted theory is that they later enter into the formation of the retinal vessels. Little is known as to the details of this process ; but the fact remains that, in the rabbit, for instance, the larger branches of the retinal vessels are internal to the inner limiting membrane, and, therefore, within the vitreous body, and that they send smaller branches into the retina (His, 80).

I. Interchange of Fluids in the Eyeball

The anterior lymph-channels of the eye comprise (i) the lymph-canaliculi of the cornea, which communicate with similar structures in the sclera ; (2) the system of the anterior chamber, which is indirectly connected, on the one hand, with the canal of Schlemm by means of the spaces of Fontana, and with the stroma iridis, into which the ligamentum pectinatum extends ; while, on the other hand, it communicates with the posterior chamber and its recesses, and with the canal of Petit.

In the posterior region of the eyeball are the lymph-channels of the retina (the perivascular spaces), those of the optic nerve, the space between the pigment layer and the remaining portion of the retina (interlaminar space, Rauber), and the lymph-spaces of the choroid and sclera. The influx and efflux of intraocular fluid occur principally by means of filtration. The influx takes place through the ciliary processes ; that the choroid has to do with this process is very improbable. The efflux takes place through the veins of the canal of Schlemm, into which the fluid filters through the cement lines of the endothelial lining of the canal of Schlemm, finally emptying into the anterior ciliary veins. A posterior efflux from the vitreous body probably does not exist, or at least occurs to a very limited extent. The anterior chamber possesses no efferent lymph-vessels (Leber, 95).

J. The Protective Organs of the Eye

1. The Lids and the Conjunctiva

At the end of the second month of embryonic life the eyelids begin to develop in the shape of two folds of skin. At the end of the third month these folds come in contact in the region of what is later the palpebral fissure, and grow together at their outer epithelial margins. Shortly before birth the two lids again separate and the definitive palpebral fissure is formed.

The eyelids show three distinct layers : (i) the external cutis, which presents special structures at its free margin and continues about I mm. inward from the inner border of the free margin ; (2) the mucous membrane, or palpebral conjunctiva, beginning from this line and covering the entire internal surface j and (3) a middle layer.

1 . The cuticular portion of the eyelid consists of a thin epidermis and a dermis poorly supplied with papillae. Fine lanugo-like hairs with small sebaceous glands and a few sweat-glands are distributed over its entire surface. The cutaneous connective tissue is very loose, contains very few elastic fibers, and is supplied with pigment cells in the superficial layers. At the lid-margin the papillae are well developed and the epidermis is somewhat thickened. The anterior margin supports several rows of larger hairs, the cilia, the posterior row of which possesses, besides the sebaceous glands, modified sweat-glands, the ciliary glands of Moll, which also empty into or near the hair follicles. The ciliary glands are readily distinguished from the sweat glands ; their tubules are relatively large, often showing alternating large vesicular segments and short narrow segments. A branching of the tubules has also been observed (Huber). The eyelids are further provided with numerous glands, known as the Meibomian or tarsal glands. About thirty of these glands are found in the upper, a slightly smaller number in the lower, lids. They lie within the tissue of the tarsus vertical to the palpebral margin. Each gland consists of a tubular duct, lined by stratified squamous epithelium, beset with numerous simple or branched alveoli lined by a stratified, cubic epithelium in every respect similar to that lining the alveoli of sebaceous glands. The ducts of these glands terminate at the palpebral margin posterior to the cilia. (See Fig. 364.)

2. The conjunctival portion of the eyelids is lined by a simple pseudostratified columnar epithelium, possessing two strata of nuclei. This is continuous with the bulbar conjunctiva at the conjunctival fornix, and is characterized by the occasional presence of folds and sulci. Longitudinal folds in the upper portion of the upper lid running parallel with the lid-margin are frequently present. Goblet cells are usually found in the epithelium. According to W. Pfitzner (97), the epithelium of the conjunctiva consists of two or three strata of cells, of which the more superficial possess a cuticular margin. Certain structures which have always been regarded as goblet cells are in all probability similar to the cells of Ley dig i. e., mucous cells, which do not pour their secretion out over the surface of the epithelium. Some lymphoid tissue is always found in the stratum proprium of the mucous membrane, and occasionally it is seen to form true lymph-nodules. It is of some interest to note that a marked production of these lymph-nodules occurs in certain diseases. Such lymph-nodules are usually associated with epithelial crypts, which fact led Henle to regard them as glandular formations. Small glands with a structure similar to that of the lacrimal glands are also present in the palpebral conjunctiva ; they are known as accessory lacrimal glands and are found in the upper eyelid, at the outer angle of the conjunctival fornix. Similar glands occur also at the mesial angle of the fornix.

