Book - The Embryology Anatomy and Histology of the Eye 3

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Brown EJ. The embryology anatomy and histology of the eye. (1906) Chicago: Hazlitt & Walker.




Histology

We will first take up the lids or palpebrae (from palpare = "to stroke"). These are two crescentic folds, which grow from above down and from below upward and cover the front of the eyeball. See Figs. 9 to 17, showing their development from the margin of the orbit. Their function is purely the protection of the eyeball and they contain many glands, all of which secrete substances which play their parts in the physiological functions of the lids. The lids also contain two semilunar plates with their convex border turned away from the palpebral slit. These are very dense, fibrous plates, known as the tarsal cartillages U, Fig. 45. However, they have nothing in common with cartillage, except their density, they being made up wholly of white fibrous tissue. However, they were named by the ancient anatomists prior to the time of our ability, by chemical analysis, to determine accurately the constituents of all tissues and bodies.

The outer or anterior surface is covered with epithelium while the inner or posterior surface is covered with mucous membrane, the epithelium changing its nature at the free margin of the lid.

Fig. 45 shows a vertical cross section of the upper lid. At A is shown the epithelium ; at B is shown the hair follicles of the small white hairs, which are scattered over the anterior surface of the lids. At C is shown the sweat glands ; at D the subepithelial tissue, or areolar tissue, which differs somewhat from that found in other parts of the body, from the fact that fat is not readily deposited in it, as is the case elsewhere in the body. Lying immediately below the areolar tissue is found the orbicularis palpebrarum muscle. The cross sections of the bundles are seen at E (also see Fig. 27) and at F are shown the hair follicles of the ciha or lashes. At G are seen the modified sweat glands of Moll and at H are .shown the sebaceous glands connected with the lash or cilia in the lids. These glands are known as Zeisse's glands. At I is seen the muscle of Riolanis. This is the involuntary muscle for closing the eye ; it also re-enforces the orbiculars and brings the margins of the lids into close and firm apposition.

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Fig. 45. Vertical Cross Section of the Upper Eyelid.

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J points to the region where the epithelium changes its nature to that of a mucous membrane, such as lines all cavities or internal openings which communicate with the external world, and in this case is known as the conjunctiva. At K is shown one of the ducts of a Meibomian Gland, and L shows the secreting portion of the gland, which is imbedded in the tarsal cartillage. M shows the palpebral conjunctiva, and at N is seen a cross-section of one of the superior palpebral arteries (see G and J, Fig. 29). There is a free anastomosis between these arteries and those of the inner or conjunctival surface formed by numerous arteries piercing the tarsal cartilage. At O are seen the post tarsal papillae, which are folds, and the depressions between them are called Henly's glands. At P are shown the glands of Waldeyer. At Q is shown the involuntary muscle of Mueller, and it is this muscular bundle which opens the eye involuntarily. At R are seen Kraus' glands, just above the fornix (arch) of the conjunctiva. S shows the fibers of the levator palpebrae superioris (see B, Fig. 28), and at T are shown the fibers from this muscle, which pass outward between the fibers of the orbicularis palpebrarum, and are attached to the skin as shown at A in Fig. 52.

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Fig. 46. Eyelid Showing Portion of a Hair Follicle.

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This fasciculus is a wise provision of nature, for when the lid is raised it keeps the skin taut between its attachment and the free margin of the lid, and draws it up with the lid, while the skin above drops down over it, making a fold in the skin at about the middle of the lid, and in this way takes care of the loose skin when the lid is raised, otherwise it would drop down over the edge of the lid and interfere with vision.


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Fig. 47. Showing Zeisse's Glands, Modified Sweat Glands of Moll and Meibomian Glands.

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Fig. 46 shows the innermost portion of a hair follicle; G the papillae in the follicle, from which the hair grows and receives its nourishment mainly, and H the cup in the end of the hair shaft.

Fig. 47 also shows a hair follicle. B, Figs. 46 and 47, shows the sebaceous gland, known as Zeisses glands. In this location, these glands are compound sacular glands, the sacks filled with secreting cells, which secrete an oily material called sebum, which is poured into the hair follicle and travels along the lashes and keeps them oiled, so they are always soft and pliable. C, Fig. 46 and 47, shows the modified sweat glands of Moll, which are tubular glands, lined with secreting cells, which in other parts of the body lie doubled up in knots in the areolar tissue with straight tubes running to the surface. These modified sweat glands lie in the muscle of Riolanis, just back of the lashes. The modification of these glands on the margin of the lids is due to the fact that instead of opening onto the surface as sweat glands do elsewhere on the body, these empty into the hair follicle and this watery secretion becomes mixed with the sebum from Zeisse's glands and thereby renders it more viscid or watery. This serves the purpose of keeping the lashes constantly covered with this thin, viscid, oily substance, which facilitates their capacity for catching dust, thereby increasing the usefulness of the lashes in protecting the cornea against dust. F, Fig. 46 and 47, shows the muscle of Riolanis, which is a small muscular bundle, which surrounds the palpebral fissure and arises from the tendo oculi (see A and B, Fig. 26), however it is really a part of the orbicularis palpebrarum and is the involuntary muscle to close the eye when the cornea becomes dry. When acting in conjunction with the orbicularis in closing the eye, it causes a folding of the free margin of the lid and reinforces the orbicularis and brings the lid margins more closely together. D, Fig. 46 and 47, shows a duct of one of the meibomian glands and at E, Fig. 46 and 47, are the gland cells, which secrete the sebaceous material which is poured out on the free margin of the lid. The meibomian glands are modified sebaceous glands, being tubular with many blind pouches or sacks connecting, filled with secreting cells. There arc from twenty to thirty of these glands in each lid. They are imbedded in the conjunctival surface of the tarsal cartillage and arc readily seen in the human Hd (when inverted) as white Hnes, and their openings are readily seen on the free margin of the lid. See Figs. 33 and 34.


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Fig. 48. Showing the Tarsal PapiUae.

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This secretion renders four important services to the eye : First, this oily substance prevents the lids from sticking together when we sleep ; second, it keeps the margins of the lids oiled and prevents the tears from flowing over their edges, when the eyelids are being closed, for it will be remembered that the lids come into apposition at the outer canthus first, then the slit is gradually closed from without inward and any tears which have accunuilated in the palpebral fissure flow along in front of the closing edges; thus they arc directed into the lakus (see C, Fig. 25) ; third, it keeps the cornea oiled, which prevents the cornea from dessication or drying so readily ; fourth, its mixing with the tears and keeping the conjunctival sac so freely lubricated, prevents friction of the structures as they glide over each other in the opening and closing of the eye lids.

