The Eye

THE EYE
Christopher Brandon
Reading: Gartner & Hiatt, Chapter 22

I. GROSS STRUCTURE OF THE EYE.

A. Tunics (Figure 1, Figure 2).

The eye is a spherical structure composed of three layers, or tunics. It functions as a sort of camera, by forming images of objects in the real world onto a layer of photo-sensitive neurons (the retina) in the back of the eye; the resulting neural impulses are eventually relayed to visual centers in the brain.

The outermost layer, the sclera (fibrous tunic) corresponds to, and is continuous with, the dura of the CNS; it represents the posterior 5/6 of the globe.

The uvea (middle tunic) is a highly vascular layer that corresponds to the pia/arachnoid of the CNS; it consists of the choroid (posteriorly), the ciliary body (under the corneo-scleral limbus), and the iris (anteriorly).

The innermost layer, the retina, has the same embryological origin as the CNS, and is anatomically continuous with the CNS. It consists of a photosensitive portion, the neural retina, that lines the posterior 2/3 of the globe, and a non-photosensitive epithelial portion that covers the ciliary body and the posterior surface of the iris. Axons from the retina leave the eye to form the optic nerve.

B. Chambers and Spaces in the Eyeball (Figure 3).

The fibrous and uveal layers are in close contact throughout most of the globe. These two layers separate at the iridocorneal angle (the junction of cornea and iris), forming the anterior chamber, or aqueous compartment. The anterior chamber is formed by the inner surface of the cornea (anteriorly), the lens, iris, and base of ciliary body (posteriorly), and the trabecular meshwork (circumferentially).

The posterior chamber is formed by the iris (anteriorly), the lens and zonule (posteriorly), and the ciliary processes (peripherally).

The vitreous chamber lies between the lens and the retina.

C. Path of Light Rays.

Light first traverses the cornea, then passes through the aqueous humor (anterior chamber), pupil, posterior chamber, lens, vitreous body, and all of the cellular layers of the neural retina before striking the photoreceptors. When we look directly at an object (fixation), the image of that object falls directly on a specialized region of the retina called the fovea, at the posterior of the globe.

D. Radius of Curvature of the Globe.

The globe is formed by the fusion of two spheres of unequal diameter. The sclera has a diameter of 12mm; the corneal diameter is 8mm. The point of intersection between these two unequal spheres is a circular zone that surrounds the eye, the corneoscleral limbus (Figure 6).

E. Blood Supply of the Eye (Figure 3).

Short and long posterior ciliary arteries; the former penetrate the sclera near the optic nerve, the latter on each side of the globe. The ciliary arteries provide blood to the middle, uveal tunic; this, in turn, nourishes the avascular portion of the overlying retina by diffusion.
Anterior ciliary arteries penetrate the sclera just posterior to the corneoscleral limbus; they provide blood to the ciliary body and iris, which are also both uveal regions.
Venous return is via vortex veins, which perforate the sclera, one for each quadrant of the eye.
The central retinal artery and central retinal vein enter the globe within the optic nerve, which passes through a perforated zone at the back of the eye, the lamina cribrosa. These vessels pass through to the interior of the globe, then run laterally and spread out over the inner surface of the retina. Smaller branches then penetrate the retina, nourishing its innermost half with a capillary plexus. The outer half of the retina is avascular and is nourished by diffusion from the underlying choroid.

II. TUNICS OF THE EYE.

A. Fibrous Tunic.

A1. Sclera (Figure 4). This is a layer of dense fibrous connective tissue, 0.3-1mm thick, except where it thickens at the scleral spur. The sclera has a specialized area near the posterior pole, the lamina cribrosa (Figure 5a, Figure 5b). This is a round, perforated zone, much like a sieve, through which ganglion cell axons leave the eye to form the optic nerve.

A second specialized scleral zone is the Canal of Schlemm; this is a small tunnel lying just under the corneo-scleral limbus, running completely around the eye, forming a ring-like structure (Figure 6, Figure 7). It functions to drain the fluid, aqueous humor, produced in the eye (see below).

