Hygiene of the eye

Front matter

iill( il!ii

Class "T ( E ^^

Book "?87

Preface

Sight is the most valuable of all the special senses and the author has endeavored to convey in the following pages, in language as free from technicalities as possible, how it may best be conserved, and the relation which the organ of vision bears to the general economy. Although general practitioners are doubtless more or less familiar with the substance of its contents, it is thought that a review of the more common diseases of the eye, with a description of the manner in which the eye is affected by the general health, and also how the latter may be influenced by eye-strain, will not be without value, both to the physician engaged in general practice and the general public.

Vision plays such an important part in education that the chapters dealing with the refraction of the eye, the illumination and equipment of the school-room, and the general care of the eyes of the scholar should invite the interest of teachers and parents.

Blindness and the diseases and conditions leading to it are fully discussed in its pages, together with the national movements which are being made towards its prevention; all of which the author hopes may lessen the labors of those who are so unselfishly leading the movement to diminish the number of blind in this country.

In view of the increasing number of ocular injuries received by workmen in foundries, factories and elsewhere, particular attention is given to this important subject, as well as to the best means of preventing them — information which should be of value to those engaged with the problem of lessening the number of industrial accidents.

iv PREFACE

It is hoped by the author that the contents of the four chapters written by distinguished speciaHsts in the subjects which they discuss may prove of interest to ophthalmologists as well as to general practitioners and laymen, as they contain information which should be of value to the specialist hitherto not available in condensed and popular form. Otherwise the author fears his professional colleagues will find the work elementary and falling far short of what a comprehensive treatise on ocular hygiene treated fully from a scientific standpoint should be. He bespeaks their co-operation, however, in his attempt to convey to the popular reader a broader comprehension of the various phases presented by ocular anomalies and diseases.

In conclusion, the author desires to express his obligation to the National Committee for the Prevention of Blindness, for the loan of a number of photographs, notably those illustrating the chapter on wounds and injuries.

The Author

2049 Chestnut Street,

Philadelphia, Pa.

Contents

CHAPTER PAGE

I. Structure of the Eye and of the Parts Adjoining . . 1 II. The Physiology of Vision — ^The Ophthalmoscope ... 12

III. Defects or Anomalies of Refraction and Their

Chapter I Structure Of The Eye And Of The Parts Adjoining

To render the descriptions of the various affections which will be treated of in the following pages more comprehensible, it has seemed desirable to give some introductory account of the anatomy and physiolog}^ of the eyes and their appendages. Naturally, any attempt at a complete or technical description of either of these subjects would be foreign to the scope of this treatise, and an effort will be made to present them in the briefest manner compatible with clearness.

Such a delicate organ as the eye necessarily demands most careful protection, not only from external injury, but also from diseases of contiguous structures. Against the former, nature has amply provided by furnishing a bony framework into which the eye is set, so that only its anterior one-third is exposed, and even this portion has the covering of the lids and the interlacing eye-lashes, to protect it when occasion demands. To give expression to the w^orkings of the mind, and to endow the countenance with charm and intelligence, the exposed part of the eye has been made the most beautiful of all the structures of the body and has well earned the designation of the poet as " the mirror of the soul."

The bony receptacles in w^hich the eyes are lodged, the orbits, are cone-shaped, with their widest portion directed forward, and as the long axis of each cone is more divergent at its opening than at the apex, a maximum of rotation is afforded to the eyes as they are pulled in various directions by the action of the group of muscles with which each is provided (Fig. 1) . In addition to the eyeball and its muscles, the orbit contains a thick bolster of fat, which surrounds the globe, protecting it from injury and permitting of its free rotation by the ocular muscles. The

Fig. 1. — Anterior orifice of the orbit with the eyeball. Natural size. The tendons of the four recti muscles are cut off near their insertion upon the eyeball, but the interior oblique, oi, and the tendon, os, of the superior oblique are left entire.

configurement of the orbit depends in large measure upon the shape of the skull and varies in different individuals and in different races, being shallow in races with skulls short in the anteroposterior axis, as exemplified in the Indian, and deep where the same axis is elongated, as may be noted in many European skulls. As the shape of the orbit determines in large measure the form of the eyeball, and, as we will see later, the state of the refraction in each eye is dependent upon its length, it follows that near-sightedness and far-sightedness are often the expression of racial pecuharities of structure.

