CHAPTER XXII - THE EYE

Sight is considered the most important of the sensations. It is the chief means of bringing the body into proper relations with its surroundings and, even more than the sensation of hearing, is an avenue for the reception of ideas. The sense organs for the production of sight are the eyes; the external stimulus is

*Light.*-Light, like sound, consists of certain vibrating movements, or waves. They differ from sound waves, however, in form, velocity, and in method of origin and transmission. Light waves are able to pa.s.s through a vacuum, thus showing that they are not dependent upon air for their transmission. They are supposed to be transmitted by what the physicist calls ether-a highly elastic and exceedingly thin substance which fills all s.p.a.ce and penetrates all matter. As a rule, light waves originate in bodies that are highly heated, being started by the vibrations of the minute particles of matter.

Light is influenced in its movements by various conditions. In a substance of uniform density it moves with an unchanging velocity and in a straight line. If it enters a less dense, or rarer, substance, its velocity increases; if one more dense, its velocity diminishes; and if it enters either the rarer or denser substance in any direction other than perpendicularly, it is bent out of its course, or _refracted_. If it strikes against a body lying in its course, it may be thrown off (_reflected_), or it may enter the body and either be pa.s.sed on through (_transmitted_) or _absorbed_ (Fig. 157). Light which is absorbed is transformed into heat.

*Kinds of Reflection.*-Waves of light striking against the smooth surface of a mirror are thrown off in definite directions, depending on the angle at which they strike. (Ill.u.s.trate by holding a mirror in the direct rays of the sun.) But light waves that strike rough surfaces are reflected in practically all directions and apparently without reference to the angle at which they strike. (Ill.u.s.trate by placing a piece of white paper in the direct rays of the sun. It matters not from what direction it is viewed, waves of light strike the eye.) This kind of reflection is called _diffusion_, and it serves the important purpose of making objects visible. The light waves pa.s.sing out in all directions from objects which have received light from the sun, or some other luminous body, enable them to be seen.

[Fig. 157]

Fig. 157-*Diagram ill.u.s.trating pa.s.sage of light waves.*On the right the light is transmitted by the gla.s.s, reflected by the mirror, refracted by the prism, and absorbed by the black cloth. On the left the light from the candle forms an image by pa.s.sing through a small hole in a cardboard and falling upon a screen.

*Formation of Images.*-Another principle necessary to seeing is that of refraction. _Refraction_ means the bending, or turning, of light from a straight course. One of the most interesting effects of refraction is the formation of images of objects, such as may be accomplished by light from them pa.s.sing in a certain manner through convex lenses. If, for example, a convex lens be moved back and forth between a candle and a screen in a dimly lighted room, a position will be found where a picture of the candle falls upon the screen. This picture, called the _image_, results from the refraction of the candle light in pa.s.sing through the lens.

[Fig. 158]

Fig. 158-*Diagram ill.u.s.trating formation of images.* On the right the image is formed by a double convex lens; on the left by the lenses of the eye. The candle flame represents a luminous, or light-giving, body; but light pa.s.ses from the large arrow by reflection. (See text.)

In order to form an image, the light waves spreading out from the object must be brought together, or focused. Focusing means literally the bringing of light to a point, but it is evident in the formation of an image that all the waves are not brought to a single point. If they were, there would be no image. In the example of the candle given above, the explanation is as follows:

The light from the candle comes from a great number of separate and distinct points in the candle flame. The lens, by its peculiar shape, bends the waves coming from any single point so that they are brought to a corresponding point on the screen. Furthermore, the points of focused light are made to occupy the same relative positions on the screen as the points from which they emanate in the candle flame (Fig. 158). This is why the area of light on the screen has the same form as the candle, or makes an image of it. The same explanation applies if, instead of the luminous candle, a body that simply reflects light, as a book, is used.

*The Problem of Seeing.*-What we call _seeing_ is vastly more than the stimulation of the brain through the action of light upon afferent neurons. It is the _perceiving _of all the different things that make up our surroundings. If one looks toward the clear sky, he receives a _sensation of light_, but sees no object. He may also get a sensation of light with the eyelids closed, if he turn the eyes toward the window or some bright light. But how different when the light from various objects enters the eyes. There is apparently no consciousness of light, but instead a consciousness of the size, form, color, and position of the objects. _Seeing is perceiving objects._ Stimulation by the light waves is only the means toward this end. The chief problem in the study of sight is that of determining _how light waves enable us to become conscious of objects._

*Sense Organs of Sight.*-The sense organs of sight consist mainly of the two eyeb.a.l.l.s. Each of these is located in a cavity of the skull bones, called the _orbit_, where it is held in position by suitable tissues and turned in different directions by a special set of muscles. A cup-shaped receptacle is provided within the orbit, by layers of fat, and a smooth surface is supplied by a double membrane that lies between the fat and the eyeball. In front the eyeb.a.l.l.s are provided with movable coverings, called the _eyelids_. These are composed of dense layers of connective tissue, covered on the outside by the skin and lined within by a sensitive membrane, called the _conjunctiva_. At the base of the lids the conjunctiva pa.s.ses to the eyeball and forms a firmly attached covering over its front surface. This membrane prevents the pa.s.sage of foreign materials back of the eyeball, and by its sensitiveness stimulates effort for the removal of irritating substances from beneath the lids. The eyelashes and the eyebrows are also a means of protecting the eyeb.a.l.l.s.