A2. The cornea (Figure 8) has the greatest refractive (light-bending) power of any ocular structure; it is, therefore, the eye's principal lens. The cornea is avascular; blood vessels would interfere with image formation. Its inner half is nourished by diffusion from the aqueous humor, while its outer half is nourished by tear fluid. It has five histological layers:

The epithelium (Figure 9, Figure 10) is stratified squamous, non-keratinized, 5-6 cell layers thick. It has many free nerve endings, and great regenerative power due to mitosis in its basal layer.
The basement membrane of this epithelium is called Bowman's membrane (Figure 9); it is a structureless collagen membrane that ends at the limbus.
The body of the cornea forms 90% of the thickness of the cornea; it is formed of many lamellae of collagen fibrils, arranged at right-angles to each other, with intervening fibroblasts (Figure 11). This structure allows the cornea to be rigid as well as transparent, and to have great light-refracting power.
Descemet's membrane (Figure 12) is the basement membrane of the corneal endothelium; it is continuous with the trabecular meshwork.
The endothelium (Figure 12) is a single layer of low cuboidal cells that transports nutrients from the aqueous humor to the cornea.
The corneo-scleral limbus (a region, not a histological layer) is well vascularized, and is about 1mm wide (Figure 6).

B. Uveal (Vascular) Tunic.

B1. The choroid (Figure 13, Figure 14) is a spongy brown layer 0.1-0.3mm thick, with several component layers:

The vessel layer comprises many arteries and veins embedded in loose connective tissue with many melanocytes. The pigment serves to absorb light that has passed through the retina and that might otherwise scatter back, reducing visual acuity (sharpness) and contrast.
The choriocapillaris is a layer of fenestrated capillaries. This layer extends anteriorly up to the ora serrata, and nourishes the outer, avascular half of the retina, which lies directly upon it.
Bruch's membrane, the innermost layer of the choroid, is the basement membrane of the retinal pigment epithelium (Figure 14). Trauma may cause the retina and the RPE to separate at this point; this is one form of retinal detachment, and will lead to retinal death if not surgically reversed. You will not be able to see this membrane with LM, but you should know where it is.

B2. Ciliary Body.

The ciliary body attaches to the sclera at the scleral spur (anteriorly) (Figure 15, Figure 16). Viewed from the optic disk, it has radial ridges (ciliary processes) that are highly vascular (Figure 17). It contains a large amount of smooth muscle (the ciliary muscle), arranged in an antero-posterior direction, and some elastic fibers and melanocytes as well (Figure 18). A capillary bed lies just under the choroidal epithelium; this area produces aqueous humor and secretes it into the posterior chamber (Figure 18) (see below).

The ciliary body is covered with a double layer of epithelium; one layer (the inner, i.e. closer to the interior of the globe) is continuous with the neural retina and is not pigmented; the other layer (outer) is continuous with the retinal pigment epithelium, and is heavily pigmented (Figure 19). The outermost layer rests on a basement membrane (a continuation of Bruch's membrane); another basement membrane covers the inner surface (between the epithelium and the vitreous body), and the apical surfaces of these two epithelial layers lie against one another (because they started as a flat sheet that later folded onto itself - see Development, below).

The epithelium extends forward to cover the posterior surface of the iris, reaching all the way to the pupil. It does NOT extend over the anterior surface, however, but doubles back on itself (arrow, Figure 20, Figure 22). At this point, both epithelial layers are pigmented. The absence of an epithelial covering may lead to glaucoma (see below).

B3. Iris.

