THE EYE

(G & H Chap.22 pp 512-524)

The eyeball is a magnificent little camera which receives and records a moving picture as long as the length of our lives, and signals its shifting exposures to the brain. Light enters through the transparent cornea, then passes through the lens situated behind it. In front of the lens there is a circular screen, the iris, that controls the width of the beam. It is adjustable, so that in dim light more may be taken for the image. This adjustment is automatic, brought about by the effects of the image itself. The eye also focuses itself automatically, by changing the shape of its lens, and it can turn in the direction of the required view. When light strikes the sensitive “film” (the retina) in the back of the eye, an electrical disturbance is created out of a chemical change, and is transmitted to the brain. It is actually there, in the dark recesses of the occipital lobe, that we finally “see” the sight before us.

Gross structure of the eyeball.

The outermost protective covering of the eye is provided by the eyelids. The Meibomian glands in the eyelid, and the lacrimal glands produce most of the constituents of tears. Tears are necessary to keep the eye moist and to nourish the cornea, as well as having an antibacterial function. Next are the tough, fibrous protective layers of the sclera and cornea. The lens is suspended from the ciliary body just behind the pupil (a hole in the iris). The cornea, lens, and the fluid media of the eye constitute the optical system which forms an image on the retina. The middle, vascular layer, pigmented to form a dark chamber, is the choroid. Innermost is the light-sensitive retina. The retina is continuous with the optic nerve, which leaves the back of the eye en route to the brain. The fluid media or humors, fill the internal cavities of the globe. One is watery, the aqueous humor, and fills the cavities anterior to the lens which are the anterior chamber in front of the iris, and the posterior chamber behind it. The space behind the lens is occupied by a gel, resembling raw egg white, the vitreous humor.

All the blood to the eye comes from the ophthalmic artery. The retina also has its own vessel, the central retinal artery. This arises from the ophthalmic artery, dividing into capillaries within the retina. Return flow of blood is via the central retinal vein, which follows the course of the artery.

Now perhaps we had better catch our breath by going back to some of the structures we have mentioned for a look at the interesting details. Peering through the transparent conjunctiva (the thin, skin-like layer coating the anterior sclera and inner aspect of the eyelids) we can see the sclera as the “white of the eye.” The sclera is white from reflection at the surfaces of many collagen fibers. Outside, the sclera is loosely attached by collagen fibers to the surrounding connective tissue and muscles, making movement of the eye possible within the bony orbit.

The junction of the sclera with the cornea is called the limbus. Here you will find the vascular loops which are the cornea’s nearest blood supply (the cornea itself is avascular). Near the inner edge of the limbus region you should look for a small vessel, called Schlemm’s canal, which carries aqueous humor out of the anterior chamber into the anterior ciliary veins. Fluid is kept moving in this direction because the pressure in the anterior chamber is greater than that in the venous channels.

The transparent cornea has little rigidity of its own, and will collapse if perforation allows the aqueous humor to escape. It maintains its’ shape due to the internal pressure of the eye. It is mainly composed of highly oriented collagen fibers, and bounded outside and inside by epithelium and a basement membrane.

The corneal epithelium is stratified squamous, with no keratinization. It is very sensitive because it contains many free nerve endings. (Sunburn of the cornea, called “snow blindness,” is excruciatingly painful, as is a scratch of the cornea.) The epithelium is constantly immersed in tear fluid. Whole sections of epithelium may be removed or damaged by a small foreign body, but these are completely regenerated within a day or two.

The epithelium rests on a rather thick layer called Bowman’s membrane, which appears homogeneous and transparent. The thickest layer of the cornea, the corneal stroma, is composed almost entirely of collagen bundles embedded in mucopolysaccharides, and arranged in layers crossing each other at angles, but all parallel to the surface. Inbetween the fibrous layers are branched fibroblasts. If the cornea is distorted by pressure, or stretching, it becomes opaque. The corneal endothelium forms the innermost layer of the

cornea.

The iris regulates the amount of light which enters the eye; in bright light it contracts, diminishing the size of the pupil, while in dim light it expands to allow more light to fall on the retina. Myoepithelial cells are arranged radially in a single sheet at the back surface of the iris and form the pupillary dilator muscle. Because of their radial arrangement with respect to the pupil, their contraction causes the pupil to dilate. The pupillary sphincter muscle is a well defined band of myoepithelial fibers arranged circularly near the front of the iris. When this muscle contracts, the size of the pupil diminishes, thereby reducing the amount of light which enters the eye.