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Fig. 49. Showing Henle's Glands.

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A, Fig. 48, shows the post tarsal papillae, which in reality are only folds or Rouga of the conjunctiva which have the appearance of being small elevations when seen in cross section and where the mucous surfaces are brought into close proximity, as is the case in the furrows or depressions, the cells change their nature, and we find these furrows lined with columnar epithelial cells as shown at A in Fig. 49, and they are called Henle's glands. These glands, or folds, become more marked as age advances. These surfaces contain many goblet cells and secrete more or less mucus.

At A, Fig. 50, are seen the glands of Waldeyer. These glands are in the nature of sweat glands and they with Kraus' glands (A, Fig. 51) secrete the tears under ordinary circumstances. At B, Fig. 51, are shown cross sections of the lachrymal gland, which is a compound tubulo racemose gland resembling serous or fluid secreting glands in other parts of the body.

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Fig. 50.

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This is a rather larger gland than any we have seen in the lid before. It is located in the upper lid, at the upper outer side of the orbit (see E, Fig. 28) ; it is almond-shaped and the size of a small almond kernel. The secretions reach the conjunctival sac by some ten or twelve ducts (C, Fig. 51), which empty into the fornix (arch) of the conjunctiva. D, Fig. 51. The lachrymal gland, only pours forth its secretions when the eye is irritated, and this washes or floods the conjunctival sac quite freely, as when the eye Is irritated by a foreign body or when we cry, and the secretion of tears is so copious, that our lachrymal apparatus cannot carry away all the fluids, and we find the tears flowing over the lower lid onto the cheek at such times.


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Fig. 51.

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The conjunctiva (joined together) is the mucous membrane which lines the conjunctival sac (the joined sac), which is really two crescentic culdesacs, one between each eyelid and the eyeball, and they are really separated by the palpebral fissure, when the lids are open. However, it is a complete oval sac when the lids are closed. This mucous membrane commences at the free margin of the lid, at B, Fig. 52, by the transformation of the epithelium into mucous epithelium, the arrangement of the cells is the same as in the epithelium in other parts of the body, the outermost cells being squamous (scaly), the middle cells being irregularly round or polyhedral (many sided) cells, while the innermost are columnar (long) cells. These lie on a loose membrane which is well supplied with blood vessels, and the tissue being loose and transparent, it gives a free flow of Lymph. That part of the conjunctiva lining the posterior surface of the lids is known as the palpebral conjunctiva (C, Fig. 52). When it reaches well back under the lids, it folds on itself and becomes adherent to the sclerotic (H, Fig. 52).


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Fig 52.

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This fold is called the fornix conjunctiva (D, Fig. 52). The portion of the conjunctiva which covers the eyeball (E, Fig. 52) is called the ocular or bulbar conjunctiva. The ocular conjunctiva is transparent and through it we can see the sclera, which is opaque and white. It is freely movable over the sclerotic (K, Fig. 52) and by manipulation we can see the blood vessels of the conjunctiva (H, Fig. 52) change their position, while the blood vessels of the sclera, which are deeper set (J, Fig. 52), remain stationary. When the conjunctiva reaches the outer margin of the cornea (G, Fig. 52) the basement tissue ends, but the epithelium continues over the front of the cornea (F, Fig. 52) and forms the anterior or stratified epithelial layer of the cornea, and is called the conjunctival portion of the cornea. The blood vessels of the conjunctiva end at the corneal margin in a circle of capillary loops (I, Fig. 52, and F, Fig. 54), very superficially placed.

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Fig. 53.

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The cornea (horn-like) A, Fig. 23, and M, Fig. 25, forms about the anterior sixth of the eyeball. It is a highly transparent structure, allowing the light from the external world to enter the eyeball, and is the first of the refractive media through which this Hght passes on its way to the retina. It is made up of five layers, as shown in cross section in Fig. 53 ; A, the anterior stratified epithehal layer ; B, Bowman's membrane, or the anterior homogeneous membrane; C, the lamina propria (proper layer) ; D, Decimet's membrane or the posterior homogeneous layer, and E, the endothelial layer ; the latter lining the cornea on its surface bounding the anterior chamber. See S, Fig. 23.

The anterior stratified epithelium, as before stated, is continuous with the epithelial layer of the conjunctiva. As its name implies, its cells differ at different depths. The outermost, F, is made of squamous (scaly) cells, G is formed by hexagonal (many-sided) cells, and the innermost layer, H, is formed by Columnar (long, square) epithelial cells and this is the layer where all new cells are formed by the division and growth of these columnar cells, and as new cells are formed the older ones are pushed outward toward the surface and become hexagonal, and as this process continues, the cells are pushed farther and farther out. They lose their nuclei and become mere flat scales and finally lose their adhesive qualities and are disquamated (thrown off) and wash away with the tears. These cells are held together both from the cement substance lying between them and by the little projections from the surface of the cells themselves. When these projections are found on a cell, they are called prickle cells, and this is the nature of these cells in the lower or inner layers.


Passing from without inward, the next layer, B, is Bowman's membrane, or the anterior homogeneous lamina. As the name, homogeneous lamina, implies, this layer when seen with the microscope reveals no structural frame work, but appears as a solid gelatinous layer. However, if this tissue be macerated (soaked) in an alkaline solution and the cement substance dissolved out, it will be found to be formed of connective tissue bundles. This layer ends at the periphery of the cornea. The next layer, C, is the Lamina Propria (proper layer) or substance of the cornea. It is formed of some sixty strata of connective tissue bundles. These cannot be stripped ofif in layers, but are made out by the microscope from the fact that the connective tissue bundles run in different directions ; that is, for instance, in one strata all the bundles run vertically across the cornea, the next layer may run horizontally, while the third strata may have its bundles laying at 45o or 135o.

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Fig. 54. Ciliary region, magnified 1,000 times.

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However, there is such a free exchange of bundles from one layer to another, in which they become lost, that the whole sixty are practically as one ; so that this arrangement forms a very firm, unyielding structure. At the sclerocorneal juncture or limbus, U, Fig. 54, the lamina propria continues backward, forming the sclerotic by continuity of tissue, the difference being simply that the nature of the tissue is changed at the Hmbiis, for in the cornea it has no blood vessels, while the sclera is fairly well supplied with blood vessels. In the cornea the tissue is quite dense and transparent, and in the sclerotic it is more loosely arranged and is opaque. In the cornea it is highly supplied with sensory nerves, while in the sclerotic it has only a moderate nerve supply, and, farther, if we should examine the cornea chemically, we would 'find it contained chondron ( found in cartilage), while the sclerotic would chow no chondron, but in its place we would find gelatin. It will then be seen that while one blends into the other, yet the two tissues are very much different.