Structure of the iris (Figure 21). The posterior portion of the iris derives from ectoderm, and is the double epithelium mentioned above (Figure 22); its anterior portion is mesodermal (uveal). The anterior surface of the iris has no epithelial covering; instead, it has a relatively porous covering of specialized fibrobasts (Figure 23). Beneath the surface is loose connective tissue with some pigmented cells (the number of these pigmented cells determines the color of the eye). Beneath this, in turn, is a layer of blood vessels. The posterior surface is covered with two layers of cells, both of which are continuous with the ciliary epithelium; both of these layers are heavily pigmented. Numerous myoepithelial cells derived from this epithelium form the dilator pupillae (Figure 24). Near the pupillary margin (i.e. the tip of the iris) lies a set of circumferentially-arranged smooth muscle cells, the sphincter pupillae (Figure 25).

Nervous Control of the Iris (Figure 26). The autonomic nervous system controls the diameter of the pupil. Sympathetic: The muscle cells comprising the dilator (above) are oriented radially, like the spokes of a wheel, and therefore their contraction increases the diameter of the pupil; these cells receive their innervation from neurons in the superior cervical ganglion. Parasympathetic: The pupillary sphincter is made up of smooth muscle cells oriented circumferentially near the pupil; their contraction reduces the diameter of the pupil. These cells receive their innervation from neurons in the ciliary ganglion.

III. RETINAL DEVELOPMENT.

As the neural tube closes, the neural tube bulges out and forms two outpocketings called optic vesicles. Each vesicle grows laterally, but remains connected to the tube by a stalk that will become the optic nerve (Figure 27). The vesicle collapses on itself, forming a double-layered cup; the innermost layer of this cup will become the retina, while the outermost layer will become the retinal pigment epithelium. Because the vesicle is a single epithelial layer, it is surrounded by a basal lamina; when it collapses, the apical surfaces of the two layers lie against one another, but very little holds them together (Figure 28). The basal lamina lies at the basal surface of the epithelium, and will eventually end up partly on the inner surface of the retina (facing the vitreous chamber), and partly on the outer, basal surface of the retinal pigment epithelium (facing the choriocapillaris) (Figure 29). The absence of any connective tissue between the two layers of epithelium (which lie apex-to-apex; Figure 30) makes them vulnerable to separation resulting from trauma to the eye.

The hyaloid artery supplies the developing eye, but degenerates once the globe has fully formed; its fragments remain in the adult eye and can cause problems (see Vitreous Body, below).

IV. FLUID DYNAMICS OF THE EYE.

Aqueous humor fills the anterior and posterior chambers, being produced by the ciliary epithelium and capillaries within the ciliary processes. It is essentially an ultrafiltrate of plasma, and is produced in the same way as CSF (it is very similar in composition to cerebrospinal fluid). Aqueous humor flows (Figure 31) from the ciliary processes (Figure 32) into the posterior chamber, then through the pupil and into the anterior chamber (it may also percolate through the iris itself). The fluid is then reabsorbed by the trabecular meshwork, from which it drains into the Canal of Schlemm within the sclera.

The trabecular meshwork is a spongy network through which aqueous humor percolates and finally drains into the Canal of Schlemm (Figure 33, Figure 34). The Canal of Schlemm encircles the eye at the limbus (Figure 35). It drains into veins within the sclera.

Glaucoma. Aqueous humor is produced at a constant rate, and drainage usually matches production. This process maintains a significant level of internal pressure (usually 10-20 mm of mercury), required to keep the eye in a fully spherical shape. If the ciliary body produces fluid at too high a rate, or if the drainage is blocked, then the pressure within the eye (the intraocular pressure, or IOP) will increase. Because the globe is a sealed chamber, abnormally high internal pressure will reduce the amount of blood flowing into the eye, eventually causing damage to the retina, and leading to blindness.

The most common cause of this condition is reduction of outward flow, caused by physical blockage of the trabecular meshwork by debris (pigment, cells or fibers sloughed from the iris, pieces of the hyaline artery, etc.), much as hair would plug a shower drain. The leaching of material from the iris into the aqueous humor probably results directly from the absence of an anterior epithelial covering on the iris. In this type of glaucoma, the iridocorneal angle is normal, so the condition is called open-angle glaucoma.