3. Besides the tarsus (fibrocartilage) the middle layer of the eyelid contains : (i) The musculus orbicularis oculi, which lies beneath the subcutaneous tissue. At the margin of the lid this structure gives off the musculus ciliaris Riolani, which is composed of two fasciculi separated by the tarsus. (2) The connective tissue between the bundles of the musculus orbicularis oculi. (3) The connective tissue lying behind the latter and the tarsus. In the upper lid the connective tissue mentioned under 2 and 3 is connected with the tendon of the musculus palpebralis superior. The latter is composed of smooth muscle-fibers, and is regarded as a continuation of the middle portion of the striated, voluntary musculus levator palpebrae superioris. The middle layer of the lower lid isstructurally analogous, except that here a fibrous expansion from the sheath of the inferior rectus muscle takes the place of the levator palpebrae.

Fig. 364. Vertical section of the upper eyelid of man; X *4 : af > arterial arcus tarseus ; c, cilia ; dgt, excretory duct of Meihomian gland ; glc, ciliary gland (Moll) ; McR, ciliary muscle of Riolani ; Mop, m. orbicularis palpebrarum ; Mt, nonstriated muscle-fibers of the tarsal muscle and tendon of the levator palpebrae superioris ; nlc, lymph-node of the conjunctiva palpebrse ; T, tarsus (Sobotta, "Atlas and Epitome of Histology.").

Fig. 365. Meibomian or tarsal gland, reconstructed after Bern's wax-plate method; X20.

The blood-vessels of the eyelid lie directly in front of the tarsus, and from this region supply adjacent parts ; they reach the posterior portion of the lid either by penetrating the tarsus or by encircling it (Waldeyer, 74). The lymph-vessels form a plexus in front and one behind the tarsus.

The " third eyelid," the plica semilunaris, contains, when well developed, a small plate of hyaline cartilage.

At the fornix the epithelium of the palpebral conjunctiva becomes continuous with the two- or three-layered squamous epithelium of the conjunctiva bulbi. Beneath this epithelium is found a loose fibre-elastic connective tissue, presenting subepithelial papillae, and quite vascular. In it are found medullated nerve-fibers, some of which terminate in free sensory nerve-endings in theconjunctival epithelium ; others terminate, especially near the corneal margin, in end-bulbs of Krause ; and still others may be traced to the cornea, to terminate in a manner previously described.

2. The Lacrimal Apparatus

The lacrimal apparatus consists of the lacrimal glands, their excretory ducts, the lacrimal puncta and canaliculi, the lacrimal sac, and the nasal duct.

The lacrimal gland, wnich is a branched tubular gland, is separated into two portions, of which the one lies laterally against the orbit and the other close to the upper lateral portion of the superior conjunctival fornix. The structure of the gland is, on the whole, that of a serous gland (parotid), with the difference that the intralobular ducts are not lined by a striated epithelium such as is found in the salivary tubules, and that those cells which are wedged in between the secretory elements and functionate as sustentacular cells (basketcells) are here much more highly developed.

The excretory ducts of the orbital division generally pass by the conjunctival half of the gland, taking up a few ducts from the latter as they go, and finally empty on the surface of the conjunctiva. Aside from these, the lateral portion of the gland possesses also independent ducts. All the excretory ducts are lined by columnar epithelium and surrounded by a relatively thick connective-tissue wall having inner longitudinal and outer circular fibers. From the lateral portion of the conjunctival culdesac, into which the secretion is brought by the excretory ducts of the lacrimal gland, the secretion passes into the capillary space of the sac, and is then evenly distributed by means of the sulci and papillae over the conjunctival surface of the lid. In this manner the secretion reaches the mesial angle of the lid, whence it passes through the lacrimal puncta into the lacrimal canals.

The nerve supply of the lacrimal glands is from the sympathetic nervous system. The neuraxes of sympathetic neurones accompany the gland ducts and form plexuses about the alveoli, the terminal branches of which may be traced to the gland cells.

The lacrimal canals are lined by stratified squamous epithelium, and possess a basement membrane as well as a connective-tissue layer containing circularly disposed elastic elements. Externally we find a layer of transversely striated muscle -fibers.

The lacrimal sac is provided with a simple pseudostratified columnar epithelium having two strata of nuclei. In it goblet cells are also found. The nasal duct is lined by a similar epithelium. The connective-tissue wall of the latter and that of the lacrimal sac come in contact with the periosteum ; between them is a welldeveloped vascular plexus. Stratified squamous and ciliated epithelium have been described as being present in the nasal duct, as well as mucous glands in both nasal duct and lacrimal sac. (See works of M. Schultze, 72 ; Schwalbe, 87.)


The eyes of the larger animals, after having been previously cleaned by removing the muscles and loose connective tissue, are placed in the fixing fluid and cut into two equal parts by means of an equatorial incision. Smaller eyes with thin walls may be fixed whole.

Muller's fluid, nitric acid, and Flemming's fluid are usually employed as fixing agents. After fixing in one of these fluids, different parts of the eyeball are imbedded in celloidin or celloidin -paraffin and then sectioned.

The corneal epithelium is best macerated in 33% alcohol ; the membrane of Descemet may be impregnated with silver. In order to bring the fibers of the latter into view, Nuel recommends an injection of i jc to 2 f formic acid into the anterior chamber of the eye of a dove or a rabbit, after having drawn off the aqueous humor. The cornea is then cut out, and fixed for from three to five minutes in osmic acid.

