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Vision/Hearing

Anatomy and Physiology of the Eye

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By John F. Gipner, M.D.

Colleague and Friend of the Cleveland Sight Center

The comparison of the eye to a camera has often been made, and is applicable from the standpoint of the design of the eye. In both the camera and the eye there is a lens system and closed box. In the camera the lens can be moved forward and backward to focus the image on the sensitive plate which is placed in the back of the camera. The image is inverted and the inside of the camera is darkened in order that no dispersion of light will be allowed. In front of the lens there is an iris diaphragm, very much like the pupil of the eye, which regulates the amount of light entering the camera.

The eye has a lens system made up of two components, the cornea and the crystalline lens. The interior of the eye is also darkened. There is a sensitive retina in the posterior part which is much like the plate in the camera. An iris diaphragm in the front regulates the size of the aperture or pupil, which is only a hole in the iris. Here again an object is focused upon the sensitive plate or the retina, forming an inverted image.

From this comparison, it can be seen that the camera and the eye are quite identical. The distance from the lens to the plate is regulated by a bellows in the camera. In the eye, the changes in focus are accomplished by changes in curvature of the crystalline lens.

As cameras are inanimate objects and the eyes are made up of living cells, one should not habitually think of the eye as a camera. Thousands upon thousands of living cells compose the structure of the eye. That being the case, the eye is influenced by all changes which take place in the human body.

Life starts when one cell is activated by another cell; a group of cells is formed. Eventually the ball of the eye develops as part of the primitive nervous system.  The eye is an outgrowth of the primitive brain.

From the surface of the fore-brain there develops two outpouchings which undergo changes and soon the rudiments of an eye appears. Then, tissue of a loose fibrous structure condenses on the surface forms the sclera, choroid, and ciliary muscle in front of the iris.

Thus the continuity of development is observed. All of the structures of the eye are developed embrylogically from one cell the eye is most intimately related to the brain.

Anatomy of the Eye

There are three primary layers. The first outer layer is the fibrous layer and is composed of cornea and sclera. It is this layer that gives shape to the eyeball. Both of these structures, the cornea, which is the glassy transparent front part of the eye, and sclera, that white porcelain-like layer, are made up of the same white fibrous connective tissue. Consequently, light strikes the surface in many directions. The color of the sclera results from this dispersion of light. Posteriorly the sclera is continuous with the sheath of the optic nerve. There are two points of weakness in an eyeball, the optic disc and the zone of transition from cornea to sclera.

The lid of the eye is stiffened by a cartilage or tarsal plate. Embedded in the tarsal plate are glands known as meibomian glands which secrete oil on the lid margin. This film of oil on the margins of both the upper and lower lid prevents tears from running over the cheek. The eyelashes are located on the skin side of the lid. Other glands are also found in the lid directly associated with these hairs, or cilia, or eyelashes. One of these is a sweat gland and the other an oil gland. An inflammation in one of these glands causes a stye; an involvement of the meibomian gland produces a chalazion or tarsal cyst.

The front surface of the cornea is covered with epithelium. This corneal epithelium, though altered in structure, is directly continuous with the conjunctiva.

The conjunctiva also lines the under surfaces of the lids. At the lid margin the conjunctiva changes over into skin. The transition zone where skin changes into mucous membrane or conjunctiva is always a potential source of malignancy or tumor.

It is important to say, however, that underneath the epithelium there is a glassy membrane called Bowman’s membrane. A cinder or a piece of hot metal flying into the eye, but burning no deeper than this membrane, will not leave a scar. But if it burns deeper than Bowman’s membrane, there is a destruction of that membrane; scar tissue fills the defect, causing a visible scar. On the posterior surface of the cornea there is another membrane which is lined on its inner surface with a single layer of cells (endothelium).

The middle layer of the eye, the uvea, is composed of chorid ciliary body and iris.

The chorid has its origin at the optic disc and extends forward as far as the ciliary body. It is made up of blood vessels of several sizes and a fine capillary layer on the innermost side. There are also pigment cells in the chorid which help to darken the interior of the eyeball.

The ciliary body is largely made up of muscle. The longitudinal muscle fibers extend backward from the point where the cornea and sclera join, and attach to the chorid. Some of the muscle fibers run in fan shape and other are circular in distribution. These circular fibers form a complete ring behind the iris and cornea.

Anterior to the ciliary body is the iris, the diaphragm that controls the size of the pupil which contains muscle fibers. There are two muscles, one called the sphincter because it makes the pupil small; the other, the dilator, as it dilates the pupil.

Of great importance is the third layer of the eye, that layer of nervous tissue which forms the retina and the optic nerve. The retina, a very delicate membrane as thin as tissue paper, lies against the wall of the eye. It is not fastened down at any point except in a circle anteriorly and the optic nerve entrance. Following a severe blow to the eye, the retina may become detached or separated. It then becomes necessary to replace the retina in its proper position.

One of the earlier operations for retinal detachment, called retinopexy, consists in passing electrically heated needles through the sclera, thus causing local areas of burn to develop at various points. As the subretinal fluid drains out, the retina floats back against the areas of induced inflammation and the sticky serum formed there holds the retina in place. Prior to the discovery of this technique, retinal detachment was a hopeless accident. But by the skillful use of this new method most of the cases are now cured. Newer techniques, such as scleral resection, scleral imbrication, sclera outfolding, vitreous implantation, and others offer cures from this form of blindness.

The retina is made up many layers. The rods and cones, curiously, are located where they are not expected to be found. They are directed away from the light, whereas in some of the lower animals the rods and cones are pointed toward the light. A chain of cells and nerve fibers run from all portion of the retina to the optic nerve.