[FYI: High intraocular pressure may also be caused by closed-angle glaucoma, which is often caused by trauma to the eye. Trauma can cause tearing of the delicate trabecular meshwork, leading to the formation of a scar-like structure that pulls the iris and cornea closer together (reducing the angle between them, hence the name 'closed angle') and may obscure or collapse the meshwork altogether (Figure 36). Glaucoma may also result from overproduction of aqueous humor, a condition somewhat analogous to high blood pressure; this form is common in people of African origin. In all forms of glaucoma, the high intraocular pressure is easily detected by routine opthalmological examination; the high internal pressure forces the fibers of the optic disk outward, causing it to be more concave, a phenomenon known as "cupping" (this may also result from thinning of the disk due to loss of axons from the optic nerve) (Figure 37).]

V. REFRACTIVE MEDIA.

A. Cornea. Light is refracted (bent) when it passes from a medium of low refractive index to a different medium with a higher refractive index. About 80% of the refractive power of the eye resides in the cornea, since it represents an interface between two media of very different refractive indices (air and water). The structure of the cornea was discussed above, in the section on "Fibrous Tunic".

B. Lens (Figure 38a, Figure 38b). The lens provides the remainder of the eye's refracting power; it contributes less than the cornea because it is essentially an aqueous (cellular) structure surrounded by other aqueous regions (posterior chamber and vitreous chamber). However, it has the advantage that its shape can be changed by muscular activity of the ciliary body, so its strength as a lens can be varied.

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38b

The lens (Figure 39a) develops from an epithelial sheet. The front part of the epithelium of the lens vesicle (the part facing the external world) remains as a thin sheet of low cuboidal cells, the subcapsular epithelium (Figure 38b, Figure 39a, Figure 39b). The lens substance is made up of many lens fibers, which are hexagonal prisms that lie parallel to the lens surface.

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39b

The lens capsule that surrounds the lens is an elastic and collagenous membrane to which the suspensory (zonule) fibers attach (Figure 40); it is actually the basement membrane of the original epithelial cells that formed the lens vesicle. Zonule fibers connect the lens to the ciliary processes that cover the ciliary body (Figure 40, Figure 41a).

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Cataract. With age, the osmotic balance of the lens fibers may change, causing them to become less transparent. The visual image then becomes hazy, and color perception is diminished (Figure 41b, Figure 41c, Figure 41d). The condition can be corrected surgically, by removal of the cloudy lens and replacement with a plastic one. Monet painted the same scene as a young man (Figure 41d, left panel) and, several decades later, after he developed cataracts (Figure 41d, right panel).

41b

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41d

C. Vitreous body. This structure is a colorless, completely transparent gel that adheres to the retina, especially at the ora serrata and optic disk, and contains hyaluronic acid and collagen fibrils, but very few cells. Remnants of the hyaline artery remain in the vitreous body throughout life; these are transparent, but they may refract light under low-contrast illumination (looking at a white wall, or into the sky) and may then be seen as "floaters". They can present a problem for pilots, who, while flying, may mistake them for something moving across their visual field.

D. Accomodation (Focusing). The eye's adjustments to form images of objects at different distances are called accomodation. Note that the eye, like the camera, forms an image that is upside-down and backwards. Therefore, objects at the top of your visual field are focused on your inferior retina and are reversed left-to-right (Figure 42).

43a

D1. Mechanism of Accomodation (Figure 43a). When the eye (i.e. the ciliary muscle) is at rest, the lens is held in a somewhat flattened shape by tension from the zonule fibers, which suspend it from the ciliary processes. The smooth muscle fibers in the ciliary muscle are oriented in the antero-posterior direction; as a result, contraction of the ciliary muscle causes it to bulge inwards, towards the lens. This reduces tension on the zonule fibers, and allows the lens to assume its own, "relaxed", more-rounded shape. When in this more spherical shape, the lens refracts light more strongly; this produces a stronger lens that can focus on nearby objects. When the ciliary muscle relaxes, tension on the zonule fibers flattens the lens and makes it less refractive; in this configuration it will form a sharp image of a distant object. Therefore, active contraction of the ciliary muscle leads to relaxation of the lens, and enables the eye to focus on close objects. The ciliary muscle is stimulated by parasympathetic nerves, which therefore control focusing.
Emmetropia. In the normal eye (Figure 43b), the cornea plus the flattened lens act to form an image of distant objects exactly in the plane of the retina. When the lens thickens, it becomes stronger, and the eye can then focus on close-by objects. This is the normal (emmetropic) condition.