The cornea is said to fit into the sclera as a watch glass fits into the bezel of a watch. This impression is given from the fact that the corneal tissue passes farther backward at its center, while the sclera runs forward farther at its outer and inner surfaces. When we view this with the microscope in stained sections, it appears as shown at U, Fig. 54. This is from the fact that the cornea being more dense than ihc sclerotic, it retains less of the stain in its preparation, so that we can make out the limits of the two tissues fairly well in this way.


Lying within the lamina propria is a network of openings or lymph channels, the lacunae, I, Fig. 53 (small lakes) and the minute canals (caniliculae) which run out in all directions from the lacunae and join the caniliculae from surrounding lacunae. Lying in these lacunae, I, Fig. 53, yet not entirely filling them, are found the fixed or corneal corpuscles. These cells in turn have very minute protoplasmic processes which run through the caniliculae and join or anastomose with the processes from the cells in the neighboring lacunae. These processes do not entirely fill the caniliculae in which they lie; thus it will be seen that we have a network of lymph channels through which the blood plasma, or nutrient lymph can have free passage to all parts of the cornea to supply it with nutrition. This lymph is given off from the capillary loops, forming a circle around the margin of the cornea, which will be described later.


Passing inward, the next layer is Decimet's Membrane, or the internal homogenous lamina, D, Fig. 53. This is a very thin, highly transparent layer and has a tendency to curl up when stripped off of the cornea. When viewed with the microscope, it is impossible to make out any ground work, it seeming to be wholly made up of a hornlike membrane, but as with Bowman's membrane, if treated properly, to remove the gelatinous substance which forms the matrix or joins the component tissues together, it will be found to be formed of connective tissue. Many functions have been ascribed to this membrane, but the chief one is its great resistance to disease, such as corneal ulcers, etc. Some writers claim this membrane breaks up into connective tissue bundles, bridges across the filtration angle and forms the pectinate (comb) ligament, K, Fig. 54.


This is composed of hundreds of connective tissue bundles which run from the periphery of the cornea to the base of the iris, K, Fig. 54, and A, Fig. 53. This angle formed by the iris and cornea, V, Fig. 54, is known as the filtration angle from the fact that the aqueous fluid passes out of the anterior chamber between the bundles of tissue, forming the pectinate ligament, to the spaces of Fontana (fountain spaces), which comprises the openings in the pectinate ligament. The posterior, or fifth layer, is known as the endothelial layer, E, Fig. 53. This is formed of a single layer of cubical (square) endothelial cells, which are placed like paving blocks and are similar to cells which are found in other parts of the body, lining closed cavities, or cavities which have no opening communicating with the external world. These cells have the faculty to withstand the dissolving qualities of the aqueous fluid, or nutrient lymph, which fills the anterior chamber.


Some anatomists divide the cornea into three portions; the conjunctival portion, consisting of the anterior stratified epitheHum, and Bowman's Membrane; the scleral portion, consisting of the lamina propria ; and the choroidal portion, consisting of Decimet's Membrane and the endothelial layer. This is from the fact that these layers are supposed to be derived from these structures.

The sclerotic (tough) coat, I, Fig. 54, forms the posterior five-sixths of the outer coat of the eyeball, except a small opening at the posterior pole, where the optic nerve pierces it. This opening is known as the choroidal fissure. See Figs. 23, 56 and 57. The sclerotic, as before stated, is continuous with the cornea by continuity of tissue. Just outside of the sclerotic is the space of Tenon, X, Fig. 54, and N, Fig. 35. This is a space between the capsule of Tenon and the sclerotic. The capsule of Tenon forms a fibrous socket for the eyeball, and this space of Tenon is a lymph space and is crossed by many connective tissue bundles passing from the capsule to the sclerotic. These are known as Trabeculae. Internal to the sclerotic, between it and the choroid, is another lymph space known as the suprachoroidal space, W, Fig. 54. This is also crossed by an abundance of trabeculae passing from the sclerotic to the choroidal coat. In fact, the trabeculae are so numerous that it is almost impossible to separate the two structures. The sclerotic, as its name implies, is very tough and opaque. The innermost portion contains quite a little pigment. It has four layers, from without inward ; they are the endothelial layer lining the space of Tenon, which is a single layer of pavement cells. Next comes the lamina propria (proper layer) ; the next layer is called the lamina fusca (Brown layer). The lamina propria and the lamina fusca are not sharply defined by any line of demarcation, but, as before stated, the innermost strata contains some pigment. It is therefore brown in color, the pigment not being sufficient to cause it to appear black. This pigment is deposited in branched cells. The innermost, or fourth layer, is the internal endothelial layer, lining the supra choroidal space, and is of the usual pavement variety. The lamina propria and lamina fusca are formed of tough fibrous tissue, the strands of which run in all directions with a general anterior posterior arrangement. Lying in the substance of the sclerotic are found lacunae, the same as in the cornea which contains the fixed or scleral corpuscles, analogous to the corneal corpuscles. In fact, the sclerotic is very similar to the cornea in the arrangement of the connective tissue, except that it is not learly so compact.

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Fig. 55. Magnified 2,500 times.

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The sclera, as before stated, is well supplied with blood vessels. These run forward and end in capillary loops at the limbus and form a complete circle extending clear around the periphery of the cornea. They, with the circle of capillaries formed by the conjunctival vessels, F, Fig. 54, and I, Fig. 52, near the outer surface, give off the nutrient lymph which flows through the lacunae (small lakes) and caniliculi (minute canals) and permeates the cornea. This lymph furnishes the nutrition for the cornea. To give the reader an idea of what is meant by capillary loops, we have taken a microphotograph of an injected section from the sole of the foot. Fig. 55, A. This is the Rete Mukosum (capillary layer, or malpighian layer),. of the skin, showing the fine capillaries running up and forming loops and passing back as venous capillaries. B shows some elevations, which form little ridges, which can be seen on the ball of the thumb so readily. C shows the branch of an artery, which breaks up into these small capillaries.