While the walls of the orbit are thin, to give less weight to the skull, their margins are dense and strong, to receive the force of blows. This is particularly true of the upper margins, where the protective power of the brow is further augmented by a cushion of hair, the eyebrow, which is formed of muscle and thick skin, covered with stiiF hairs. The brow forms a conspicuous setting for the eye and enters largely into facial expression. It also affords protection, by shedding perspiration.

With but one exception, the eocternal muscles of the eye, six in number, arise from the apex of the orbit and are attached to the globe near its equator. This arrangement permits of the free movement of the eye in all directions, while the one muscle which has its origin near the opening of the orbit is concerned chiefly with its easy rotation ( Fig. 2 ) . These muscles receive their stimuli from nerves which, together with the principal blood-vessels, gain access to the orbit through a fissure in the apex of the orbit. Another hole in the same position, the optic foramen, permits the passage of the optic nerve with the chief retinal blood-vessels into the skull. The muscles of both eyes work in perfect harmony with one another, and, by means of a wonderful complex mechanism of correlated activity, rotate the eyes, either subconsciously or at the will of the individual, in all directions. Any considerable incoordination in their action is productive of double vision, and the nice parallelism which is maintained by the perfect balance of all the muscles is destroyed and the affected eye turns or squints (see p. 52).

The eyelids when closed form a perfect curtain for the eye, and when open an attractive setting for the organ of vision. The lashes, with their graceful curve, are decorative as well as protective to mechanical injury, and when the lids are partially closed, their interlacing without excluding vision serves as a protection against excessive light. They attain their maximum growth in about five months and then drop out, being succeeded by new ones. Inflam-

FiG. 2. — Muscles of the right orbit. ("Motor Apparatus of the Eye," Stephens, Courtesy

F. A.Davis Co.)

matory conditions of the edge of the lids sometimes cause the incoming lashes to assume a wrong direction and to turn inward against the eye. They are then commonly known as " wild hairs," and cause great distress and may even give rise to serious inflammation of the eyeball. Assuming that other inflammatory conditions of the eye which are attended by sensations of pricking and irritation are also caused by wild hairs, it happens not infrequently that the ignorant pull out their lashes in the hope of obtaining relief from the annoying symptoms, a useless procedure, as the lashes in such instances are not at fault.

The movements of the lids are performed almost entirely by the upper, the lower being almost stationary. Upon the width of the opening between the lids depends the apparent size of the eye and the expression of the individual. Racial characteristics find expression in the almond eye of the Mongolian. In certain diseases of the eye and the nervous system, the upper lids droop, giving an expression of drowsiness and frequently necessitating the throwing back of the head to enable the rays of light to gain entrance to the pupils.

The eyelids, which are composed of muscle, elastic tissue, and plates of cartilage which give form and firmness to the lids, are covered by skin, which is but loosely attached to the sublying tissues. This laxity of the superficial tissues accounts for the great swelling which frequently attends inflammations or follows injury of these structures, and permits of the pufiiness under the lower lids which frequently accompanies diseases of the heart and kidneys.

The thin, delicate tissue, the conjunctiva, which covers the anterior part of the globe and also lines the lids, is continually lubricated by tears derived from the lachrymal gland, a secreting organ the size of a filbert, hidden under the outer portion of the roof of the orbit, and by mucus derived from the conjunctiva itself and from a series of small glands embedded in the edge of the lids. After lubricating the eye and the conjunctival sac, the secretions are sucked by two small apertures situated upon the inner portion of each lid into the tear sac, with which they are connected by two fine tubes. This sac, the size of a small pea, is situated just below the ligament which connects the lids with the orbit, so that its emptying is facilitated not only by the contraction of its own muscular coat, but also by the tension exerted upon it by the ligament of the lids during the act of winking. The lachrymal sac empties into the nose, through a bony canal at the lower inner portion of the orbit. This canal and sac are lined by a mucous membrane continuous with that of the nose and

