Special Senses I: The Eye


The Eye

Let's first consider the overall structure of eyes. The vertebrate eye is constructed pretty much the same way in all species of domestic mammals, in birds, and in primates, though there do appear to be some specific structural and functional adaptations in each of these groups. To a large extent, however, similarity in structure is to be expected. After all, the same laws of optics apply to everybody, and it's therefore likely that all of the final designs would be pretty much variations on the same theme. In a mechanistic sense designing a functional eye is really an engineering problem, and there are only a few ways in which that problem can be solved.

For a more comprehensive treatment of the physiology of the eye and the nature of vision, click here.

Terminology and Orientation

A bit of terminology is in order at this point. Since the eye is globular, it's customary to refer to its structures with respect to 1) its optical axis, and 2) its imaginary central point. To speak of a structure as being "inner" or "outer" means it's nearer to (or farther away from) the physical central point of the spherical eyeball. The structural asymmetry resulting from the placement of the lens and the retina defines the "front" and "back." A section of the eyeball taken parallel to the optical axis is a "horizontal" section; one at right angles to the optical axis is a "vertical" section.

General Structure

Most of this exercise will be done on slide 268. This is a horizontal section, and if you will examine it off the microscope, you'll be able to make out the general shape and several major structures.

The Three Tunics of the Eye

Now turn to the microscope with slide 267 or 268. The wall of the eyeball is made up of three coats, or tunics, one inside the other. From outermost to innermost, these are the corneoscleral tunic, the uvea, and the retina.

Corneoscleral Tunic: Sclera

The first and largest part of the corneoscleral tunic is the sclera, the tough, collagenous, outer part of the eyeball. This is what we call the "whites of the eyes," and this tightly woven bag of collagen fibers forms a flexible, watertight, and protective covering. It's also the site of attachment of muscles that rotate the eye.

Corneoscleral Tunic: Cornea

The second part of this tunic, the cornea, is the principal light refracting structure. It forms the anterior part of the corneoscleral tunic, joining the sclera at the limbus or corneoscleral junction.

To see the structure of the cornea, click here.

The cornea is exposed to the environment on its outer surface, and is epithelial in nature. The outermost part of the cornea is a stratified squamous layer, the corneal epithelium. This layer isn't normally keratinized, of course. The curvature of the cornea is what refracts light coming into the eye and provides for most of the focusing; alterations in the shape of the cornea can have profound effects on vision, and surgical procedures to correct vision problems take advantage of this fact. It is possible to remove the cornea, freeze it, and actually machine it in a small lathe-like device to alter its curvature, then stitch it back into place, with different focusing characteristics!

The cells of the corneal epithelium sit on a thick basal lamina, Bowman's membrane, (named for Sir William Bowman, 1816-1892, the English ophthalmologist, anatomist and physiologist).

The corneal epithelium varies in thickness among different species, and if you have trouble finding in on this slide, try slide 268, where it should be very evident. The bulk of the tissue in the cornea is the substantia propria, a thick collagenous CT in the form of regular lamellae. The inner surface of the cornea has a very thin, single layer of epithelium, and between these cells and the substantia propria you will find Descemet's membrane, their basal lamina (after Jean Descemet, 1732-1810, a French physician).

Uveal Tunic

The uvea is the middle tunic, and it's composed of three elements. These are the choroid, which comprises most of it, the ciliary body, and the iris.

Uveal Tunic: Choroid

The choroid is the highly pigmented and vascular layer of the uveal tunic that underlies the sclera. It provides nutritive support, and so there are numerous blood vessels and large lymphatic channels visible in it. In most cases the melanin pigmentation in very obvious; the heavy pigmentation serves as a "light trap" in much the same way the black matte finish of the inside of a camera does. It minimizes internal reflections and increases contrast by absorbing stray light passing through the retina, thus increasing the acuity of vision.

To see the choroid, click here.

Uveal Tunic: Ciliary Body

The ciliary body is part of the uveal tunic, too. It's located in the angle between the end of the cornea and the lens, and runs circularly around the anterior part of the eye.