At J, Fig. 54, is seen the canal of Schlemm. This is a canal lying near the inner surface of the sclerotic, just at the limbus. It is circular in course, running clear around the margin of the cornea. It may be single, or may be composed of several small canals. They branch from and return to the main opening, so that it forms one continuous sinus, it is lined with endothelial cells, and is drained by the anterior ciliary veins. The aqueous humor passes from the spaces of fontana to the canal of Schlemm and eventually is carried back into the circulation by the anterior ciliary veins.


As before stated the sclerotic forms the posterior fivesixths of the outer coat of the eyeball. To it are attached the six recti (straight) muscles. See Figs. 35, 36 and 37. It is pierced by the anterior ciliary arteries and veins at these points of attachment. (See D, Fig. 39.) These pass through about eight to ten millimeters back of the limbus; then just back of the equator it is pierced by the vena vorticosa (whorl veins) four to six in number. (See H, Fig. 39, and B. and C, Fig. 40) Then posteriorly it is pierced by the cihary arteries and nerves, there being twelve to twenty of each. These pass through just outside of the optic nerve A, Fig. 39, and at A, Fig. 56, is seen one of these vessels passing through this structure. When the sclerotic reaches the optic nerve, it divides into three portions. The innermost, B, Fig. 56 and 57, breaks up into individual bundles. These pass across the choroidal fissure and form the lamina cribrosa (sieve layer), C, Figs. 56 and 57. These bundles pass across in all directions and reinforce the eyeball at this otherwise weak point, leaving meshes or openings through which the optic nerve fibers pass out of the eyeball. It is also pierced by the arteria centralis retinae (central artery of the retina) L, Fig. 57. The opening through the lamina cribrosa, through which the arteria centralis retinae and vein pass, is known as the porus opticus.

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Fig. 56.

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The middle portion passes to and blends with the pia mater of the optic nerve D, Figs. 56 and 57. The outer portion passes into the sheath of the optic nerve, F, Fig. 56 and 57. At E is shown the intervaginal space of the optic nerve, which is continuous with the sub-dural space of the brain at the optic foramen and contains cerebro spinal fluid.


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Fig. 57.

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H, Figs. 56 and 57, shows the choroid, and J, Figs. 56 and 57, shows the retina detached from the choroid. K shows the physiological cup ; M shows the optic nerve, and in Fig. 57 the nerve bundles are extremely well shown with the myelin sheaths surrounding them. These sheaths end normally just behind the lamina cribrosa, C. The choroid is continuous from the optic nerve to the free margin of .the iris, or to the pupillary opening. It lies inside of the schlerotic and is the second grand tunic or coat of the eye. From the ora serratta of the retina (saw tooth mouth of the retina) to the choroidal fissure it Hes in touch with the sclerotic, only being separated from it by the supra choroidal space and intimately attached to the sclerotic by the interchange of trabeculae passing across the supra choroidal space from one to the other. It is a pigmented and highly vascular tissue, as its name implies, and supplies the greater part of the nutrition and secretions of the eyeball.

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Fig. 58. Showing Section of Choroid.

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It is composed of five layers of fibrous tissue with branched pigment cells in the meshes between the connective tissue fibers. First from without inward we have the endothelial layer lining the supra choroidal space, A, Fig. 58. Below that is the lamina supra choroidea (upper layer of the choroid), B, Fig. 58, also called the lamina fusca of the choroid, on account of its pigmentation and brown color.


The next layer is the layer of large blood vessels, C, Fig. 58. The next layer is known as the layer of small blood vessels, D, Fig. 58. The layer of large and small blood vessels are composed of the posterior ciliary arteries as they pass forward in the structure, giving off branches all along their course ; also the veins which go to form the vena vorticosa lie in these two layers. The next layer is called the choriocapillario (capillary layer of the choroid), E, Fig. 58. These capillaries are separated from the retina by the bacillary layer (basement layer), also called Bracks' Membrane or lamina vitrea, F, Fig. 58. The capillary layer of the choroid furnishes much of the nutrition to the outer layers of the retina, it reaching them by osmosis (passing through) Brucks' Membrane. This layer is free from pigment and is rich in cement substance, so much so that it is a homogeneous membrane or layer.


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Showing Ora Seratta, Ciliary Processes and Bodies and the Iris from Posterior Aspect.

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Fig.


Showing Same as Fig. 59 in Cross Section.

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The choroid is highly pigmented to prevent the light penetrating the wall of the eyeball, thus making an absolutely dark chamber of it. The choroid is extremely susceptible to disease on account of its extreme vascularity.


As before stated the choroid is continuous from the head of the optic nerve forward to the free margin of the iris. However, it is divided into the choroid ciHary process, ciHary bodies and iris. The choroid, I, Fig. 60, extends from the optic nerve to the ora serratta or anterior margin of the retina, A, Figs. 59 and 60. It then becomes somewhat ridged on its inner surface, B, Figs. 59 and 60. These ridges have an anterior posterior direction, and these ridges, about seventy in number, are known as the ciHary processes.

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Fig. 61. The CapiUaries of the Ciliary Processes.

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These end in blunt endings which project into the cavity of the eyeball towards the lens, H, Fig. 60, and are known as the ciliary bodies, C, Figs, 59 and 60. From the outer angle of the bases of the ciliary bodies, J, Fig. 60, the choroid or uvea leaves the outer wall of the eyeball and takes a transverse direction. This transverse portion is called the iris (rainbow), D, Figs. 59 and 60. At the center of the (Doll so called from the diminutive image of oneself as seen in the pupillary area when looking into anyone's eye), transverse portion there is an opening known as the pupil, G, Figs. 59 and 60. The free margin of the iris, F, Figs. 59 and 60, Hes free and rests on the anterior surface of the lens, H, Fig. 60. The short, posterior ciliary arteries run forward through the choroid in the layer of large blood vessels, C, Fig. 58, and B, Fig. 39, being bunched in straight vessels in the ciliary processes and end in capillary tufts, Fig. 61, in the ciliary bodies, turning back as venous capillaries, A, Fig. 40. From the capillaries are given out the fluids of the blood which passes into the canal of Petit, N, Fig. 60, and the posterior chamber, M, Fig. 60. This fluid is known as the aqueous humor.

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Fig. 62. The Blood Vessels of the Iris.