Superior lacrimal gland Inferior lacrimal gland

Upper eyelid partly divested of skin

Upper punctum

Lacrimal sac, near it9 fundus

T. . X ■ ^ I II iiwiMiii I Common duct, formed

Ducts from supenor ^ ^^^^^^^tIIpO^^^^MIL by junction of upper

gland ^/^///Afllrv \^ ^^^^^^MT and lower ducts

Lower punctum Naso-lacrimal duct

Fig. 3. — The lachrymal apparatus. ("Modern Ophthalmology," Ball, F. A. Davis Co.)

of the conjunctival sac, which accounts for the sympathetic watering of the eye in so many nasal affections (Fig. 3) .

The eye itself is spheroid in form and about an inch in diameter, being somewhat longer in its anteroposterior than its vertical axis. It is suspended in the orbit by its muscles and by a series of membranes or fascia to the walls of the surrounding orbit. The layer of fat referred to above fills out the remaining orbital space.

The outer layer, or envelope, the sclerotic, is a firm, opaque, tough membrane, into the anterior part of which the transparent cornea is set. The sclerotic maintains the form of the eyeball and serves for attachment of the

PLATE I.

CANAL OF/ SCHLEMN

C/UARY BODY

Horizontal section of the eyeball. Magnified about ZYi times (see PP- 7-10). ("Manual of the Diseases of the Eye," Charles H. May, M.D.; Wm. Wood <k Co.)

muscles which move the eye. That portion immediately smrounding the cornea and which is alone visible, forms the so-called " white of the eye," the rest being hidden behind the conjunctiva and orbital tissues. The cornea is also tough and resisting, but perfectly transparent, to permit the passage of rays of light into the eye (Plate I) . It may be designated as " the window of the eye." Its outer sm-face is smooth and preserves a high degree of polish, and, being curved, returns, as a convex mirror, an erect image of diminished size of any object before it.

Lying within the sclerotic, is the choroid, a membrane rich in blood-vessels and pigment. The blood-vessels nourish the interior of the eye and the pigment aids in the absorption of excessive light. The quantity of pigment varies, being plentiful in brunettes and sparse in blondes. In albinos it is entirely absent. The choroid does not extend quite so far forward as the sclerotic, and merges anteriorly into the ciliary body.

The third and essential coat is the retina. This is a delicate sheet of nervous tissue, continuous with the brain, through the medium of the optic nerve and its cerebral prolongations, and the rest of the eye may be regarded merely as a mechanism for its protection and means of exposure to external impressions ( Fig. 4 ) . The light-perceiving elements of the retina are arranged in ten distinct layers, held together by a connective-tissue framework. The most important layer is that of the rods and cones, the latter elements being especially concerned in the perception of visual sensations, as these bodies are alone represented in the macular region, a very minute area upon which the rays concerned with the most direct and acute vision are focussed. External to the laver of rods and cones is the pigment layer, in which is stored the retinal purple, a photochemical substance which absorbs light and is concerned in the transformation of the rays into visual impulses. Just what the nature of this transformation is has not been clearly determined, but, of course, there is no ir n o

Fig. 4. — Section of human retina (Bohm and v. Davidoff) : a, a', outer and inner segments of rods; b, b', outer and inner segments of cones; c, membrana limitans externa; d, outer nuclear layer; c, outer plexiform layer; /, inner nuclear layer; g, inner plexiform layer; h, layer of ganglion cells; i, fibre layer. Magnified 700 diameters.

actual picture capable of perception by another person formed there.

The claim sometimes made, therefore, of the possibility of detecting by posthumous examination of the retina the object last regarded before death, is extravagant and absurd.