Most of the ciliary body is composed of smooth muscle whose contractions work the lens accommodation mechanism via the tension imposed on the zonule fibers.

To see the relationship between ciliary body, zonule fibers, and the lens, click here.

In addition to its role in the mechanism of accommodation (see below), the ciliary body is a secretory structure. The inner surface of the ciliary body is lined on its inner surface with a double layer of cells. These are continuous with the retinal pigmented epithelium and are derived from the optic cup. This ciliary epithelium is the site of production of the fluid which fills the anterior and posterior chambers of the eye, the aqueous humor.

To see the ciliary epithelium, click here.

The epithelium on the finger-like projections of the ciliary body into the posterior chamber (the ciliary processes) produce the aqueous humor, secreting it into the posterior chamber. It then passes through the opening in the iris (the pupil) and is drained away via a series of channels in the angle between the iris and the cornea.

The principal site of drainage is the canal of Schlemm (Friedrich S. Schlemm, 1795-1858, a German anatomist). The canal is easily seen in the iridio-corneal angle as an irregularly shaped opening lined with simple squamous epithelium. It looks much like a small lymphatic duct or vein, but of course you would not expect to see cells in it!

To take a ride on the Canal of Schlemm, click here.

The continued production of the aqueous humor and its removal is important in function; the intraocular pressure thus produced helps to maintain the normal shape and size of the eyeball, which is vital to keeping an image in focus. Blurry vision and "seeing stars" are premonitory symptoms of increased intraocular pressure, which can result from overproduction of aqueous humor, or damage to the drainage system, and this is the condition of glaucoma. Uncorrected, pressure can build up and damage the eye, causing eventual blindness.

Uveal Tunic: Iris

Projecting from the ciliary body is the iris. This forms a pigmented, diaphragm-like structure around the eye. It has a hole of variable aperture in it, the pupil, through which light passes. The iris also serves to divide the eye into anterior and posterior chambers in front of the lens.

The iris contains a fair amount of muscle, and can contract in bright light or expand in dim light to provide the light sensitive retina with the proper level of illumination. The iris forms the colored part of the eye. Sometimes the formation of the iris is incomplete, a developmental defect called coloboma iridis. It's frequently associated with an incomplete choroid coat (coloboma choroidea) and it may be the result of trauma or surgical intervention.

The Retina

The retina is the innermost tunic of the eye, in which the light sensitive elements are located and on which the image is formed. The vertebrate retina has nine true layers, and a tenth pigmented layer is closely associated with it. The vertebrate retina is "reversed." That is, the image is actually formed at the back (the outer part) and the nervous impulse it generates is brought inward by the converging axons that form the optic nerve. The optic nerve physically passes back through the entire thickness of the retina, and thence on to the brain. This situation results from the way the eye forms in the embryo, by invagination of the optic vesicle into a double walled optic cup.

Layers of the Retina: Pigment Layer

We'll look at the retina's layers from the outermost to the innermost, in order of their occurrence.

To see these layers of the retina, click here.

The cells of the pigment layer are impregnated with melanin, and they also contain lipofuscin. The light sensitive parts of the retina are continually "turned over" to maintain optimum function. The pigment layer cells phagocytose the ends of the rods and cones as they are renewed: hence the accumulation of lipofuscin. The pigmentation this activity produces helps to increase the contrast of the visual image by absorbing light that would otherwise be reflected back inwards towards the rods and cones. These cells comprise a layer of cuboidal cells that have round nuclei and an aggregation of melanin and lipofuscin at their inner ends.

The pigment layer is not considered by all authorities to be part of the retina proper, because its embryological origin is different from the retina's. It comes from the outer wall of the optic cup, not the inner one. But the association is so close and so vital to normal retinal function that a strong case can be made for including it as a part of the retina.

Layers of the Retina: Rods and Cones

The outermost layer of the retina proper is the bacillary layer or the layer of rods and cones. These are the actual light sensitive elements.

Rods and cones are cellular structures derived from cilia, and they represent another example of the modification of cilia for sensory purposes. Each is a fairly large structure, enclosing a stack of light sensitive membranes.