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The two long posterior ciliary arteries run forward in the choroid in the layer of large blood vessels, C, Fig. 58, and C, Fig. 39. They join the anterior ciliary arteries, D, Fig. 39, and form one arterial trunk, which lies right at the base of the iris, A, Fig. 62, K, Fig. 60, and E, Fig. 39. This arterial trunk formed by the anastomosis (joining) of the long posterior and anterior ciliary arteries, is known as the circulus major (larger circle) of the iris, A, Fig. 62. From the circulus major is given off branches which run radially inward toward the free margin of the iris, B, Fig. 62, These radially coursing arteries in the iris may be likened to the spokes in a wheel. When they come near to the free margin of the iris, they anastomose (join) and form another circle known as the circulus minor (smaller circle) of the iris, C, Fig. 62. From the circulus minor is given off capillaries which run inward toward the free margin of the iris. They double back as venous capillaries, D, Fig. 62. The drainage from the iris is by the anterior ciliary veins which leave the eyeball at the attachments of the extrinsic muscles, while the drainage from ciliary bodies and the rest of the uveal tract is drained by the vena vortacosa (vortex veins). See Fig. 40,


Anterior to the ora serratta the outermost or pigment layer of the retina continues forward in two layers of columnar epithelial pigmented cells and lines the inner wall of the eyeball over the pars ciliaris, retina, ciliary processes, and bodies, O, Fig. 60, also continuing over the posterior surface of the iris, clear to the free margin at F, Fig. 60. This is known as the retinal portion of these structures and the amount of pigment contained in this layer over the posterior of the iris largely determines the color of the eye, for if there is no pigment in this layer, or the stroma of the iris, we would have the pink or albino eye. If there is a small amount of pigment in the retinal portion, then we would have a light blue eye ; a little more pigment and it will produce the dark blue eye, and so on as more pigment is deposited, the eye is gray, brown or black. However, in the brown and black eyes there is much pigmentation of the stroma of the iris.


Fig. 63 shows a cross section of an iris in which the retinal portion, A, is well pigmented, while the stroma, B, has but a small amount of pigment. This would have a tendency to produce a light grayish color when the iris is viewed from the front. Fig. 64 shows a cross section of an iris, which is highly pigmented, both in the retinal layer, A, as well as the stroma, B. This would produce a dark brown or black iris if viewed from the front.


The iris is a very delicate structure formed of a very thin network of connective tissue, with a large amount of cells filling in the spaces between the connective tissue fibers. In dark eyes these cells become more or less pigmented, C, Fig. 64. The iris contains two muscles, the Sphincter (binder) Pupillae, A, Fig. 65, and the Dilator (enlarger) Pupillae, B. It has four layers from within outward ; they are the pigment, or retinal layer, F, muscular, B, the stroma proper in which lie the blood vessels, E, and the endothelial or corneal layer, D. The front of the iris has deep depressions or crypts ; these run radially, or from the base toward the free margin. These depressions, or crypts, lie between the blood vessels, see Fig. 62, and in medium or light colored eyes this causes the stellate (star like) or radially spoke-like appearance of the anterior surface of the iris as seen in those eyes. Fig, 66 represents a quadrant of the front surface of an iris; A, the pigment layer at the free margin ; B, the circulus minor and capillaries ; C, the circulus major; D, a trabeculae or ridge in which runs a blood vessel; E, a depression or crypt; and F, the pectinate ligament. The spoke or stellate appearance is caused by the vessels being so near the surface that the reflection is greater over them than from the spaces between them. However, in very dark eyes, the pigment is so densely deposited that it. hides the blood vessels; therefore, in dark eyes this spokelike appearance is absent.

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Fig. 63. Cross section from iris of a light colored eye.

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Fig. 64. A cross section of Iris from dark Eye.

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Fig. 65. Showing cross section of Iris, its muscles and layers.

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Lying just behind the iris is found the crystalline lens (pea or lentil), and as the name implies, it is a transparent body shown at A, Fig. 6y. This lies in the Fossae Patilaris (dish like depression) in the anterior surface of the vitreous body and is held in place by the suspensory ligament, B. The lens has two portions, however, not divisible or sharply outlined; the central or nuclear portion and the outer or cortical portion. The central or nuclear portion is more dense than the cortical portion. The nuclear portion is composed of the elongated or spindle cells which first fill the lens vesicle by the elongation of the cells composing the posterior wall of the lens vesicle; see Fig. 8. These cells, as before stated, fill the whole cavity as shown in Fig. 68, D. The nuclei of these cells are pushed forward, as shown at K. After these first formed spindle cells have filled the lens vesicle, then at the transitional (transforming) zone, the original cells of the lens vesicle continue to elongate and grow around the ends of the cells which have formed the nucleus of the lens as shown at J, and in this section the cells, which will form the cortical portion, are just beginning to grow and elongate. These cells then form the outer or cortical portion of the lens and the ends of the fibers butt together, as shown at A, Fig. 69. These fibers, or spindle cells, have a diamond shape and these again are formed in layers bound together by transparent cement substance. These layers are then laid one on another, as the layers of an onion are found, and these layers in turn are bound together by the cement substance.

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Fig. 66. A quadrant of the front surface of the Iris.

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Fig. 67. Cross section of the liuman eye.

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Over the anterior surface of the crystalline lens is formed a single layer of columnar cells, which are the cells composing the original lens vesicle wall. This layer extends back to the equator of the lens and then they become transformed into the spindle cells, which compose the lens substance. The area of transformation is known as the transitional Zone; see G, Fig. 60. Surrounding the whole lens is a thin transparent membrane known as the capsule of the lens, C, Fig. 67, and to this capsule is attached the suspensory ligament, the anterior fibers just in front of the equator and the posterior fibers just behind the equator.


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Fig. 68. Human embryo eye, 2 months. Magnified 1,080 times.

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The suspensory ligament of the lens or Zonule or Zinn. C and D, Figs. 70 to 74, is imbedded in the outer layer of the hyaloid membrane. This membrane divides into two layers at the ora seratta of the retina (saw tooth mouth), F, Figs. 70 to 74. The inner layer continues over the front of the vitreous body, while the outer layer in which the fibers of the suspensory ligament, I, Figs. 70 to 74, are imbedded, is firmly bound down to the inner surface of the pars ciliaris retina, ciliary processes and bodies G and H, Figs. 70 to 74. From the ciliary processes, H, the fibers and membrane leave the outer wall of the eyeball and turn transversely across toward the equator of the lens, P. The outer layer of the hyaloid membrane, I, Figs. 70 to 73, becomes very thin and fluid passes through it very readily. It passes across with the fibers of the suspensory ligament, which are attached in front of the equator of the lens, and the triangular space bounded by it in front and the hyaloid membrane behind with its base at the equator of the lens. The apex at the ciliary bodies is called the canal of Petit, E, Figs. 70 to 73.


