325. The Eye. The eye, the outer instrument of vision, is a most beautiful and ingenious machine. All its parts are arranged with such a delicate adjustment to one another, and such an exquisite adaptation of every part to the great object of the whole, that the eye is properly regarded as one of the wonders of nature.

The eyeball is nearly spherical in shape, but is slightly elongated from before backwards. The front part is clear and transparent, and bulges somewhat prominently to allow the entrance of the rays of light. The eye rests in a bowl-shaped socket, called the orbit, formed by parts of various bones of the head and face. The margins of this cavity are formed of strong bone which can withstand heavy blows. The socket is padded with loose, fatty tissue, and certain membranes, which serve as a soft and yielding bed in which the eyeball can rest and move without injury. In a severe sickness this fatty tissue is absorbed, and this fact explains the sunken appearance of the eyes.

The orbit is pierced through its posterior surface by an opening through which the nerve of sight, the optic, pa.s.ses to the eyeball. We may think of the optic nerve holding the eyeball much as the stem holds the apple. It is the function of this most important nerve to transmit retinal impressions to the seat of consciousness in the brain, where they are interpreted.

The eye is bathed with a watery fluid, and protected by the eyelids and the eyebrows; it is moved in various directions, by muscles, all of which will soon be described.

[Ill.u.s.tration: Fig. 127.--Section of the Human Eye.]

326. The Coats of the Eyeball. The eyeball proper is elastic but firm, and is composed of three coats, or layers, each of which performs important functions. These coats are the sclerotic, the choroid, and the retina.

The sclerotic coat is the outside layer and enclosing membrane of the eyeball. It is a tough, fibrous coat for the protection and maintenance of the shape of the eye. It is white and glistening in appearance, and is in part visible, to which the phrase, "the white of the eye," is applied. To this coat, which serves as a kind of framework for the eye, are attached the muscles which move the eyeball. In front of the globe, the sclerotic pa.s.ses into a transparent circular portion forming a window through which one can see into the interior. This is the cornea.

The cornea, a clear, transparent, circular disk, fits into the sclerotic, somewhat as the crystal fits into the metallic case of a watch, forming a covering for its dial. It projects from the general contour of the eyeball, not unlike a rounded bay-window, and is often spoken of as the "window of the eye."

Lining the inner surface of the sclerotic is the second coat, the choroid. It is dark in color and fragile in structure, and is made up almost entirely of blood-vessels and nerves. As the choroid approaches the front part of the eyeball, its parts become folded upon themselves into a series of ridges, called ciliary processes. These folds gradually become larger, and at last merge into the ciliary or accommodation muscle of the eye. The circular s.p.a.ce thus left in front by the termination of the choroid is occupied by the iris, a thin, circular curtain, suspended in the aqueous humor behind the cornea and in front of the crystalline lens. In its center is a round opening for the admission of light.

This is the pupil, which appears as if it were a black spot. The back of the iris is lined with dark pigment, and as the coloring matter is more or less abundant, we may have a variety of colors. This pigment layer and that of the choroid and retina absorb the light entering the eye, so that little is reflected.

The pupil appears black, just as the open doorway to a dark closet seems black. The margin of the iris is firmly connected with the eyeball all round, at the junction of the sclerotic and the cornea.

327. The Retina. The third and innermost coat of the eyeball is the retina. This is the perceptive coat, without which it would be impossible to see, and upon which the images of external objects are received. It lines nearly the whole of the inner surface of the posterior chamber, resting on the inner surface of the choroid. It is with the retina, therefore, that the vitreous humor is in contact.

The retina is a very thin, delicate membrane. Although very thin, it is made up of ten distinct layers, and is so complicated in structure that not even a general description will be attempted in this book. It does not extend quite to the front limits of the posterior chamber, but stops short in a scalloped border, a little behind the ciliary processes. This is the nerve coat of the eye, and forms the terminal organ of vision. It is really an expansion of the ultimate fibers of the optic nerve, by means of which impressions are sent to the brain.

The retina contains curious structures which can be seen only with the aid of the microscope. For instance, a layer near the choroid is made up of nerve cells arranged in innumerable cylinders called "rods and cones," and packed together not unlike the seeds of a sunflower. These rods and cones are to be regarded as the peculiar modes of termination of the nerve filaments of the eye, just as the taste buds are the modes of termination of the nerve of taste in the tongue, and just as the touch corpuscles are the terminations of the nerves in the skin.

Experiment 148. Close one eye and look steadily at the small a in the figure below. The other letters will also be visible at the same time. If now the page be brought slowly nearer to the eye while the eye is kept steadily looking at the small a, the large A will disappear at a certain point, reappearing when the book is brought still nearer.