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43c

D2. Disorders of Accomodation

Myopia. If the eye is too long (Figure 43c), then the in-focus image is formed in front of the retina, and the image formed on the retina is out of focus. This condition can be corrected with a negative (reducing) lens of the proper strength. This condition is also called "near-sightedness".
Hypermetropia. If the eye is too short, then the in-focus image plane is actually behind the eye (Figure 43d), and the image formed on the retina is out of focus. This condition can be corrected with a positive (magnifying) lens of the proper strength. This condition is also called "far-sightedness".
Presbyopia. The lens loses its elasticity with age. As a result, it can no longer assume the rounded shape needed for close focusing. Once an individual loses the ability to focus at normal reading distance (0.3 - 0.5 meters, Figure 43e), which normally happens during the fifth decade of life, "reading glasses" can be used to correct this condition; these are similar to the magnifying lenses used for far-sightedness.

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43e

VI. RETINA.

A. Layers of the retina. The retina is a thin (200-375 microns) layer of CNS tissue that lines the inner surface of the posterior two-thirds of the eye (Figure 44). The neural retina (optic retina) contains the actual light-sensitive elements of the eye (the photoreceptors), and extends anteriorly from the optic root to the ora serrata (Figure 45). Anteriorly from the ora serrata, the retina continues as a non-neural layer that is not light-sensitive (i.e. the ciliary epithelium, described above). Classically, ten layers of the retina have been defined; layers 2-10 comprise the neural retina, in which the cells and processes of the retina are segregated into discrete layers (Figure 46). You will be responsible for the following layers:

LAYER 1: Retinal pigment epithelium (RPE). This is a layer of simple cuboidal epithelium (Figure 47) that rests on a basement membrane lying on the choroid (Bruch's membrane, Figure 14); the cells contain melanin granules that serve to absorb stray light that has passed undetected through the photoreceptor layer. Processes from these epithelial cells surround the outer segments of the photoreceptors, nourishing them by diffusion of nutrients, scavenging discarded photoreceptor outer segments, and recycling the visual pigment. The most common type of retinal detachment involves separation of the retina from the RPE; as mentioned above, diabetes and mechanical trauma are the most common causes of retinal detachment.

LAYER 2: Photoreceptor outer segments (Figure 47). The outer segments of photoreceptors are specialized for the absorption of light. Photoreceptor cells with rod-shaped outer segments (i.e. rods) are more sensitive, and are used for night vision; cells with conical outer segments (i.e. cones) are less sensitive, but can distinguish different ranges of wavelengths, thereby permitting perception of color (see Photoreceptors, below).

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Material below is optional (Figure 46):

LAYER 3: Outer limiting membrane.

LAYER 4: Outer nuclear layer contains the cell bodies of the photoreceptor cells; axons extend from these cells in the vitreal (inner) direction.

LAYER 5: Within the outer plexiform layer, axon terminals from the photoreceptor cells interact with dendrites of bipolar and horizontal cells.

LAYER 6: The inner nuclear layer contains the somas of three neuronal cell types:

Bipolar cells, which receive input from the photoreceptor cells and transmit it to more vitreal layers.
Horizontal cells, which are interneurons found in the outer third of the inner nuclear layer.
Amacrine cells, which are interneurons found in the inner third of the inner nuclear layer.

LAYER 7: In the inner plexiform layer, processes from bipolar cells, amacrine cells, and ganglion cells interact synaptically.

end of optional section
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LAYER 8: The ganglion cell layer (Figure 46) contains the cell bodies of ganglion cells.