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Fig. 69. Human embryo eye, 5 months. Magnified 7,000 times.

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The fibers of the suspensory ligament, C and D, Figs. 70 to 74, arise from the retina at the ora seratta, F, and are continuous clear to their attachments to the lens, A and B. These fibers are believed to be specialized elongated fibers of Mueller, which are of a very elastic nature. These fibers become attached to the lens capsule during the development of the eye and as the eye enlarges become elongated. When they leave the ciliary bodies they divide and a part of them, C, Figs. 70 and 73, pass to their attachment to the lens capsule in front of the equator and others, D, Figs. 70 to 74, pass to their attachment to the lens capsule back of the equator, while a few pass across in the canal of Petit, E, Figs. 70 to 73. These are attached to the lens at its equator. By glancing at Fig. 70 and noting the attachment of the suspensory ligament, C and D, it will readily be understood that tension on the suspensory ligament, C and D, of the lens, P, Fig. 70, would have a tendency to flatten it in its anterior posterior diameter and enlarge its transverse diameter, thus increasing and decreasing its convexity, as this tension was exerted or relaxed.


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Fig. 70. Showing the suspensory Ligament.

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Fig. 71. Showing ora seratta and ciliary processes. +++++++++++++++++++++++++++++++++++++


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Fig 72. Enlarged view of ciliary muscle.

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Fig. 73. Showing ciliary processes and bodies.

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Lying between the ciliary processes, bodies and choroid, G, H and I, Figs. 70 to 73, and the sclerotic, K, Figs, ^^2 and y^, is found the ciliary muscle, L and M, Figs. 70 to 73 (hair-like muscle), composed of plain muscular fibers. It is composed of two portions, the longitudinal or the outer portion, L, Figs. 70 to yi, and the circular portion, M, Figs. 70 to 73. The fibers of the first or outer portion, L, run anterior posterior, arising at the limbus (seam), N,

Figs. 70 and "j^i^ a portion of them in front of and a portion posterior to the canal of Schlemm, O, Figs. 70 and 73. The circular portion, M, has the same origin, but takes a circular course and lies just ouiside of the ciliary bodies, H, Figs. 70 to y2). The longitudinal fibers are attached to the outer surface of the choroid, J, Figs. 71 to y2i- They spread out fan shaped, some being attached as far forward as the posterior ends of the ciliary bodies, H, Figs. 70 to 74. Others extend backward and arc attached as far backward as the ora scratta, F, Figs. 70 to 74; thus it is seen they have a very extensive attachment to the choroid. The function of the ciliary muscle is to put the choroid, J, on the stretch. This is possible owing to the supra choroidal space, Figs. 70 to 73, separating the choroid and sclerotic, and the circular fibers, M, press the ciliary bodies, H, nearer to the equator of the lens, B, Fig. 74. As the suspensory ligament, C and D, is bound down to the choroid ciliary processes and bodies, and bridges across the space between the ciliary bodies, H, and the lens, P, the action of the muscle when it contracts is to slacken the suspensory ligament, allowing the lens P to become more convex by virtue of its elasticity or resiliency.

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Fig. 74. Suspensory ligaments and lens drawn in.

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Fig. 75. Vitreous darkened to show hyaloid canal.

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However, the main strain in accommodation seems to fall upon the circular portion M in Figs. 70 to y^, from the fact that in myopic eyes where the far point is within thirteen inches of the eye, where accommodation is never necessary, but few if any of these circular fibers are found upon examining the ciliary muscle after death, whereas in the hypermetropic eyes where accommodation is necessary for all vision, these fibers will be found to be very plentiful. In fact they have been known to make up as much as seventyfive per cent of the bulk of the muscle. The ciliary muscle receives its nerve supply from the posterior ciliary nerves, which arise from the lenticular or ciliary ganglion (to knit or weave), which receives its motor roots from the third cranial or motor oculi nerve. See Figs. 42, 43 and 44. The terminal portions of the posterior ciliary nerves break up into small anastomosing branches and form the ciliary plexus, which lies in the ciliary muscle.

The vitreous body, B, Fig. 75, composes the greater portion of the eyeball, filling all the cavity posterior to the lens. It is composed of shapeless transparent cells, very loosely arranged, so that it resembles a sponge and is filled with fluid resembling the aqueous humor, and is about of the density as the white of an egg, running through the vitreous body. Antero posteriorly from the posterior of the lens to the head of the optic nerve is found a lymph canal. A, Fig. 75, which was the space occupied by the hyaloid artery, which is present during the development of the lens during foetal life. See B, Fig. 16. This canal is known as the hyaloid canal or the canal of Stilling. The lens is imbedded in the anterior surface of the vitreous body, lying in a depression called the Fossae Patellaris (saucer-like depression), C, Fig. 75. The whole body is surrounded by the hyaloid membrane (glass-like membrane), which is transparent and homogenous (structureless), D, Fig. 75. This membrane divides at the ora seratta, F, Figs. 70 to 74, the inner layer covering the anterior of the vitreous and lining the fossae patellaris, C, Fig. 75, while the outer layer is intimately attached to the ciliary processes and bodies and leaves the ciliary bodies and extends to the lens. In this outer layer is imbedded the fibers of the suspensory ligament, I, Figs. 70 to 73. Posterior to the ora seratta the hyaloid membrane is very intimately attached to the retina, F, Fig. 75. This attachment is so firm that when the vitreous body is disturbed the nine innermost layers of the retina are usually detached.

The retina (net) lines the inner wall of the eye ball, it extends, properly speaking, from the head of the optic nerve, M. Fig. 'j^), to the Ora Serratta (Saw Tooth Mouth) X ; however, it is continuous clear to the free margin of the Iris E, by means of a double layer of pigmented epithelial cells which cover the inner surface of the pars ciliaris retina (the part between the ciliary bodies and the retina), G, Fig. ^^, ciliary processes G, Fig. 74, and ciliary bodies H. Figs. 73 and 74, as well as the inner or posterior surface of the iris, E. Fig. 76. This anterior or pigmented portion is called the Uvea (Grape Skin) ; it is formed by the continuation forward of the outer or pigment layer of the retina and the anterior portion of the secondary optic vesicle which does not take part in the formation of the nine innermost layers of the retina or more properly speaking, the receiving and transmitting portion of this structure.