[Ill.u.s.tration: a oAx]

On the reappearance of the A it will be noted that it comes into view from the inner side, the x being seen before it. If now we move the book towards its original place, the A will again disappear, coming again into view from the outer side when the o is seen before it.

328. Inner Structure of the Eye. Let us imagine an eyeball divided through the middle from above downwards. Let us now start in front and observe its parts (Fig. 127). We come first to the cornea, which has just been described. The iris forms a sort of vertical part.i.tion, dividing the cavity of the eyeball into two chambers.

[Ill.u.s.tration: Fig. 128.--Diagram ill.u.s.trating the Manner in which the Image of an Object is brought to a Focus on the Retina.]

The anterior chamber occupies the s.p.a.ce between the cornea and the iris, and is filled with a thin, watery fluid called the aqueous humor.

The portion behind the iris forms the posterior chamber, and contains the crystalline lens and a transparent, jelly-like fluid, the vitreous humor. This fluid is never renewed, and its loss is popularly described by the phrase, "when the eye runs out."

Experiment 149. The retina is not sensitive where the optic nerve enters the eyeball. This is called the "blind spot." Put two ink-bottles about two feet apart, on a table covered with white paper. Close the left eye, and fix the right steadily on the left-hand inkstand, gradually varying the distance from the eye to the ink-bottle. At a certain distance the right-hand bottle will disappear; but nearer or farther than that, it will be plainly seen.

The vitreous humor fills about four-fifths of the eyeball and prevents it from falling into a shapeless ma.s.s. It also serves to hold the choroid and the retina in position, and to maintain the proper relations of the inner structures of the eye.

The iris consists of a framework of connective tissue, the surface of which is lined by cells containing pigment, which gives color to the eye.

Bundles of involuntary muscular fibers are found in the substance of the iris. Some are arranged in a ring round the margin of the pupil; others radiate from it like the spokes of a wheel. When the circular fibers contract, the pupil is made smaller, but if these fibers relax, the radiating fibers cause the pupil to dilate more or less widely.

329. The Crystalline Lens. Just behind the pupil and close to the iris is a semi-solid, double-convex body, called the crystalline lens. It is shaped like a magnifying gla.s.s, convex on each side, but with the posterior surface more convex than the anterior. In health it is perfectly clear and transparent, and highly elastic. When the lens becomes opaque, from change in old age, or from ulcers or wounds, we have the disease known as _cataract_.

[Ill.u.s.tration: Fig. 129.--Diagram showing the Change in the Lens during Accommodation.

On the right the lens is arranged for distant vision, the ciliary muscle is relaxed and the ligament D is tense, so flattening by its compression the front of the lens C; on the left the muscle A is acting, and this relaxes the ligament and allows the lens B to become more convex, and so fitted for the vision of near objects.]

The lens is not placed loosely in the eyeball, but is enclosed in a transparent and elastic capsule suspended throughout its circ.u.mference by a ligament called the suspensory ligament. This ligament not only retains the lens in place, but is capable of altering its shape. In ordinary conditions of the eye, this ligament is kept tense so that the front part of the lens is flattened somewhat by the pressure on it.

All around the edge, where the cornea, sclerotic, and choroid meet, is a ring of involuntary muscular fibers, forming the ciliary muscle. When these fibers contract, they draw forwards the attachment of the suspensory ligament of the lens, the pressure of which on the lens is consequently diminished. The elasticity of the lens causes it at once to bulge forwards, and it becomes more convex.

The ciliary muscle is thus known as the muscle of accommodation, because it has the power to accommodate the eye to near and distant objects. In this respect it corresponds in its use to the adjusting screw in the opera-gla.s.s and the microscope.

330. The Eye Compared to the Photographic Camera. As an optical instrument, the eye may be aptly compared, in many particulars, to the photographic camera. The latter, of course, is much simpler in structure. The eyelid forms the cap, which being removed, the light from the object streams through the eye and pa.s.ses across the dark chamber to the retina behind, which corresponds to the sensitive plate of the camera.

The transparent structures through which the rays of light pa.s.s represent the lenses. To prevent any reflected light from striking the plate and interfering with the sharpness of the picture, the interior of the photographic camera box is darkened. The pigmented layer of the choroid coat represents this blackened lining.

In the camera, the artist uses a thumb-screw to bring to a focus on the sensitive plate the rays of light coming from objects at different distances. Thus the lens of the camera may be moved nearer to or farther from the object. In order to obtain clear images, the same result must be accomplished by the eye. When the eye is focused for near objects, those at a distance are blurred, and when focused for distant objects, those near at hand are indistinct. Now, in the eye there is no arrangement to alter the position of the lenses, as in the camera, but the same result is obtained by what is called "accommodation."