LAYER 9: The optic nerve fiber layer (Figure 46) contains the axons of ganglion cells; these axons leave the retina at the optic disk, and form the optic nerve, which projects to higher visual centers in the brain.

LAYER 10: The inner limiting membrane (Figure 46) is formed by the processes of retinal glial cells called Müller cells; processes from Müller cells span the entire thickness of the retina. The innermost surface of the retina, which is formed largely by the end-foot processes of Müller cells, is covered by a basement membrane that is part of the tenth layer (see Development, above, to understand how this B.M. got here). This basement membrane is important clinically because collagen fibers of the vitreous attach to it; if they pull too strongly, they may cause retinal detachment.

B. Retinal function.

B1. Photoreceptors.

a. Photoreceptor structure.

When an image is formed on the retina, light energy is first absorbed by specialized neurons called photoreceptors, causing a change in their membrane potential and, ultimately, a change in the amount of neurotransmitter that they release onto other retinal neurons. Thus, the photoreceptors convert, or transduce, light into a form that can be transmitted by chemical (neurotransmitter) pathways; this process is called phototransduction.

The distal half of the photoreceptor (Figure 48, Figure 49) has a specialized region, the outer segment, that contains the biochemical apparatus for phototransduction. The distal region of the outer segment is much enlarged, with many infoldings, called discs, of the plasma membrane that dramatically increase its the surface area. These discs are oriented at right-angles to the path of light; this arrangement maximizes the chance that a photon of light will pass through the membrane and be detected. In addition, to further increase sensitivity, outer segments are packed together at very high densities.

b. Phototransduction.

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The light-absorbing complex is dissolved in the plasma membrane of the discs. This complex contains a membrane protein, opsin, and a small molecule, retinal. Retinal is a Vitamin A derivative. A photon of light strikes the complex and is absorbed by retinal, which changes its conformation (Figure 50a) and dissociates from the opsin. This, in turn, causes a change in opsin's configuration that, through a second-messenger cascade, reduces the conductance of a sodium channel in the outer plasma membrane of the outer segment (Figure 50b). Ultimately, this change causes the photoreceptor to hyperpolarize in response to light.

c. Visual acuity and color vision.

The vertebrate retina has two types of photoreceptors, rods and cones; the names relate to the shape of the outer segments (Figure 49). There is only one type of rod; it can detect a broad range of visible light wavelengths in the green range, but, since there is only one type, it cannot distinguish among lights of different colors (Figure 51). Rods are extremely sensitive to light, being able to detect a single photon. Because of this, rods are specialised for night vision (in fact, they do not function in bright light). Human cones come in three types; each type can detect either red, green, or blue light, and all of them require bright light to function. They are also packed at very high densities within the retina, so they provide a very high-resolution, fine-grained image. Therefore, cones are specialised for daylight, high-resolution color vision.

While rods and cones co-exist in most retinal regions (Figure 52), the rod:cone ratio varies across the retina. Cones are concentrated in the fovea of the retina (Figure 53); this area provides the image of the object we look directly at. They provide us with high-resolution color vision during the day. The peripheral retina provides us with peripheral vision (outside the center of our visual field). Cone densities are much lower here, which means that our peripheral vision is largely color-blind. Rods are absent from our foveal pit, which means that our central vision has low sensitivity; rods are absent from the fovea, but increase in number dramatically in the peripheral retina, so that our peripheral retina is much more sensitive to dim light.

[FYI: This is the reason that you cannot see a dim star on a dark night if you look directly at it - the image falls on relatively low-sensitivity cones, which simply can't see it. Moving your eye slightly away from the star will make it appear, because the image falls on a retinal region that contains rods, which are sensitive enough to detect the star.]

[FYI: Rods cannot detect red light, whereas red cones can (Figure 51). Therefore, if you stay in a room lit only by deep-red light, your rods 'think' they are in the dark, and therefore remain fully activated (they shut down in bright light). This means that military pilots can remain in a state of night-readiness, while still being able to see relatively well, by staying in a room lit by deep-red light.]