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Fig. 76. Cross section of the human eye, showing its construction. +++++++++++++++++++++++++++++++++++++


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Fig. 77. +++++++++++++++++++++++++++++++++++++

The retina is a very thin, delicate structure, being onehalf millimeter thick at its thickest portion near the optic nerve, and gradually becoming thinner toward the ora serratta, where it is but one-tenth millimeter thick. It is firmly attached to the choroid at the ora serratta, and is firmly bound down at the head of the optic nerve by virtue of the optic fibers passing from it through the choroidal fissure (the opening of the choroid), L. Fig. "jd. There is a less secure attachment at the macula lutea (yellow spot), J. Fig. 78. In all other portions of the retina the nine innermost layers are very loosely attached to the outer or pigment layer; this attachment is accomplished simply by the interlacing of the rods and cones with the processes which project inward from the cells forming the pigment or outer layer. It is held in place mainly by the interocular pressure.

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Fig. 78. Cross section of the Eye, showing its construction.

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The retina receives its blood supply from the arteria centralis retina (central artery of the retina) which reaches it through the choroidal fissure after having traversed the optic nerve for some ten millimeters back of the eye ball, L. Fig. yy. This artery is an end artery, or in other words, it is not joined by any other set of arteries, but it sends its branches to all parts of the retina, A. Fig. 78, terminating ill arterial capillaries and turning back as venous capillaries ; these keep joining and rejoining and form the vena centralis retina (the central vein of the retina), which leaves the eye ball through the choroidal fissure by the side of the entrance of the artery. See darker vessels in Fig. 78.

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Fig. 79.

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By staining cross sections of the retina it is shown to be divisable into ten layers. Seven of these are nervous tissue, two of neuroglia or nervous connective tissue and one of pigmented epithelium. The nine innermost layers are transparent and are bound together by the fibers of Mueller, which is the nervous connective tissue of the retina. The outermost or pigmented layer is more intimately attached to the choroid than it is to the other layers.


The layers from within outward are: First, the inner limiting membrane, A. Fig. 79. Second, the layer of nerve fibers, B. Third, the layer of ganglionic cells or ganglionic (knot like) layers, C. Fourth, the mner molecular or plexiform layer, D. Fifth, the inner nuclear or granular layer, E. Sixth, the outer molecular or plexiform layer, F. Seventh, the outer nuclear or granular layer, G. Eighth, the outer limiting layer, H. Ninth, the layer of rods and cones, I. Tenth, the pigment layer, J. K. shows the hyaloid membrane which lies just inside of the retina and L shows the choroid ^which is the structure just outside of the retina. In this section the choroid is somewhat torn and separated.


The pigment layer, as before stated, is composed of a smgle layer of columnar epithelial cells which are long hexagonal cells separated from each other by a well defined, clear, cement substance. They have long protoplasmic processes which project inward and interlace with the rods and cones. In these cells are deposited pigment granules which remain in the base or outer portions of the cells when the eye is closed or in darkness. See G, Fig. 80. F is the lamina vitrea or Bruck's membrane of the choroid. However, when the retina is exposed to the light these pigment granules flow into the processes which lie amongst the rods and cones (C, Fig. 81), thus protecting these delicate structures from destruction by too intense light as well as forming a screen right amongst the rods and cones, to receive the image which is formed by the refracting surfaces of the eye. See Fig. 81. A is the choroid, B the bases of the pigment cells and C the processes lying amongst the rods and cones.


The layer of rods and cones (I Fig. 79), especially the cones, are the real sensory cells of the retina, as it is their function to produce the impulse which is transmitted to the brain and there produces the sense of sight. Each rod and each cone is at the end of a process which comes from a cell in the outer nuclear layer (G, Fig. 79). These pass through openings in the outer limiting membrane (H, Fig. 79).


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Fig. 80. Showing Section of Choroid.

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The cones, as their name implies, are of a conical shape, shorter than the rods ; they have a large oval inner portion with a finer tapering point extending outward into, and interlacing with the processes extending inward from the pigment layer. The oval or enlarged inner portion is striated longitudinally, while the outer or tapering portion is formed apparently of discs. The rods are long cylindrical cells striated longitudinally, and are divided into two segments at about their middle. Their function is not clearly established. There are estimated to be about three niilHon cones in the human retina, and the rods exceed this many times. The cones predominate in the macula or most acute area of sight, while the rods predominate in all other portions of the retina, thus proving the cones to be the real sensory elements.

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Fig. 81.

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The next layer from without inward is the outer limiting layer. (EP, Fig. 79.) This is formed by the overlapping of the flattened ends or feet of the outer extremities of the fibers of Mueller or nervous connective tissue, which will be explained later; this layer is punctured by millions of openings through which pass the processes on the distal ends of which the rods and cones are found. The next layer inside of the outer limiting membrane is the outer nuclear or granular layer. (G, Fig. 79.) It is almost wholly composed of bipolar cells; that is, they have two processes, one runs outward through the outer limiting membrane and ends in a rod or cone, whilst the other runs inward and ends in a brush like end or tuft in the outer molecular layer. The single layer of cells seen just inside of the inner limiting membrane in Fig. 79, are supposed to be the cells connected with the cones whilst the cells connected with the rods lie in the middle and inner portion of this layer. There are several varieties of nerve cells found in this layer, the functions of which are undetermined, and will be omitted in this description. The next innermost layer is the outer molecular or plexiform layer. (F Fig. 79.) This layer is composed of the end arborisations of the bipolar cells in the outer nuclear layer, which run inward, and the distal end tufts on the processes from the bipolar cells in the inner nuclear layer, which run outward, as well as some other nerve cells which have processes which extend to a greater or less extent in this layer. They are known as amacrine (long fiber) cells; their function is undetermined, but they seem to be association elements to join different portions of the same layer.