The remaining portions of the eye are adapted to the proper focussing of the rays of light entering the eye through the cornea upon the retina, and in the regulation of the amount of light entering the eye. The chief of these is the crystalline lens ( Fig. 5 ) . This structure is a double transparent convex lens or ordinary magnifying glass, but of such consistency that its convexity may be readily altered. It consists of connective layers composed of minute fibrils, the outermost layers, or cortex, being softer than at the central zone, or nucleus. As age advances, the lens becomes harder and more resistant. It is contained within an elastic covering and is suspended in the anterior

Fig. 5. — Crystalline lens of new-born child, seen from the side, showing the course of the lens fibres. (Arnold.) Magnified 6 diameters.

part of the eye by a delicate ligament, a fine transparent structure of fibrous tissue, and surrounded by a small circular structure, the ciliary body, the termination of the anterior extremity of the choroid coat. This body is divided into two portions, a circular band of muscular tissue, the ciliary muscle, and the ciliary processes. These latter, 60 or 70 in number, merge with the anterior portion of the choroid and possess the power of pulling that membrane forward. The ciliary body is a structure of great importance, as will appear later, for upon its action depends the alteration in the shape of the crystalline lens and the changing focus of the eye.

The lens rests posteriorly in a cup-shaped depression in the vitreous humor, a transparent gelatinous structure which occupies four-fifths of the interior of the ball. This humor fills out the form of the eyeball and gives support to the retina, spread over its outer surface.

The amount of light entering the eye is regulated by means of a circular curtain, the iris, its central aperture forming the pupil. The iris lies upon the front of the lens, its base of support and action being at the margin of the posterior surface of the cornea, and the anterior portion of the ciliary muscle. In addition to muscle fibres, the iris contains pigment, the quantity of which determines the color of the eye, the designation of the eyes as gray, blue or black depending upon that factor. In albinos, the pigmentation of the iris is absent, and the pinkish hue is dependent upon the reflection through it of the red blood in the vessels of the choroid. All eyes are blue at birth, the commencement of permanent coloration taking place about the sixth week. There is no truth in the popular idea that dark eyes are stronger than light ones, except in so far as they are better protected against excessive light. It is a wise provision of Nature, that among Southern races dark eyes prevail, the better to protect against the glare of a tropical sun, while in the North, light eyes predominate.

The circular aperture in the iris, the pupil, is adapted for the transmission of light into the eye, its size constantly varying in response to variations in the quantity of light which falls upon it. The change in the shape of the pupil is effected by a double set of muscles in the iris, one of circular fibres, contracting to make the pupil small in the light, and one of dilating powers, to enlarge the pupil in darkness. The pupil varies in size in different individuals, and at different ages, growing smaller as age advances. The pupil is larger, also, in near-sightedness than in farsightedness. Local affections within the eye, as well as diseased conditions elsewhere in the body, affect its size; various drugs have the same power also.

The space between the iris and lens and the arching surface of the cornea is filled with an aqueous humor ^ which has some refracting properties. The cornea and vitreous also possess like qualities. These are constant, and but slight in comparison with those of the lens.

The eyeball is richly supplied with blood-vessels and nerves, apart from the great nerve of sight, the optic, and it is by means of some of these latter that the change in the size of the pupil is accomplished, acting under stimuli from the brain centres. The chief source of vascular supply to the retina is by means of a small artery which enters the eye through the centre of the optic nerve. In case of sudden stoppage of this vessel by a clot, instant blindness ensues.

Chapter II The Physiology Of Vision— The Ophthalmoscope

Of all the special senses, sight, perhaps, is the most valuable and highly prized. It comprises the perception and translation of impressions derived from external objects into cerebral conceptions. The explanation of the visual act must therefore include the manner by which impulses are conveyed from the exterior to the brain centres, and something at least regarding the nature of the impressions. How physical impulses are transformed into vision we cannot say, nor have we, indeed, any knowledge of other kinds of perceptions.

From an optical standpoint, all objects in nature are divided roughly into luminous and non-luminous bodies; light emitted from the former or reflected from the surface of the latter moves in straight lines, and the smallest conceivable line of light is called a ray. Of these rays we will speak more hereafter. It will suffice at this time to say that the great function of the eye is to collect these rays from surrounding bodies, to focus them upon one of its parts, the retina, whence the impressions are conveyed along the optic nerve and higher brain tracts to the various visual centres. The eye, therefore, resembles in its action a photographer's camera, of such intricate and wonderful construction that it is always exposed for the receipt of impressions from all distances, and can act either alone or in harmony with its fellow, with a swiftness and accuracy that no mechanical device can even approach, much less equal.