In the embryo there are two layers of the primitive retina in which a depression has been formed with an inner and outer layer This can be likened to a soft rubber ball in which one has poked a finger. The outer surface becomes a layer of pigment cells which lies between the chorid and the retina and extends over the ciliary body and the posterior surface of the iris. The inner layer becomes the true retina.

Perhaps the most interesting part of the eye structure is the crystalline lens, which is held in place by a very delicate suspensory ligament extending from the surface of the ciliary body. Unlike the strong fibrous end of a muscle, such as the ligaments found in arms and legs, this transparent ligament of the lens is more delicate than the finest cellophane. This is the structure acted upon when a chymotrypsin is employed in cataract surgery. The enzyme dissolves the ligament so that the lens is more easily removed. The large space behind the lens within the eye is filled with a jelly-like material called the vitreous humor. In the normal state, it has the consistency of jelly but becomes fluid in disease. Anterior to the lens are two small chambers in the eye, filled with a watery fluid called aqueous, an anterior and a posterior chamber.

A cataract is an opaque, clouded lens. When an eye is opened for the removal of a cataract, a cut is made in the eyeball at the edge of the cornea. Naturally, with that cut or with any perforation, the aqueous fluid rushes out. After the wound is healed, however, aqueous is reformed from the fluid portion of the blood.

Relationship of Eye to Body

All through the middle layer of the blood vessels, in the chorid, in the ciliary body and in the iris. The fluid portion of the blood in these structures is the source of the aqueous humor. Since the blood flows to all parts of the body, germs or their toxins which originate from other parts of the body can readily gain access to the eye. Poisons always enter the eye through the blood stream. Therefore, any disease from another part of the body may affect the eye secondarily.

At the corneo-scleral juncture there is a channel called the Canal of Schlemm, which acts as a drainage system for the outflow to the aqueous.

It can be readily understood now that the eye is not an isolated independent little organ, but that it is intimately associated with the entire body and influenced by any change in the rest of the body.

Factors in Seeing

In discussing the action foal the eye when a ray of light enters, one asks the question, “How do we see?” The first reply may be, “We see with our eyes.” That is about one third of the answer. The first requirement is light. Without light there is no sight. That is obvious. This factor, however, has been grossly neglected until the present time. Sufficient and proper illumination is as important to good seeing as any other factor.

The second factor is a healthy, normal eye, or if the eye is not healthy and normal, one that is rendered so by some method or aid such as a correcting lens.

The third factor we have called good health, but is a broader thing than good health – it is a normal and healthy body.

Taking these three points up individually, in illustration is made by assuming the light is coming from an infinite source. The rays of light enter the eye and come to a focus on the retina. By definition, a normal eye is one in which parallel rays of light entering an eye in which there is no action or contraction of the ciliary muscle (i.e., with the accommodation at rest) come to a focus on the retina.

Essential to the normal eye are parallel rays of light, the focusing mechanism of the eye at rest, and the focus of the image on the retina. In other words, no effort is required for the normal eye to see clearly. It looks and sees perfectly.

The eye is the sensory organ of vision. It changes light rays, which are physical, into physico-chemical nervous energy. This is transmitted through the optic nerve and relayed through an elaborate telegraphy-like system to the brain. This telegraphic system is relayed through a number of channels. A marvelous example is shown as the impressions received by the eye are sent way back to the occipital lobe of the brain. Without a normal, healthy, and properly functioning brain one would not be able to interpret what is seen with the eyes.

This statement is true of one eye. But normal sight is always binocular, never monocular. Therefore, a definition of normal vision must include some statement regarding the use of the two eyes, i.e., that normal vision is stereoscopic vision. In other words, two eyes function together. They see the same object, transmit images of the object to the brain, and the brain interprets what it perceives as an object in space and its proper space relationship to the rest of the world.

The act of seeing requires a coordination of the muscles that regulate the position of the eye. Each eye has six muscles, which must be coordinated in order that the two eyeballs move together and move harmoniously – harmonious eye muscle action.

It is possible that a normal eye cannot be used because of lack of a cerebrum or a defective occipital lobe of the brain. It is also possible to have a normal brain and a sightless eye. This condition is more easily understandable. People are often blinded by accident or some other cause. One can equally well be blinded by something that has happened to the occipital lobe of the brain. When I say, “General Health with Good Health,” I mean that all the nervous pathways are intact. Those nervous pathways are associated with different structures at the base of the brain.

In addition to an intact nervous system structure, we must have a good heart with a healthy circulation of blood through the blood vessels of the eye. Likewise the body must be free from infection because toxins or germs can be carried to the eyes and set up trouble there.

Accommodation

To comprehend the normal physiological action of the eye in accommodation, one asks the questions, “How does the eye change its focus?” After one has looked off into the distance and then returned the gaze to a near object, something has taken place; the eyes have converged. Accommodation has taken place, the focus has changed from distance to near. How does the eye accomplish this? It is done entirely by the action of the muscle in the ciliary body. When the circular muscle contracts the suspensory ligament becomes loose – the tension is less – and the lens assumes a more globular shape, which is permitted by the result of change in shape of the crystalline lens.

Consider the normal eye which wishes to view a closer object. Some change has to occur in the lens, which up to this point has been in focus for distance. A stronger converging lens is required. The crystalline lens is altered in shape, which is indirectly the result of the contraction of the muscle which narrows the ciliary muscle ring and relaxes the tension on the lens, allowing the lens to assume a more globular shape by its own elasticity.

The action of the individual eye has now been emphasized. In addition, it has been explained that many factors are involved in normal sight; light, two healthy eyes moving in coordination, and proper nervous connections to the brain, all working together to produce what is termed normal, binocular, stereoscopic vision.

Any defect in any one of these points, any change or alteration in this whole chain, will cause defective sight.

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