THE STKUCTUEE AND FUNCTIONS OF THE EYE, ILLUSTRATIVE OF THE POWEE,. WISDOM, AND GOODNESS OF GOD. SPENCER THOMSON, M.D., L.R.C.S., EDIN. “ The light of the body is the eye.” “Nature, therefore, is nothing else but God’s instrument.” Richard Hooker. LONDON : GROOMBRIDGE AND SONS, PATERNOSTER ROW. 1857. INTRODUCTION. j ■ It is impossible to imagine an intelligent man engaging ^ in tlie investigation and study of any portion of crea- tion's wide domain^ prosecuting tlie survey even of a I single division of the grand Cosmos^ without his becom- ing deeply interested in the world of use and of beauty thus opened to his intellectual view — without his feeling that the pursuit had^ like the hoof of the winged horse, opened in the higher places of his intellectual being a fount whose streams yielded deep draughts of the true ^ and the beautiful, a never-failing source of pleasurable ^ happiness independent of external circumstances, and ^ unsullied by the turbid waters of worldly striving. ^Before him lies an inexhaustible field, and never, like ^ the great conqueror, need he weep because he has j \ ^nothing more to overcome. Health and length of days, OoPO^er of intellect, and industry unwearied, can but place him on the border-land of the vast realms of VI INTRODUCTION. knowledge^ which stretchy boundless, away into the ages of eternity. The man who studies forms of Nature has before him, so far as these forms are concerned, models of perfection — he has before him truth. His sole business is to analyse all the parts and all the bear- ings of that truth, and make them known to the world. The models and materials of his study are divine, and how much more they exceed those of any human artist will be manifested by a blade of grass, compared with which the most exquisite carvings in stone and ivory sink into insignificance/^* It is a question whether any object in creation pre- sents, in equally small space, so much structural beauty and exquisite adaptation, as the Eye, either of man or animal, or affords so much illustrative proof of having for its Author a being of infinite love, wisdom, and power. Elevated, equally with the ear, above the other senses, by virtue of its mental and psychological fulfil- ments, the eye certainly offers more calculated to arrest the attention and excite the admiration of inquirers generally, whilst the readiness with which much of its organization may be seen, even in active, living opera- tion, renders the whole subject more comprehensible to those who are unaccustomed to the consideration of Swainson. INTRODUCTION. Vll animal structures. The author has endeavoured_, with- out passing too lightly over the subject^ to render it^ both by description and illustration, as generally intelligible as possible. At the same time_, there inevitably is much which cannot be comprehended or appreciated without some amount of thought and trouble ; but the object is worthy of the means^ however feeble or inadequate the pencil or pen of expositors* may be to do it justice. The Chapter on Light has been confined within limits as narrow as are eonsistent with the consideration of the laws necessarily involved in the physiology of visual phenomena, and the anatomieal detail has been princi- pally directed to the elucidation of those points of struc- ture most interestingly and directly connected with the function of vision itself ; whilst throughout it has been the one endeavour to connect, in the mind of the reader, the perfections of the instrument with the attributes of its Maker, His power, His wisdom, His love, in nothing more visible than in the animal happiness, the intellec- tual and spiritual enjoyment which He has bounteously linked with the exercise of the organ of vision in all his creatures, from the creeping things of the earth, up to man, formed after his own image and likeness. ^ In the “ List of Illustrations” will be found the names of authors from whose works the figures are derived. 1 CONTENTS. CHAPTEE I. ' SEl^fSATIOlN. PAGE Definition of Sensation — Conditions of Sensation — Speciality of Sense — ^Modification of Sense . . 