The lens of the eye is a transparent, bi-convex disc, the shape of which can be changed slightly in order to accommodate the eye for near vision. The only evidence of cells at all is a single sheet of flattened cells on the anterior surface--the lens epithelium. These cells divide giving rise to elongated lens fibers.

Usually, in a growing epithelium, the older cells are continually shed and replaced by new ones (think of the skin!). In this respect the lens is unique. As growth proceeds, there is no shedding of the senior citizens. Instead, they become squeezed to death in the center of the lens by young cells at the periphery. The young, growing lens fibers fill themselves with protein, most of which is water soluble. The older fibers, buried alive and receiving only tissue fluids which have been picked over by younger generations near the surface, deprived of oxygen, glucose, and medicare, experience an insolubilization of their intracellular protein which leads to a gradual hardening of the lens nucleus, a progressive loss of flexibility, and consequently of focusing ability. Already by middle life the eye begins to have difficulty in focusing at the usual reading distance. This is called presbyopia.

The lens is completely enclosed by a non-cellular, elastic membrane called the lens capsule, secreted by the epithelial cells in contact with it.

The lens gets the materials it needs for growth and survival from the aqueous humor. If the normal flow of aqueous is interrupted, the lens soon becomes cloudy. Such opacity of the lens is call cataract. It results from numerous causes.. Cataract is the leading cause of blindness in the world. It can be cured by surgical removal of the opaque lens; however in many third world countries, this simple operation is not widely available, and millions of people are unnecessarily blind.

In the lens, as in the cornea, transparency can be maintained only by the constant and unflagging efforts of healthy, living cells. These cells, like dental students, have to work very hard just to survive where they are, never mind getting anyplace.

At the periphery of the lens, the middle layer of the eye (the choroid) expands to form the ciliary body. In three dimensions, the ciliary body forms a ring to which the suspensory ligaments of the lens are attached. Its surface is thrown up into ridges called the ciliary processes.

The epithelium of the ciliary processes secretes most of the constituents of the aqueous humor.

This aqueous humor is actively secreted into the posterior chamber behind the iris. This watery fluid nourishes the lens, then passes through the pupil to the anterior chamber, where it percolates through the trabecular meshwork into Schlemm’s canal, and then back into the vascular system. Obstruction of the canal or its surroundings raises intraocular pressure, giving rise to the condition of glaucoma. Glaucoma is the second major cause of blindness in the USA. If detected early, it can be controlled.

The lens is suspended from the ciliary body by delicate, fibers radiating from the equatorial region of the lens capsule, and extending to the ciliary body. This fiber system, called the suspensory ligament, is fused with the outer layer of lens capsule and with the basement membrane on the free surface of the ciliary epithelium.

The bulk of the ciliary body is composed of the ciliary muscle which is important in the act of accommodation for near vision.

Accommodation for near vision is taken care of by the parasympathetic innervation of the eye. When we are looking at a distant object, the ciliary muscle is relaxed, and the tension on the suspensory ligament of the lens keeps the lens relatively flattened out. When we look at something close up, the ciliary muscle contracts. This lessens the tension in the suspensory ligament, allowing the lens to bulge, so that the image may be correctly focused on the retina.

Distant vision is effortless because the ciliary muscle is relaxed, whereas near vision, as in reading, requires accommodation and therefore, muscular effort. Consequently, when you grow tired while reading, the lines become indistinct because the ciliary muscle relaxes and the image on your retina goes out of focus.

After passing through the cornea, aqueous humor and the lens, light must traverse the vitreous to reach the retina. About 80% of the globe is occupied by the vitreous, which is a transparent, colorless gel. Its jelly-like state is maintained by the presence of hyaluronic acid, which has an immense capacity for water absorption.

The retina is a piece of the brain within the eyeball, and is the key structure of all. The image received by the retina has two dimensions, length and breadth. The third dimension, depth, is somehow mysteriously preserved, to be recreated in the brain out of nerve impulses transmitted to it along the optic nerve. Furthermore these nerve impulses are able to transmit to the brain the concept of color.

The retinal cells which contribute to the process of vision are arranged in a number of layers, the outermost of which is the pigment epithelium. These pigmented cells envelop the ends of the rods and cones in the next layer, and play a vital role in the maintaining the viability of these light-sensitive cells.