*The Eyeball*, or globe of the eye, is a device for _focusing_ light upon a sensitized nervous surface which it incloses and protects. In shape it is nearly spherical, being about an inch in diameter from right to left and nine tenths of an inch both in its vertical diameter and from front to back. It has the appearance of having been formed by the union of two spherical segments of different size. The smaller segment, which forms about one sixth of the whole, is set upon the larger and forms the projecting transparent portion in front. The walls of the eyeb.a.l.l.s are made up of three separate layers, or coats-an _outer coat_, a _middle coat_, and an _inner coat_ (Fig. 159).

[Fig. 159]

Fig. 159-*Diagram of the eyeball in position.* 1. Yellow spot. 2. Blind spot. 3. Retina. 4. Choroid coat. 5. Sclerotic coat. 6. Crystalline lens.

7. Suspensory ligament. 8. Ciliary processes and ciliary muscle. 9. Iris containing the pupil. 10. Cornea. 11. Lymph duct. 12. Conjunctiva. 13.

Inferior and superior recti muscles. 14. Optic nerve. 15. Elevator muscle of eyelid. 16. Bone. _A._ Posterior chamber containing the vitreous humor.

_B._ Anterior chamber containing the aqueous humor.

*The Outer Coat* surrounds the entire globe of the eye and consists of two parts-the sclerotic coat and the cornea. The _sclerotic coat_ covers the greater portion of the larger spherical segment and is recognized in front as "the white of the eye." It is composed mainly of fibrous connective tissue and is dense, opaque, and tough. It preserves the form of the eyeball and protects the portions within. It is pierced at the back by a small opening which admits the optic nerve, and in front it becomes changed into the peculiar tissue that makes up the cornea.

The _cornea_ forms the transparent covering over the lesser spherical segment of the eyeball, shading into the sclerotic coat at its edges. It has a complex structure, consisting in the main of a transparent form of connective tissue. It serves the purpose of admitting light into the eyeball.

*The Middle Coat* consists of three connected portions-the _choroid coat_, the _ciliary processes_, and the _iris_. These surround the larger spherical segment. All three parts are rich in blood vessels, containing the blood supply to the greater portion of the eyeball.

The _choroid coat_ lies immediately beneath the sclerotic coat at all places except a small margin toward the front of the eyeball. It is composed chiefly of blood vessels and a delicate form of connective tissue that holds them in place. It contains numerous pigment cells which give it a dark appearance and serve to absorb surplus light. Near where the sclerotic coat joins the cornea, the choroid coat separates from the outer wall and, by folding, forms many slight projections into the interior s.p.a.ce. These are known as the _ciliary processes_. The effect of these folds is to collect a large number of capillaries into a small s.p.a.ce and to give this part of the eyeball an extra supply of blood. Between the ciliary processes and the sclerotic coat is a small muscle, containing both circular and longitudinal fibers, called the _ciliary muscle_.

The _iris_ is a continuation of the choroid coat across the front of the eyeball. It forms a dividing curtain between the two spherical segments and gives the color to the eye. At its center is a circular opening, called the _pupil_, which admits light to the back of the eyeball. By varying the size of the pupil, the iris is able to regulate the amount of light which pa.s.ses through and it employs for this purpose two sets of muscular fibers. One set of fibers forms a thin band which encircles the pupil and serves as a sphincter to diminish the opening. Opposing this are radiating fibers which are attached between the inner and outer margins of the iris. By their contraction the size of the opening is increased. Both sets of fibers act reflexively and are stimulated by variations in the light falling upon the retina.

[Fig. 160]

Fig. 160-*Diagram showing main nervous elements in the retina.* Light waves stimulate the rods and cones at back surface of the retina, starting impulses which excite the ganglion cells at the front surface. Fibers from the ganglion cells pa.s.s into the optic nerve.

*The Inner Coat, or Retina.*-This is a delicate membrane containing the expanded termination of the optic nerve. It rests upon the choroid coat and spreads over about two thirds of the back surface of the eyeball.