17 CHAPTEE II, LiaHT. Light — Hypothesis of Nature — Velocity, &c. — Eo- lation to Bodies — Luminous — Opaque — Trans- parent — Colour — Eefraction — Laws of — Plane Surfaces — Curved Surfaces — Density of Media — Lenses — Pocal Points — ^Eormation of Image- Spherical APicrration — ^Eeflection of Light — Chro- matic or Colour Phenomena — Prism — ^Eecapitula- tion 27 CHAPTEE III. AKATOMT or HUMAN EYE. Organ of Vision generally — Orbits — External Ap- pendages — Eyelids — Orbicular Muscle — Con- X CONTENTS. junctiva — Meibomian Grlands — Lachrymal Canals — Eyelashes — Lachrymal Grland CHAPTEE IV. AXATOMY or HUMAX EYE. Internal Appendages — Elevator Muscle of Upper Eyelid — Straight Alnscles of Eye — Oblique Mus- cles — Eascia — Vessels — Nerves CHAPTEE V. AXATOMY OE HUMAX EYE — THE OLOBE. Form of Eye — Sclerotic Coat and Cornea — Optic Nerve — Structure of Cornea — Anterior Epithe- lium — Anterior Elastic Layer — Lamellated Layer “Posterior Elastic — Posterior Epithelium— Cho- roid Coat— Vessels — Pigment — Ciliary Processes — -Eetina — 'Structure — Jacob’s Membrane — Sen- sible Spot — Macula Lutea — Hyaloid Membrane and Vitreous Humour — Crystalline Lens — Sus- pensory Ligament — Structure — Divisions — Fi- bres— Ciliary Muscle — Aqueous Chamber— Iris and Pupil CHAPTEE YL PHYSIOLOGY OF HUMAX VISIOX. Essentials of an Organ of Vision— Course of Eays entering Eye — Cornea and Aqueous — Lens — Vitreous — Eetina — Choroid — Iris, its actions, CONTENTS. XI perfection, and beauty — Lens — Eefracting Powers — Vitreous — Cboroid — Absorption of Light — Vision — Local Convergence — Adaptation — Dis- tant Vision — Myopia — N’ear Vision — Presbyopia — Eange of Vision — Vision with two Eyes — Optic Commissure — ^Double Vision— Perspective Dis- crimination — Appreciation of Distance — Chroma- tic Correction — Deflections on Evidence of Crea- tive Power and Wisdom • . . 155 CHAPTER VII. COMPAKATIVE AXATOMT AXD PHTSIOLO&T OP THE EYE. IXVEETEBEATE AISTIMALS. Sensibility to Light of lowest Animal Tribe — Ocelli of Medusa and Star-fish — Polygastric and Wheel Animalcules — Parasites — Earnacles — Annelida, or Worm Tribe — Centipedes — Eyes of Insects — Compound and Simple Mode of Vision — Spider Tribes — Crustacea or Crab Tribe — Simple — Agglomerated — Compound — Mollusca or Soft- bodied Tribe . 181 CHAPTER VIII. COMPAEATIVE AXATOMY OF THE EYE. VEETEBEATE AXIMALS. Eyes generally — Eishes — Adaptation to Watery Me- dium — Elat Eorm — Spherical Lens — Cartilaginous Plates — Plaited Nerve — Amphibia — Reptiles- - Xll CONTENTS. Birds — Adaptation of Eorm of Eye — Osseous Plates — Pecten — Adaptation in Eapid Motion — Owls — Aquatic Birds — Eyelids and Muscles — Mammalia — Eyes generally — ^Whale Tribe — Am- phibious Mammalia — Mole — Orbits — Pupils — Tapetum lucidum 199 CHAPTEE IX. ox THE WISDOM OE GOD Displayed in the Structure of the Eye 237 CHAPTEE X. THE PHILOSOPHY OE YISIOX, As Illustrative of the Beneficence of God .... 247 LIST OF ILLUSTKATIONS. DIACRAMS. No. PAGE I. Eefraction of Diverging Eays of Light hy Plane Dioptric Medium 35 II. Eefraction of Converging Eays hy ditto . . 37 III. Eefraction of Parallel Eays hy Curved Sur- faces — Double Convex Lens .... 40 IV. Eefraction of Diverging Eays by ditto . . 43 V. Pormation of Inverted Image by Double Convex Lens 45 VI. Poreshortened View of Double Convex Lens — Spherical Aberration 47 VII. Eeflection of Eays from Plane and Curved Surface 49 VIII. Prismatic Spectrum 50 IX. Optical Essentials of Human Eye .... 159 X. Pormation of Image in Human Eye . . . 167 XIV LIST or ILLUSTRATIONS. FIGURES. No. COPIKD PROM PAGE I. Orbits Wilson 63 II. Orbicularis Muscle 65 III. Diagram of Optic Nerves 69 IV. A, Internal view of Eyelids and Lachry- mal Gland. Soemmering. B, Tarsal Cartilages and Lachrymal Apparatus . 73 V. Meibomian Glands — Lachrymal Appara- tus Zinn 77 VI. Perpendicular Section of Orbits and Con- tents Grant 85 VII. Muscular Apparatus of Eye . . Wilson 87 VIII. Horizontal Section of Left Eyeball . . 