The substantia propria is examined either by means of sections or by means of teased preparations from a cornea macerated in limewater or potassium permanganate. The sections are stained with picrocarmin (Ranvier). The corneal spaces and canaliculi may be demonstrated in two ways with the aid of silver nitrate ; either the fresh cornea of a small animal is stripped of its epithelium, cauterized with a solid stick of silver nitrate, and then examined in water, in which case the corneal spaces and their canaliculi show light upon a dark ground (negative impregnation) ; or the corneae of larger animals are treated in the same manner, after which tangential sections are made with a razor, and placed in water for a few days ; in this case the corneal spaces and their canaliculi show dark upon a light ground (positive impregnation, Ranvier, 89).

By means of Altmann's oil method casts of the corneal spaces and their canaliculi may be made. Treatment by the gold method often brings out not only the nerves, but also the corneal corpuscles and their processes.

Ranvier (89) especially recommends a i% solution of the double chlorid of gold and potassium for the corneal nerves. The cornea of the frog is treated for five minutes with lemon-juice, then for a quarter of an hour with i % potassium-gold chlorid solution, and, finally, for one or two days with water weakly acidulated with acetic acid drops to 30 c.c. of water), the whole process taking place in the light. Golgi's method may also be used, but the gold method is more certain. The sclera is treated in a similar manner.

The pigmentation of the vascular layer interferes with examination, and albinotic animals should therefore be selected ; or the pigment may be removed from the previously fixed eyeball with hydrogen peroxid or nascent chlorin. The latter method is applied exactly as in cases where the removal of osmic acid is desired.

The adult lens is sectioned with difficulty, as it becomes very hard in all fixing fluids. The anterior capsule of the lens may be removed from previously fixed specimens and examined by itself. The lens-fibers are demonstrated by maceration in y^ alcohol (twenty-four hours) or in strong nitric acid. Before immersion the lens-capsule is opened by a puncture.

The retina can rarely be kept unwrinkled in eyes that have been fixed whole. The eyeball should therefore be opened in the fixing fluid and the latter permitted to act internally ; or the external tunics are removed, thereby enabling the fixing fluid to act externally.

Ranvier recommends subjecting the eyes of smaller animals (mouse, triton) for a quarter or half hour to the action of osmic acid fumes (see p. 24), after which the eyes are opened in yi alcohol with the scissors. At the end of three or four hours the posterior half of the eye is stained for some time in picrocarmin (p. 44), then carried over into osmic acid for twelve hours, washed with water, treated with alcohol, and cut.

In osmic acid preparations the rod-nuclei show dark transverse bands, a condition due to the fact that the end-regions of the nuclei stain more deeply.

The retina is a good object for differential staining, as, for instance, with hematoxylin-eosin, hematoxylin -orange G, etc. The latter combination is particularly successful in staining the rod- and cone-ellipsoids. The examination of tangential sections should not be omitted.

With the retina the best results are obtained by means of Golgi's method. Attention must be called to the fact that the supporting structures of the retina are more easily impregnated than the nervous elements, and that the latter can be demonstrated to any extent only in very young eyes.

Ramon y Cajal (94) recommends the following method, modified after Golgi : After the removal of the vitreous humor the posterior half of the eyeball is placed for one or two days in a mixture containing 2,% potassium bichromate 20 c.c. and i% osmic acid 5 or 6 c.c. The pieces are then dried with tissue paper and placed in a 0.75% silver nitrate solution for an equal length of time. Without washing, the pieces are immersed for from twenty-four to thirty -six hours in a mixture containing 3% potassium bichromate 20 c.c., and i% osmic acid 2 or 3 c.c., and then again carried over into a 0.75% silver nitrate solution for twenty-four hours. In order to prevent precipitation it is advisable to roll up the retina before treating, and to cover it with a thin layer of a thin celloidin solution, which prevents it from again unrolling.

The methylene-blue method (p. 184) will also bring out the nervous elements of the retina, although the results are not quite so satisfactory as those obtained by Golgi's method.

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A Textbook of Histology (1910): Introduction To Microscopic Technic | General Histology | I. The Cell | II. Tissues | Special Histology | I. Blood And Blood-Forming Organs, Heart, Blood-Vessels, And Lymph- Vessels | II. Circulatory System | III. Digestive Organs | IV. Organs Of Respiration | V. Genito-Urinary Organs | VI. The Skin and its Appendages | VII. The Central Nervous System | VIII. Eye | IX. Organ of Hearing | X. Organ of Smell | Illustrations - Online Histology

Reference: Böhm AA. and M. Von Davidoff. (translated Huber GC.) A textbook of histology, including microscopic technic. (1910) Second Edn. W. B. Saunders Company, Philadelphia and London.

Cite this page: Hill, M.A. (2021, August 5) Embryology Book - A textbook of histology, including microscopic technic (1910) Special Histology 8. Retrieved from,_including_microscopic_technic_(1910)_Special_Histology_8

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