The nerve layers of the retina serve to receive the light stimuli and transmit them to the optic nerve on their way to the visual cortex of the brain. This is basically a three cell relay system. The first cell, the light receptor is either a rod or cone. The second cell is a bipolar cell which relays information to the third cell, the ganglion cell. This cell is a nerve cell whose axons are continued out of the eye as the optic nerve. Thus the retina is an upside-down organ. In order to reach the light-sensitive rods and cones, light must traverse all the other layers of the retina. The impulses generated in these cells are transmitted back to the ganglion cells, and then out of the eye via the axons of the ganglion cells.

There are about 120 million rods and 6 million cones in the human eye. The rods and cones are divided into an outer segment, an inner segment and a synaptic ending. The visual pigments, rhodopsin and the cone opsins, are in the discs of the outer segment. The outer segment has one function--to act as a marvelously efficient light trap, and to convert the energy of light into a chemical impulse, and then into an electrical impulse.

Rods are very, very sensitive to light and will generate a nerve signal when their visual pigment absorbs a single photon of light. They operate in dim light. Cone vision, which takes over in bright light, is much less sensitive than rod vision, but operates over a far wider range of light intensity than does rod vision. Cones also transmit the stimuli of color. Each cone contains a pigment that is maximally sensitive to one of the three primary colors: red, green or blue. The color that is “seen” in the visual cortex depends on the proportion of nerve impulses originating from each class of cone, as well as very complex processing in the retina and the brain.

The choroid layer, situated between the sclera and the retina in the posterior part of the eye is a vascular, pigmented layer containing a rich plexus of blood vessels and capillaries. These capillaries are separated from the pigment epithelium by a thin, non-cellular layer called Bruch’s membrane. Nutrients from the blood contribute to the nourishment of this epithelium and of the outer retina. Because of its deep pigmentation, the eye is black inside, like the inside of a camera, to prevent internal scattering of light, or its entry by any other route than through the optical system.,

At the point of exit of the optic nerve from the globe, the optic disc, no sensory elements are present. This constitutes the “blind spot”. (Make a little “x” on a sheet of paper and a black dot about three inches to the right of it. Cover your left eye and look at the “x”. Now move the paper slowly back and forth. About 10 inches away, the dot will disappear as its image falls on the disc).

The nerve fibers pass through the sclera and immediately acquire myelin sheaths. The optic nerve is coated with pia, arachnoid and dura. Septa of pia divide the optic nerve into bundles. There are up to one million fibers in the optic nerve, carrying the coded information gathered by 126 million visual cells.

PATHOLOGICAL POSTSCRIPTS (for interest only)

One million Americans are functionally blind--they cannot read ordinary newspaper type even with the aid of glasses. Eighty percent of all blindness is the result of diseases whose causes are unknown. Less than 5% is caused by injuries, and 3% by poisoning.

Cataracts (lens opacity) causes 20% of the cases of blindness in the U.S. Almost 60% of people over 60 have some cataract formation, and almost 100% of all who reach age 80 have some cataract formation.

Glaucoma (increased intraocular pressure due to faulty circulation of aqueous humor) accounts for 15% of the cases of blindness in the USA. Nearly two million Americans over 40 years of age (about 2% of this age group) have glaucoma.

About half of the population of the United States has some ocular malfunction. Many of these are optical defects:

Presbyopia is the gradual loss of accommodation for near vision which occurs with aging due to progressive hardening of the lens. If the eye is optically normal, parallel rays of light coming from a distance of six meters or more from the eye should be focused on the retina. Hyperopia (farsightedness) is a refractive error in which rays of light from distant objects come to a focus in back of the retina. The individual can see the distant objects by accommodating, but cannot accommodate enough to see close objects. Hyperopia results from the eyeball being relatively too short from front to back or from the cornea not being spherical enough. The farsighted eye can be corrected with a convex lens, which converges the light rays. Myopia (nearsightedness) is that form of refractive error in which parallel rays of light come to a focus in front of the retina, usually because the eye is relatively too large. The individual will have defective distant vision, but may be able to see close objects without accommodation. The corrective lens will be concave, diverging the light rays before they reach the eye. In astigmatism, the cornea (usually) or the lens (rarely) has a surface which is not perfectly spherical, thereby preventing light rays from coming to a single focus on the retina. Clear vision is unobtainable at any distance unless proper corrective spectacles are worn.