99 IX. Ciliary Apparatus .... JBoivman 103 X. Choroid and Vascular System of Eye- ball Zinn 107 XI. A, Internal Vie’sv of Choroid. B, Ciliary Processes Zinn 111 XII. Nerves and Vorticose Veins of Eye- ball Zinn 114 XIII. Vorticose Veins Wilson 119 XIV. Ketina Jacob 123 f Todd and i XV. Microscopic Anatomy of Cornea J ^(9 I 127 \IIassal j XVI. Microscopic Anatomy of Cornea . Bowman 131 LIST OP ILLUSTRATIOKS. XV COPIED FROM XYII. Vessels, &c., of Cornea . . . Carpenter XYIII. Laminated and Tube Structure of Cornea magnified . . . Todd and Boivman XIX. Pigment Cells of Choroid (ToddandBotc- \man, Hassal, magnified ( Carpenter XX. Microscopic Structure of f -p) I • j i? T 1 5 1 Tacob ^ Todd Eetina and oi Jacobs ■{ . J, and Bowman Membrane [ XXI. Iris and Corona Ciliaris .... Jacob XXII. Structure of Lens . Todd and Bowman [ ToddandBow- XXIII. Microscopic Structure of Lens J man, Muller, ( Zinn,IIassal ( Cyclopcedia XXIY. Simple Eyes of Invertebrata \ ^ •’ and FnysioL, I Muller . XXY. Compound Eyes of Insects . Bymer Jones XXYI. Eyes of Eish Bymer Jones XXYII. Eyes of Eep tiles . . Cyclop. A. and B. XXYIII. Eye of Owl Bymer Jones XXIX. Muscular Apparatus of Eye of Bird Bymer Jones XXX . Section of Eye of Whale Cyclop. A. andB. XXXI. Section of Eye of Cuttle-fish Bymer Jones 135 139 1 143 \ 147 j 149 151 1 153 1 185 1 ) 189 203 211 216 224 228 231 \ 1 ' / ■ CHAPTER I. DEFINITION OE SENSATION — CONDITIONS OE SENSATION — SPECIALITY OE SENSE — modification OE SENSE. I / /•; 1 THE EYE. CHAPTER I. THE SENSES, AND SENSATION GENERALLY. The word sensation, like other similar words which express ideas of an abstract rather than of a tangible nature, has been, and is often now, used in two different senses : the first, to express the communication conveyed to the brain or sensorium by the sense acted on ; the second, the impression communicated to the conscious mind of the individual by the brain. How the impression receired by the physical organ is taken up by the mental power we know not ; to us it is an ultimate fact, beyond our finite powers of comprehension. All we know is the actual connection of the mental phenomena with that perfect and wonderful 20 THE EYE. system of nerrous telegraph by which the spiri- tual of man, the mental of the animal, holds communion with the natural and material. Knowing, then, that the action upon the sensorium, and the consciousness excited thereby, are inseparably connected ; moreover, that the word sensation is used to express both actions separately — a separation, nevertheless, of whieh we cannot form an idea in our own minds — it is thought better, in using the word in this treatise, to keep that inseparability in view, and to regard the impression exerted upon the sensorium, or material part, as necessarily exerted upon the mind, or imma- terial part, at the same time, and thus to inelude both actions in the same expression. At the same time, there is in this no ground for interference with the metaphysical idea — no countenance for the illogical absurdity, which would make mental phenomena merely the result of physical organisation. In accordance with the above, the defini- tion is adopted, that sensation consists in the reception by the sensorium (and mind), through the medium of the nerves, of certain definite impressions, these impressions being the result of changes in the nerves themselves — depen- SENSATION. 21 dent for their effect upon the sensorinm— upon the peculiar energy or quality of the individual nerve, which is capable only of ^ being affected by, and of conveying, one certain species of impression, and no other. Thus, it being the peculiar energy or quality of the optic nerve to convey the sensation of light or colour, it is precluded, by virtue of this endowment, from conveying any other description of sensation. “The essential nature of these conditions of the nerves, by virtue of which they see light and hear sound*- — the essential nature of sound as a property of the auditory nerve, and of light as a property of the optic nerve, of taste, of smell, and of feeling — remains, like the ulti- mate causes of natural phenomena generally, a problem incapable of solution.” Although every man is conscious of being in his own person susceptible of the action of the senses, it can only he matter of inference that other sentient beings are capable of the same impressions — undoubtedly of certain in- ference as regards our fellow-men, and of When we say sensation and thought are functions of the nervous system, we mean only that this system furnishes the conditions under which sensation and thought in the living body take place.”