Photoreceptors differ in their wavelength sensitivity because they contain different types of opsins; these various opsins make opsin-retinal complexes that differ in their light-absorption maxima. Several different genes produce the several different types of opsins. All of these genes are located on the X chromosome; as a result, males are much more susceptible to genetic deficits in color vision because they have only one copy of each gene (Figure 54). The most common deficit involves the absence of a functional green opsin, which prevents the eye from distinguising red from green ("red-green color blindness"). Clinically, this can easily be detected with colored diagrams called Ishihara plates (Figure 55a). A simple filter can be used to simulate color blindness (Figure 55b).

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B2. Retinal pigment epithelium (RPE).

The photoreceptor's outer-segment discs are renewed constantly; new ones are made at the base of the outer segment, and older ones are shed at the tip. The discs are phagocytized and digested by cells of the RPE, which have long, pigmented processes that surround the outer segments (Figure 56, Figure 57). In addition, the RPE carries out part of the biochemical conversion of inactive retinal to active retinal, and Vitamin A to retinal. A genetic deficit in RPE function underlies the familial disease Retinitis Pigmentosa; phagocytised discs accumulate without being digested, and the RPE eventually degenerates, causing death of the retina and leading to blindness.

B3. Ganglion cells (Figure 58).

The original light-induced hyperpolarization of the photoreceptor is conveyed synaptically across a series of retinal neurons (you will study these later). All of these neuronal types modify the original signal in a variety of ways; eventually, the transduced information reaches the innermost layer of cells (i.e. closest to the vitreous), the ganglion cells. The ganglion cell axons reach to the surface layer of the retina (nerve fiber layer), then run toward the optic disk, pass through the lamina cribrosa of the sclera, and form the optic nerve. This heavily-myelinated nerve (cranial nerve II) projects into the cranium through the optic foramen, carrying visual information to higher brain regions.

C. Specialized areas of the retina.

C1. The fovea (fovea centralis) (Figure 59) is the region of the retina that is specialized for high-resolution, color vision. Here, the innermost retinal layers are present, but have been displaced to the sides, away from the fovea (Figure 60). As a result, light has fewer layers to pass through (Figure 61), and is therefore scattered less; this increases the sharpness of the image in this area. The higher primates (including man) have foveal retinas; most other animal species lack a fovea.

C2. The macula is a retinal region about 3mm in diameter that surrounds (and includes) the fovea, and that contains a yellow pigment (predominantly lipofuscin) (Figure 62). The pigment functions to absorb blue light, which would otherwise be scattered (which is what makes the sky blue); the scattered blue light would reduce image sharpness.

C3. Optic disk. At the optic disk (optic papilla), axons of ganglion cells gather and exit the eye (Figure 63), traveling through perforations in the sclera (the lamina cribrosa) (Figure 64). No retinal neurons are found in this region, which is therefore called the "blind spot".

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Finding your blind spot (Figure 65a).

The retina cannot provide us with visual information about light falling on the optic disk, and yet we are not aware of the missing information, i.e. we do not see the spot (Figure 65b). To identify your own blind spot, look at the dark 'plus' in the upper part of the diagram in Figure 65a with your left eye open and your right eye closed. At the correct viewing distance, the image of the dark circle falls on your optic disk, and simply disappears.

[FYI: Our lack of awareness of the blind spot cannot be due simply to the fact that the images from our two eyes overlap and fill in the missing information, because the blind spot does not appear in one eye even when we cover the other eye; in fact, the reasons are obscure, but psychophysical tests show that higher brain centers "fill in" the missing information based on what they see in the surrounding areas. To demonstrate this to yourself, find your blind spot as above (Figure 65a), then move down to the dark 'plus' in the lower part of the diagram. The gap in the line falls on your blind spot; your brain receives no information about what is in the blind spot (it cannot know that the gap is there), so it "assumes" that the line is continuous across the spot, and you see an unbroken (but unreal) line.