The names "rod" and "cone" reflect the general shape each type of light receptor takes, and in good preparations for the LM, the shapes are generally visible. Remember that each of these is actually only part of a cell; there is a narrow "waist" between the rod or cone and the cell body of which it's a part. Rods and cones are transducers and their function is to take the physical signal of electromagnetic radiation and transform it into a neural one.

Rods are associated with low light vision, and are most numerous in nocturnal animals. Cones are required for color vision (there are three types, one sensitive to each of the primary colors) and are most numerous in diurnal species. You would expect this, since the spectral range of daylight is much broader than the light of the moon or the stars is. Although Benjamin Franklin had something else in mind when he made the remark, "In the dark, all cats are gray," nevertheless the statement is true in and of itself. It's extremely difficult, even for animals with good color vision, to make out colors in low light environments. Humans have excellent color vision, and you can prove this to yourself by going out on a moonlit night and trying to distinguish between different colors.

Color vision is best in the birds and primates. You will frequently hear the statement made that domestic animals are "color blind," but this has yet to be definitively proven. As a matter of fact, there is no way we can tell exactly what some other animal sees, and such data can only be derived from indirect experimentation. There seems to be no doubt that birds can see colors quite well (though whether they see them exactly as we do is open to debate) and from behavioral studies there's some evidence that other mammals besides primates have at least rudimentary color vision. There is fairly good evidence that deer can see blaze orange, and seeing eye dogs can certainly tell red traffic signals from green ones.

To finish off this train of thought, I'll note that cones are present in animals that have long been held to be "color blind" like dogs and cats, though they are less numerous than in primates. If cones are part of the color vision process, as they seem certainly to be, it's a bit premature to assume that any animal that has them "can't see color."

Layers of the Retina: Outer Limiting Membrane

Moving inward, you'll find the next layer, the outer limiting membrane. This isn't really a membrane. It's a place where the rod and cone cells are intersected by the "feet" of the quasi-glial element, the Müller cells. This has the appearance of a membrane in the light microscope because everything is closely packed together. The Müller cells form junctions with the photoreceptors and with other Müller cells. This "membrane" may be a way to isolate the subretinal space, creating a space for the outer segments of the rods and cones and limiting diffusion of nutrients, growth factors, and waste products between the choriocapllaris and the outer segments.

Layers of the Retina: Outer Nuclear Layer

Immediately inwards of the outer limiting membrane, you'll find a group of nuclei. This layer is the location for the nuclei and cell bodies of the rod and cone cells (whose sensitive elements we saw projecting outwards as the bacillary layer). Collectively the nuclei of these cells constitute the outer nuclear layer.

Layers of the Retina: Outer Plexiform, Inner Nuclear, and Inner Plexiform Layers

Inwards of the outer nuclear layer you'll find a relatively clear zone. This is the site of numerous synapses between the rod and cone cells and the processes of various integrator neurons. This is the outer plexiform layer. While the initial neural impulse is generated in the rods and cones, they must pass the signal along to other neural elements for processing and further handling. The dendrites of these cells and the axons of the rod and cone cells constitute the outer plexiform layer.

The integrating elements are the horizontal, bipolar, and amacrine cells. They pass along the signal to the next neural element in the chain. The cell bodies and nuclei of these integrator neurons are located in the inner nuclear layer, next inwards from the outer plexiform layer. The bipolar cells are particularly important, as they have one pole thrust outwards into the outer plexiform layer, and the other—their axon—thrust inwards to the next layer, the inner plexiform layer.

The inner plexiform layer, like the outer one, is a region of synapses. Here the bipolar cell processes synapse with the processes of ganglion cells. Ganglion cells are also neurons, and they are specifically those neurons whose axons will collect to form the optic nerve. Thus they form the final intra-retinal element in the neuron chain, and their axons run back into the visual centers of the brain proper.

Layers of the Retina: Ganglion Cell Layer

While the ganglion cells have synapses with bipolar cells in the inner plexiform layer, their cell bodies aren't located there. The cell bodies and nuclei of the ganglion cells are located in the next retinal layer, the ganglion cell layer, which has far fewer nuclei than the inner or outer nuclear layers. Like any other neurons, these ganglion cells have axons, and the axons form the next layer inwards.