The next innermost layer is the inner nuclear or granular layer, E, Fig. 79. This layer is mainly formed of bipolar cells; they send one process outward into the outer molecular layer which ends in a brush-like end or tuft interlacing with the tufts on the inner ends of the inner processes from the bipolars of the outer nuclear layer and send another process inward into the inner molecular layer which ends in an end tuft or arborisation. There are other nerve cells in this layer also, the function of which has not been determined. The next innermost layer is the inner molecular or plexiform layer, D, Fig. 79. This, like the outer plexiform layer, is almost wholly composed of the end tufts of the processes from the bipolar cells ; however these come from the bipolars in the inner nuclear layer which run inward and the processes which run outward from the ganglionic cells in the ganglionic layer and, as explained about the other cells found in the outer molecular layer, those found in the inner molecular layer have not been thoroughly studied and their functions ascertained farther than that they associate different areas of the same layer. The next innermost layer is the ganglionic (knotlike) cell layer, C. These cells might well be called relay cells, for they are very large ; they send from two to three processes outward into the inner molecular layer from each cell, which form tufts and interlace with the tufts on the inner ends of the processes from the bipolar cells in the inner nuclear layer. It is from these ganglionic cells that the axis cylinder processes grow which form the next innermost layer, which is called the nerve fiber layer, B. These axis cylinder processes are continuous from the ganglion cells of the retina into the nerve fiber layer. They pass out of the eyeball through the choroidal fissure and form the optic nerve, which will be described later, and are continuous from the ganglion cells in the retina to the nuclei at the base of the brain. The next innermost layer is the inner limiting membrane, A. It is formed by the expanded or foot-like inner ends of the fibers of Mueller. The fibers of Mueller are the sustentacular (sustaining or binding) tissue of the retina and are the same as the neuroglia cells found in the brain and spinal cord. They are long, branching, connective tissue cells which extend from the inner to the outer limiting membranes and the overlapping of their expanded, or foot-like, ends form both the inner and outer limiting membranes. Their function is to bind the nine innermost layers of the retina together. The retina becomes quite thin at the macula and the cells which otherwise would occupy the space are piled up around it. The processes from these displaced cells, as well as the fibers of Mueller, run obliquely outward and toward its center.


The optic nerve, M, Figs, "j^ and 'JJ, leaves the eyeball at the choroidal fissure (opening through the choroid) and is made up of the axis cylinder processes, which arise from the ganglionic layer of the retina C, Fig. 79, and lie between this layer and the inner limiting membrane, A, forming the nerve fiber layer of the retina B. These nerve fibers, or axis cylinder processes, pass through the openings in the lamina cribrosa (sieve layer), C, Fig. 77, just back of the choroidal fissure. The fibers are bare, or, in other words, devoid of the myeline (marrow) sheaths or white substance of Sw^an, until after they pass through the lamina cribrosa (sieve layer). This covering is then added and this addition adds greatly to the bulk or size of the nerve at the choroidal fissure and at points posterior to the lamina cribrosa. All the fibers which arise from the ganglionic cells in the retina transmit visual impulses toward the brain. However, in the optic nerve are found many fibers which grow from the brain to the retina. These are sensory fibers of association and carry sensory impulses which cause the closure of the pupil when the retina is exposed to bright light, as well as causing the dilation of the pupil when the eye is in darkness and govern co-ordinate movements of the two eyes.

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Fig. Cross section of optic nerve showing neuroglia stained dark and nerve fibers light.

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Fig. 83. Cross section of optic nerve showing nerve fibers stained dark and the neuroglia stained light.

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The arteria centralis retina (central artery of the retina), L, Fig. ^"j, B, Fig. 78, and H, Fig. 82, and the vena centralis retinae (central vein of the retina), I, Fig. 82, and dark vessels in Fig. 78, enter and leave the eyeball with the optic nerve, after entering its substance some ten or twelve millimeters back of the eyeball.


The optic nerve is surrounded by three coverings ; the outermost being the optic nerve sheath, A, Figs. 82 and 83, and F, Fig. yy. This covering is formed by the continuation backward around the nerve of the outermost portion of the sclerotic, Y, Fig. jd, and F, Fig. yj. This sheath is continuous backward to the optic foramen (open mg), where it is continuous with the dura mater (hard or firm mother) of the brain. The optic nerve sheath is quite firm and is composed of connective tissue bundles. Beneath the optic nerve sheath is found a space surrounding the nerve which is known as the intervaginal space, E, Fig. jy, and B, Figs. 82 and 83. This space is continuous through the optic foramen with the sub-dural and sub-arachnoidal spaces of the brain, and this intervaginal space is filled with the cerebro spinal fluid.


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Fig. 84. Showing cross section of the head of a bird,

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Lying within the intervaginal space is found the arachnodial sheath (spider web sheath), G. Fig. 82. This is a very thin, web-like membrane, joined quite intimately to the outer and inner sheaths of the optic nerve, by trabeculse (beams), which cross the intervaginal space.


The innermost covering or sheath is known as the pia mater (thin mother) or pial sheath, D, Fig. "jj, and C, Figs. 82 and 83. This is formed of glial tissue (nervous connective tissue) and from it is given off the septa or trabeculae (beams) which surrounds the bundles of nerve fibers and forms the frame work of the optic nerve and holds it together, E, Figs. 82 and 83, and darker longitudinal striations in M, Fig. J'j, The pial sheath and trabeculse is highly supplied with minute arteries and veins which furnish it with nutrition.


The optic nerve is composed of about eight hundred bundles of medulated (covered with myelin) nerve fibers, D, Figs. 82 and 83, and light longitudinal striations in M, Fig. 77, each bundle being composed of from six to seven hundred axis cylinder processes or nerve fibers, each of which are insulated or covered by the myelin (marrow) sheaths.


The optic nerves, B, Fig. 84, leave the eyeballs. A, Fig. 84, just internal to the posterior poles of the eyeballs, and run obliquely backward and inward through the orbit and pass into the cranial cavity through the optic foramen, then join together and form the optic commissure (uniting band), C, Fig. 84. In the commissure a part of the nerve fibers decussate (cross over) and pass backward in the optic tract of the opposite side, while a portion pass into the optic tract of the same side.


The optic tracts extend from the optic commissure to the base of the brain, where a part of the optic fibers enter the external and internal geniculate (knee-like) bodies, others, the optic thalmus (bed), and the rest go to the anterior corpora quadrigemina (meaning the four bodies). These latter fibers are supposed to be the sensory association fibers, which communicate with the different centers of the brain and their function is for co-ordinate movements of the two eyes as well as reflex movements and sensibilities, while the optic fibers which enter the other basilar (lower) nuclei (nut) come in contact with the protoplasmic processes of the ganglion (enlarged or swollen) cells in these bodies. From these ganglion cells extend the axis cylinder processes, which run upward and backward through the optic radiations to reach the centers of sight which are situated along the calcarian fissure in the cuniate lobe of the brain, which is located in the posterior or occipital region. It is by the interpretation of the impulses created by the cones in the retina and transmitted through the conducting elements in the retina, optic nerve, optic commissure, optic tracts, external and internal geniculate bodies, optic thalmus, and optic radiations to these centers, that sight is accomplished by man.



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