65b

Papilledema.

The optic nerve is NOT a peripheral nerve, but rather a tract within the CNS. This arrangement provides a very useful way to detect intracranial pressure that is dangerously high. Because the sclera and dura are continuous, the underlying subarachnoid space around the brain is continuous with the corresponding space under the sheath of the optic nerve (Figure 66). Because of this continuity, high pressure in the cranial cavity (due, e.g., to bleeding or to a tumor) will result in high pressure around the optic nerve. At a high enough pressure, the central retinal vein will collapse, but the central retinal artery will remain open (recall the differences in the structure of the walls of arteries and veins). Blood continues to enter the globe but cannot drain out; this causes fluid to be extruded into the extracellular spaces of the optic nerve head and retina, causing them to swell. The normally-concave optic disk bulges inward, into the eye, becoming convex. This condition, papilledema, is easily recognized on routine ophthalmological examination (Figure 67).

[Another consequence of the embryological origin of the retina: Because the retina is CNS tissue, diseases that affect the CNS also may affect the optic nerve. The most common example is Multiple Sclerosis, an autoimmune disease in which CNS myelin is destroyed. Optic nerve inflammation and damage (and consequent visual loss) is usually the first noticeable symptom of MS.]

D. Nourishment of the retina (Figure 3).

The inner half of the human retina is vascular, containing two capillary plexuses that derive from the central retinal artery. The retinal plexuses branch in the same plane as the retina. The innermost plexus lies within the nerve fiber layer, and partially within the ganglion cell layer; the outer plexus runs in the outer plexiform layer. Diabetes often leads to damage of these capillaries.

The outer half of the retina is avascular, receiving its nourishment by diffusion from the underlying choriocapillaris. Because of this arrangement, separation of the retina from the RPE, or separation of the RPE from the choriocapillaris, cuts off the outer retina's normal supply of nutrients and oxygen, and very rapidly leads to death of the separated region of retina. Separation may be caused by trauma (the tennis ball is a particularly effective agent for this), or by several systemic diseases (e.g. diabetes).

The development of the retinal vascular capillary networks is disturbed in premature infants who are exposed to high oxygen levels within incubators (to alleviate Respiratory Distress Syndrome, which, you should recall, is caused by inadequate production of pulmonary surfactant before 24 weeks of gestation). In response to the high oxygen (hyperoxic) environment within the incubator, the retinal capillary networks do not develop (because they are not needed). Then, when the infant is returned to room air, the change to a (relatively) hypoxic environment causes capillary development to be switched on; unfortunately, growth is often excessive (hypervascularization), completely covering the inner surface of the retina, and leading to blindness (Retinopathy of Prematurity). Careful monitoring of oxygen levels has reduced the incidence of this condition, which nonetheless remains clinically significant.

VII. CONJUNCTIVA.

At the corneal margins, the epithelium of the anterior portion of the sclera (the bulbar conjunctiva) is continuous with the mucous membrane that lines the eyelids (the palpebral conjunctiva) (Figure 68). This epithelium ranges from stratified squamous (at the corneal margin) to stratified cuboidal with mucous goblet cells. The underlying stroma of the lamina propria is very loose, and rests on a cushion of intraorbital fat; this arrangement permits free rotation of the eyeball. This conjunctiva also functions to seal off the orbit from the outside world. The goblet cell secretions become part of the tear fluid.

VIII. EYELIDS.

The inner surface of the eyelid contains ducts from several deep glands. Modified sebaceous glands (Meibomian glands) open on the free edge of the lid (Figure 69, Figure 70); their oily secretion mixes with tear fluid and reduces the rate of evaporation of that fluid. Other modified sebaceous glands (Glands of Moll), as well as modified sebaceous glands (Glands of Zeis) empty into the follicles of the eyelashes.

IX. LACRIMAL GLAND.

The lacrimal gland is a compound tubuloalveolar gland very similar to the parotid gland. It produces a serous secretion rich in the antibacterial enzyme lysozyme.

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