Again, every camera has an arrangement of diaphragms regulating the amount of light. This is a rude contrivance compared with the iris, which by means of its muscular fibers can in a moment alter the size of the pupil, thus serving a similar purpose.

[Ill.u.s.tration: Fig. 130.--Ill.u.s.trating the manner in which the Image of an Object is brought to a Focus in a Photographer's Camera.]

331. The Refractive Media of the Eye. The eye is a closed chamber into which no light can pa.s.s but through the cornea. All the rays that enter the eye must also pa.s.s through the crystalline lens, which brings them to a focus, as any ordinary lens would do.

Now, if the media through which the light from an object pa.s.ses to reach the retina were all of the same density as the air, and were also plane surfaces, an impression would be produced, but the image would not be distinct. The action of the lens is aided by several refractive media in the eye. These media are the cornea, the aqueous humor, and the vitreous humor. By reason of their shape and density these media refract the rays of light, and bring them to a focus upon the retina, thus aiding in producing a sharp and distinct image of the object. Each point of the image being the focus or meeting-place of a vast number of rays coming from the corresponding point of the object is sufficiently bright to stimulate the retina to action.[44]

Thus, the moment rays of light enter the eye they are bent out of their course. By the action of the crystalline lens, aided by the refractive media, the rays of light that are parallel when they fall upon the normal eye are brought to a focus on the retina.

If the entire optical apparatus of the eye were rigid and immovable, one of three things would be necessary, in order to obtain a clear image of an object; for only parallel rays (that is, rays coming from objects distant about thirty feet or more), are brought to a focus in the average normal eye, unless some change is brought about in the refractive media. First, the posterior wall of the eye must be moved further back, or the lens would have to be capable of movement, or there must be some way of increasing the focusing power of the lens. In the eye it is the convexity of the lens that is altered so that the eye is capable of adjusting itself to different distances.[45]

[Ill.u.s.tration: Fig. 131.--The Actual Size of the Test-Type, which should be seen by the Normal Eye at a Distance of Twenty Feet.]

332. The More Common Defects of Vision. The eye may be free from disease and perfectly sound, and yet vision be indistinct, because the rays of light are not accurately brought to a focus on the retina. "Old sight," known as presbyopia, is a common defect of vision in advancing years. This is a partial loss of the power to accommodate the eye to different distances. This defect is caused by an increase in the density of the crystalline lens, and an accompanying diminution in the ability to change its form. The far point of vision is not changed, but the near point is removed so far from the eye, that small objects are no longer visible.

[Ill.u.s.tration: Fig. 132.--Diagram ill.u.s.trating the Hypermetropic (far-sighted) Eye.

The image P' of a point P falls behind the retina in the unaccommodated eye. By means of a convex lens it may be focused on the retina without accommodation (dotted lines). (To save s.p.a.ce P is placed much too near the eye.)]

Hence, when a person about forty-five years of age complains of dim light, poor print, and tired eyes, the time has come to seek the advice of an optician. A convex lens may be needed to aid the failing power to increase the convexity of the lens, and to a.s.sist it in bringing the divergent rays of light to a focus.

In "long sight," or hypermetropia both the near and far point of vision are concerned, and there is no distinct vision at any distance without a strain. It is a defect in the focus, dependent upon the form of the eyes, and exists in childhood. The axis of the eyeball is too short, and the focus falls beyond the retina, which is too near the cornea. In childhood this strain may pa.s.s unnoticed, but, sooner or later it manifests itself by a sense of fatigue, dizziness, and a blurred and indistinct vision. The remedy is in the use of convex gla.s.ses to converge parallel rays of light before they enter the eye. The muscles of accommodation are thus relieved of their extra work.

"Short sight," known as myopia, is one of the commonest defects of vision. In this defect the axis of the eye, or the distance between the cornea and the retina, is too long and the rays of light are brought to a focus in front of the retina. The tendency to short-sightedness exists in many cases at birth, and is largely hereditary. It is alarmingly common with those who make a severe demand upon the eyes. During childhood there is a marked increase of near-sightedness. The results of imprudence and abuse, in matters of eyesight, are so disastrous, especially during school life, that the question of short sight becomes one of paramount importance.

Experiment 150. With a hand-mirror reflect the sunlight on a white wall. Look steadily at the spot for a full minute, and then let the mirror suddenly be removed. The "complementary" color--a dark spot--will appear.

Experiment 151. _To show that impressions made upon the retina do not disappear at once_. Look steadily at a bright light for a moment or two, and then turn away suddenly, or shut the eyes. A gleam of light will be seen for a second or two.

Look steadily at a well-lighted window for a few seconds, and then turn the eyes suddenly to a darkened wall. The window frame may be plainly seen for a moment.