In the camera, images of external objects are focussed upon a sensitive plate and become permanent in consequence of chemical changes induced by light; in the eye,

Fig. 6. — Camera obscura of a photographic apparatus. AB, object; Z>, diaphragm for shutting off too divergent rays; L, lens for refracting the rays so that they will form the image ab upon the sensitive plate R.

they fall upon the retina, where, after undergoing biochemical change, they are conveyed to the visual centres in the brain by the optic nerve. The eyeball itself, therefore,

Fig. 7.— The eye as a camera. AB, object; C, cornea, where the rays undergo a first refraction; D, the iris, that acts as a diaphragm for shutting off too divergent rays; L, lens, where the rays are again refracted; R, retina, upon which the image ah is projected; a'fe' represents the surface of a hypermetropic eye, and shows that the rays are not completely focussed, and consequently the image must be blurred and indistinct; o"b" represents the surface of a myopic eye, and shows a similar condition.

does not see, the act of vision being performed by the brain. Figs. 6 and 7 compare the mechanism of a camera with that of the eye. The sclerotic represents the sides of the box, the choroid its blackened inner surface, the pupil forms the opening, and the crystalline lens and cornea the focussing lens (e) , The iris serves as a diaphragm. The rays of light fall in the one case on the sensitive plate, in the other on the retina. As the rays of light cross in the eye, the retinal picture is an inverted one, just as is the image in the magic lantern. The transposition of the image doubtless occurs in the brain, though there are some who assert that the righting process takes place in the retina itself.

Lenses. — A lens may be defined as a portion of glass

Fig. 8. — A, plano-convex: one side convex, the other plane. B, double convex: both sides convex. When both sides are equally convex, as represented in the figure, the lens is called biconvex. C, concavo-convex: one side concave, the other more convex. D, planoconcave: one side concave, the other plane. E, double-concave: both sides concave. When they are equally concave, as in the figure, the lens is called biconcave. F, convexo-concave: one side convex, the other more concave. Either C ot F ia called a meniscus, or periscopic lens.

or other substance capable of transmitting light rays bounded by one or more curved surfaces. Lenses that are thickest at the centre are designated as convex lenses, those thinnest at the centre as concave lenses. Convex lenses have the property of collecting light rays, concave lenses of dispersing them. According to the degree in which they divert rays from their original source, lenses are designated as strong and weak, the former bringing rays quickly to a focus, the latter having their focus at a greater distance. The standard now universally adopted in the notation of lenses is based upon the metric system and the unit from which the gradations in the strength of lenses is reckoned is the Dioptre, i.e.„ the focussing power required to bring parallel rays to a focus at the distance of 1 metre. A lens with twice this strength is designated as a 2 D. lens, and has a focal distance of one-half metre. An 0.50 D. lens is one with a focal distance of 2 metres (Fig. 8).

Lenses may be either spheric or cylindric. In the former, light is refracted equally in all its planes, in the latter, in one meridian only.

Prisms. — By a prism is meant a section of glass consisting of two plane surfaces so inclined to each other that

Fig.

-Passage of parallel rays through a prism.

they form an angle. As it is the property of prisms to deviate rays of light from the apex toward the base of the prism, a prism placed before either eye with its base toward the nose will deflect rays of light entering the eye, and lessen convergence effort (Fig. 9). Prisms may be ground into the lens, correcting any kind of refraction error, and as their base may be placed at any axis, and their strength accurately prescribed, they form one of the most valuable means of correcting eye-strain in the oculist's armamentarium. Prisms are not only of service in removing the strain from weak muscles, but are often employed in exercising such muscles, the base of the prism in such cases being opposed in direction to the action of the muscle which is to be strengthened.

All luminous rays reflected from a source at a distance greater than 15 or 20 feet are considered to be parallel. When the object from which light is reflected is brought nearer, rays entering the eye are divergent and

Fig. 10. — An emmetropic or normal eye. Parallel rays a and & are focussed upon the retina, while divergent rays, as those proceeding from the point B, come to a focus behind the retina.

require to be more strongly bent or collected before they focus upon the retina. This focussing property, of bending parallel rays of light from the distance and diverging rays from near objects so that they meet upon the retina, is termed refraction.