Layers of the Retina: Nerve Fiber Layer

The axons of ganglion cells are bundled together to form the nerve fiber layer. These axons are afferent fibers from the ganglion cells, and after running radially around the inner surface of the retina, they all come together at the site where the optic nerve penetrates the retina (see below). This is the "blind spot" of the vertebrate eye.

Layers of the Retina: Inner Limiting Membrane

The innermost layer is the inner limiting membrane, which again isn't really a membrane, but is instead a place where the Müller cell feet are in close approximation to each other.

Remember that most of the cells of the retina are in fact neural elements; they are either transducers (the rods and cones), integrators (bipolar and horizontal cells) or the terminal sensory neuron of the chain (the ganglion cells). As is the case with other neurons, all of these cells are pretty much helpless to defend themselves against the insults of the outside world. They have to be cosseted and coddled by neuroglia, or at least their equivalent. The cells involved in the formation of the inner limiting membrane are Müller cells, tall cells which (like astrocytes in the CNS) serve to support and protect the neurons. The Müller cells run from the outer limiting membrane to the inner limiting membrane, and are sealed at both ends. At their outer ends they are fused to the "waists" of the rod and cone cells, and at the inner end to each other.

The Optic Nerve and the Ora Serrata

You may be able to see a section of the optic nerve on your slide 267, if there is one in your box. The optic nerve begins at the optic disc, the point where all the axons of the ganglion cells come together, and projects back into the main portion of the brain. This is the "blind spot," of course, because no light sensitive elements can located at the point of perforation.

To see the optic nerve, and the "blind spot" click here.

There is another non light sensitive portion of the retina, that part between the ciliary body and the posterior part of the iris. The margin between the sensitive and insensitive parts of the retina is the ora serrata, normally visible only in gross anatomy preparations.

The Fovea

Vertebrate eyes have a region of most acute vision, the fovea. The retina is thinner here, and receptors more crowded together. The fovea may not be present on your slide, but some slides in the set do show one. It's an area where the thickness of the retina is reduced but the elements in it are noticeably more orderly and tightly packed.

At first thought you might guess that the fovea would be centrally located, and that gazing straight at an object would put its image in the area of most acute vision, but that isn't the case. The fovea is slightly off the optical center of the eye, and rays that come in at an angle to the central axis are focused directly on it.

Why should this be so? The reason is simple. This arrangement gives an animal much better peripheral vision, which is of real survival value. Think in terms of an animal who lives in fear of attack from predators and relies on vision for warning of an approach. That animal is using its eyes for other things as well, and any predator who knows his job will normally sidle up alongside his prey, hoping to avoid detection. By arranging the visual organ so that the most acute area is off the main axis, any object moving through the edge of an animal's visual field will more readily be detected.

An old trick, long taught in the military, is to tell sentries to keep their eyes moving and not to stare out at one spot. If they perceive movement and are interested in seeing some small or distant object, they are taught not to look straight at it, but rather to look past it.

The Lens

The lens is the largest and most obvious part of the eye. It's a biconvex, transparent structure composed of numerous layers of cells. It's located between the large vitreous chamber of the eye and the smaller posterior chamber. The anterior chamber is demarcated by the cornea on the outside surface, and by the iris between it and the lens.

To see the relationship of the lens, iris, and anterior chamber, click here.

The lens, because of its dense construction, is usually poorly infiltrated by paraffin, and histological sections usually show some damage and considerable shrinkage; however, you should be able to make out the major features of its histology.

The lens is covered by a thin lens capsule, an amorphous layer that is really the basement membrane of the underlying lens epithelium. The capsule is thickest on that side of the lens facing the iris, and thinnest on the posterior surface. The lens epithelium that produced it is a single layer of cuboidal cells, which are the source not only of the capsule, but of the lens cells that constitute the bulk of the lens' material. At the equator of the lens these epithelial cells proliferate, elongate, and transform themselves into lens cells.