Although not more than one fiftieth of an inch in thickness, it presents a very complex structure, essentially nervous, and is made up of several distinct layers. Of chief importance in the outer layer are the cells which are acted upon directly by the light and are named, from their shape, the _rods_ and _cones_. In contact with these, but occupying a separate layer, are the ends of small afferent nerve cells. These in turn communicate with nerve cells in a third layer, known as the ganglion cells, that send their fibers into the optic nerve (Fig. 160).

In the center of the retina is a slight oval depression having a faint yellowish color, and called, on that account, the _yellow spot_. This is the part of the retina which is most sensitive to light. Directly over the place of entrance of the optic nerve is a small area from which the rods and cones are absent and which, therefore, is not sensitive to light. This is called the _blind spot_. (See Practical Work.)

*The Crystalline Lens.*-Immediately back of the iris and touching it is a transparent, rounded body, called the crystalline lens. This is about one fourth of an inch thick and one third of an inch through its long diameter, and is more curved on the back than on the front surface. It is inclosed in a thin sheath, called the _membranous capsule_, which connects with a divided sheath from the sides of the eyeball, called the _suspensory ligament_ (Fig. 159). Both the lens and the capsule are highly elastic.

*Chambers and "Humors" of the Eyeball.*-The crystalline lens together with the suspensory ligament and the ciliary processes form a part.i.tion across the eyeball. This divides the eye s.p.a.ce into two separate compartments, which are filled with the so-called "humors" of the eye. The front cavity of the eyeball, which is again divided in part by the iris, is filled with the _aqueous_ humor. This is a clear, lymph-like liquid which contains an occasional white corpuscle. It has a feeble motion and is slowly added to and withdrawn from the eye. It is supplied mainly by the blood vessels in the ciliary processes and finds a place of exit through a small lymph duct at the edge of the cornea (Fig. 159).

The back portion of the eyeball is filled with a soft, transparent, jelly-like substance, called the _vitreous_ humor. It is in contact with the surface of the retina at the back and with the attachments of the lens in front, being surrounded by a thin covering of its own, called the _hyaloid membrane_. The aqueous and vitreous humors aid in keeping the eyeball in shape and also in focusing.

*How we see Objects.*-To see an object at least four things must happen:

1. Light must pa.s.s from the object into the eye. Objects cannot be seen where there is no light or where, for some reason, it is kept from entering the eye.

2. The light from the object must be focused (made to form an image) on the retina. In forming the image, an area of the retina is stimulated which corresponds to _the form of the object_.

3. Impulses must pa.s.s from the retina to the brain, stimulating it to produce the sensations.

4. The sensations must be so interpreted by the mind as to give an impression of the object.

*Focusing Power of the Eyeball.*-The eyeball is essentially a device for focusing light. All of its transparent portions are directly concerned in this work, and the portions that are not transparent serve to protect and operate these parts and hold them in place. Of chief importance are the crystalline lens and the cornea. Both of these are lenses. The cornea with its inclosed liquid is a plano-convex lens, while the crystalline lens is double convex.(123) Because of the great difference in density between the air on the outside and the aqueous humor within, the cornea is the more powerful of the two. The crystalline lens, however, performs a special work in focusing which is of great importance. The iris also aids in focusing since it, through the pupil, regulates the amount of light entering the back chamber of the eyeball and causes it to fall in the center of the crystalline lens, the part which focuses most accurately.

[Fig. 161]

Fig. 161-*Diagram showing changes in shape of crystalline lens* to adapt it to near and distant vision.

*Accommodation.*-A difficulty in focusing arises from the fact that the degree of divergence of the light waves entering the eye from different objects, varies according to their distance. Since the waves from any given point on an object pa.s.s out in straight lines in all directions, the waves that enter the eye from distant objects are at a different angle from those that enter from near objects. In reality waves from distant objects are practically parallel, while those from very near objects diverge to a considerable degree. To adjust the eye to different distances requires some change in the focusing parts that corresponds to the differences in the divergence of the light. This change, called _accommodation_, occurs in the crystalline lens.(124) In the process of accommodation, changes occur in the shape of the crystalline lens, as follows:

1. In looking from a distant to a near object, the lens becomes more convex, _i.e._, rounder and thicker (Fig. 161). This change is necessary because the greater divergence of the light from the near objects requires a greater converging power on the part of the lens.(125)

2. In looking from near to distant objects, the lens becomes flatter and thinner (Fig. 161). This change is necessary because the less divergent waves from the distant objects require less converging power on the part of the lens.

The method employed in changing the shape of the lens is difficult to determine and different theories have been advanced to account for it. The following, proposed by Helmholtz, is the theory most generally accepted:

The lens is held in place back of the pupil by the suspensory ligament.

This is attached at its inner margin to the membranous capsule, and at its outer margin to the sides of the eyeball, and entirely surrounds the lens.