All eyes are not of equal length, some being shorter

FiQ. 11. — A hypermetropic or far-sighted eye. Parallel rays converge behind the retina, Rays

must be convergent in order to focus there.

and some longer than the average, or what may be considered to be the standard, eye. According, therefore, to this variation in length, ocuhsts designate by the term emmetropic, the eye of 7i07^mal length (Fig. 10), in which rays of light entering the eye from a distance are brought to a focus exactly on the retina. By the hypermetropic or farsighted eye (Fig. 11) is meant too short an eye, one in

PHYSIOLOGY OF \^SION— THE OPHTHALMOSCOPE 17

which similar rays of hght are brought to a focus behind the retina, and by myopic or near-sighted eye (Fig. 12), too long an eye is designated, one in which the rays from a distance are focussed in front of the retina.

Xot only may eyes vary in length, but, owing to a lack of uniformity in the curvature of the cornea, different meridians of the same eye have a different focus. In this refractive condition, which is termed astigmatism, rays of light w^hich pass through the meridian of greatest convexity are brought to a focus sooner than those passing through the meridian of least curvature. According to the degree of corneal asymmetry, the type of astigmatism may vary. In simple astigmatism, for example, one of the

Fig. 12. — A myopic or near-sighted eye. Parallel rays converge before reaching the retina. Rays must be divergent in order to focus there.

meridians is emmetropic, the other hypermetropic or myopic. In compound astigmatism, both meridians are faulty, but unequal in degree. In mixed astigmatism, one meridian is hypermetropic, the other myopic.

Accommodation. — In describing the various parts of the eye, the crystalline lens was designated as a transparent biconvex lens, enclosed in a fibrous capsule, adaptable to change of shape, by action of the circular structure which surrounds it, the ciliary body. Unlike the camera, whose lenticular focal distance may be changed by movement of the sensitive plate backwards and forwards, the length of the eyeball is fixed and constant, and alterations in its focussing power performed by a change in strength of the refracting surfaces within the eye (Fig. 13) . As has been stated, hght rays coming from a distance greater than 15 or 20 feet are parallel; those emanating from an object situated nearer are divergent. To change the focus of the eye, therefore, from the perception of distant objects to those for near, its refracting power must be increased, and the convexity of the crystalline lens increased by action of the ciliary muscle. This adaptability of the eye is known as the power of accommodation, and is made use of whenever there is the slightest change in the distance of any

PAR

NEAR

Fig. 13. — Diagram to illustrate accomraodation (after Helmholtz) . C.P., ciliary process; 7, iris; Sp.l, suspensory ligament; Z. cm., longitudinal ciliary muscle; c.c.m., circukr ciliary muscle; c.S., canal of Schlemm. The left half represents the arrangement for viewing far objects and the right half for viewing near objects. (Foster's "Text-book of Physiology," The Macmillan Company.)

near object we look at. As life advances, accommodation steadily declines. At 30 years of age, half of its power is gone and at 45 years it is so weakened that small objects near at hand are not perceived without additional focussing power. At 60 years, accommodation is practically nil. This decline in power is due to the increasing hardness in the lens already referred to, in consequence of which the ciliary muscle loses its power to increase its convexity. This loss in power of accommodation due to advancing years is termed presbyopia or old sight.

Visual Acuity. — ^The normal acuteness of vision has been determined to be the power the eye has of distinguishing objects which subtend an angle of 5' upon the eye. With this as a basis, Snellen, a distinguished Dutch ophthalmologist, constructed a series of letters made to conform to this standard. The accompanying diagram (Fig. 14) demonstrates that an object, in order to subtend the same angle, must be larger the farther it is removed from the eye. In conducting the test for visual acuity, a card containing graduated series of letters of different sizes is placed 6 metres distant from the observer and record made of the smallest line of letters which the observer is able to distinguish with each eye separately. If vision is normal,