The lens grows in size throughout life, and a constant production of new lens cells is necessary. Mitoses of the lens epithelium and differentiation provide for this. At the equator of the lens the proliferating cells begin to elongate and send out long processes that "dig" under the adjacent lens epithelium. The nuclei of these cells remain in the region of the equator, and you will see them scattered there. The extrusion of processes causes new layers to be added to the lens continually. Eventually the nuclei of the transformed lens cells will fragment and disappear, but their cytoplasmic boundaries can usually be made out fairly easily.

The lens is responsible for accommodation, i.e. the adjustment of the eye for observing near or far objects. It does this by changing its shape. This changes the focal length of the lens, and permits refraction of the rays coming from near or far objects.

The lens is suspended from the muscular ciliary body by a number of delicate zonule fibers, which you should be able to make out as wispy strands between the edge of the lens and the pigmented ciliary body. Contraction of the muscles of the ciliary body tugs at the fibers and changes the shape of the lens. In the relaxed condition (that is, with no tension on the fibers beyond that needed to support the lens) the shape is such as to refract rays from distant objects, and the eye is focused at infinity. Accommodation for close vision requires tension to be exerted to deform the eye.

This system makes a lot of sense for an organ with the role of a long range warning system: most predators (and prey) spend their time scanning the far horizon, looking for something to eat (or for something that might want to eat them). By having long distance vision as the "default" condition, the amount of exertion needed is minimized. Incidentally, this system is also related to the deterioration of eyesight with advancing age. Older lenses are stiffer and less deformable, and more effort is needed. So close vision becomes more difficult with each year. Humans get around this by equipping themselves with artificial lenses; animals don't worry about it. Most wild animals don't live long enough in any event for accommodation problems to become serious; but the situation does arise in domestic animals whose protected environments make it possible for them to live lives much longer than "normal" for their species.

Another age related degenerative change in the lens is the development of cataracts, opacities that impair the passage of light and cause loss of vision. There are many kinds of cataracts, and many causes for them, but they are a fairly common consequence of living beyond the "normal" life span. They're quite common in older domestic animal, especially dogs, because pets tend to live much longer lives than their feral counterparts.

Accessory Structures

Accessory structures of the eye include the eyelids and the lachrymal glands.


The eyelid (and the that part of the eye socket which faces it) is lined with an interesting and unusual conjunctival epithelium. The part lining the inside of the lid is the tarsal conjunctiva, which continues onto the outer surface of the eyeball as the bulbar conjunctiva. The conjunctival epithelium is unusual in that it's classified as stratified columnar, and it also contains goblet cells. The tarsal conjunctiva forms a mucocutaneous junction with the integument at the edge of the eyelid, and the bulbar conjunctiva transitions to the stratified squamous epithelium of the cornea.

Slide 266 is a vertical section through an eyelid, showing a typical stratified squamous epithelium and adnexal structures of skin on the outside surface, and a transition region of smooth conjunctiva on the bulbar side. The center of the eyelid is filled with bundles of skeletal muscle, blood vessels, nerves, glands, etc. most of which are visible on slide 266.

To see the conjunctiva, click here.

To see the eyelid, click here.

The large tarsal glands (also called Meibomian glands, after Hendrick Meibom, 1638-1700, a German anatomist) are quite obvious. These are modified sebaceous glands, opening into a long central duct which isn't visible on this slide. (If your eyes have ever been gummed shut after a hard night at the local Brauhaus, that crud that held them closed came from these glands.)

There are also modified sweat glands present, the glands of Moll (Jacob A. Moll, 1832-1914, a Dutch oculist) whose secretions are released into the follicles of the eyelashes.

Lachrymal Glands

It's necessary to keep the surface of the eye moist, to prevent abrasion and damage to the cornea. The eye produces a fluid secretion, the tears, in glands located nearby. These secretions are conducted via ducts to the surface of the eyeball. None of the slides in the set have lachrymal glands, but a demonstration slide (which also shows all of the other features of the eye) is available on which you can see these. Lachrymal glands are very similar to serous type salivary glands in structure and operation. The cells closely resemble those in salivary glands, too. Tears, in addition to providing lubrication, have an antibacterial effect because of the inclusion of lysozyme in their composition.

To see an example of the lachrymal gland, click here.

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