— Physiology, p. 121. / 22 THE EYE. most probable inference as regards the higher animals. As we descend in the scale of crea- tion, however, and find either an extremely low development of the nervous system, or no perceptible development at all, we have no data by which to measure the probable amount of sensibility to impressions, either external or internal, “although there is good reason to believe that all beings of a truly animal nature possess a consciousness of their own existence.” It is evident, however, that this simple con- sciousness of existence, which we judge to be possessed by even the lowest tribes of animals, is very different from the arrangements of special sense enjoyed by the higher, For the due exercise of the special senses — touch (a portion of general sensation), taste, smell, hearing, sight — two conditions appear to be more especially necessary ; the integrity of the connection of their nerves with the central mass of the nervous system, and the free cir- culation of arterial blood throughout that sys- tem. Moreover, it is evident that the trans- mission of the special sensation to the senso- rium, by the special nerve, is not simply as by a conductor, but is dependent upon certain changes peculiar to the nerve itself. This we SENSATION. 23 know to be the fact ; the nerves are capable of conveying to the sensorium impressions similar to those excited by external influences, when no such influences have been exerted. Thus, individuals will frequently describe sensations as if experienced in tbe toes or fingers long after the amputation of the limb. The sensa- tion of disagreeable odours may result from functional disorder of the olfactory nerves, or luminous appearances be apparently visible to an eye which had been extirpated. Further, the single stimulus of electricity, properly applied, is capable of exciting in each nerve its own peculiar energy. Applied to the eye, the sensorium receives the impression of light ; * to the ear, of sound;! to the nostrils, of the odour of phosphorus ; to the tongue, of acid ; to the skin, of prickling. Not only, however, are the special sensations of the nerve liable to excitation by these external agencies, but also by internal means. Disorder of the bodily health, whether occurring spontaneously or as Humboldt. t Yolta states, that while his ears were included between the poles of a battery of forty pairs of plates, he heard a hissing and pulsating sound, which continued as long as the circle was closed. — Muller's Physiology , p. 1063. .THE EXE, 2i \ the result; of medicinal or other substances, ' more especially of narcotics, may have a similar effect. : * ' It has been remarked that one nerve of special sensation cannot become substituted for another ; the optic nerve cannot convey the impression of sound, nor the auditory that of light. This might have been ex- pected, but we also find that these nerves have not the faculty of conveying common sensation. The optic nerve may be divided in operation without pain; the only sensa- tion — if the function be not destroyed by disease — ^being that of a flash of light. In Magendie’s experiments, the olfactory and optic nerves, and the retina, were found in- susceptible of excitement by mechanical irrita- tion. In the higher tribes of animals, in which the five senses exist in what may be con- sidered perfect condition, one of the five is frequently more or less modified or deve- loped, according to the requirements of the creature, whether for the attainment of sub- sistence, the preservation from injury, or both, and an acuteness of perception possessed which is not experienced by man, at least SENSATION.. 25 in a civilized condition. It is certainly true that, amid savage nations, the constant incen- tive to the keen exercise of the senses in proenring subsistence and guarding against danger, must and does call forth in them a power of discrimination they would not other- wise have; hut it is a question whether this power ever equals in intensity the natural instinctive sense of the hrute. The mere animal aeuteness of perception, more- over, is very different from the edueahility of the senses bestowed upon man — the faculty of distinguishing the useful and the beautiful; in short, of surrounding himself with all that can add to the comfort and gratification of sense, of appropriating all that can elevate him as a social, moral, an intellectual and religious being. CHAPTER II. iLtsfit. LIGHT — HYPOTHESIS OE NATURE — VELOCITY, ETC. — RELATION TO BODIES — LUMINOUS- OPAQUE — TRANSPARENT — COLOUR — REFRAC- TION — LAWS OP — PLANE SURFACES— CURVED SURFACES — DENSITY OF MEDIA — LENSES— FOCAL POINTS — FORMATION OF IMAGE — SPHERICAL ABERRATION — REFLECTION OF LIGHT — CHROMATIC OR COLOUR PHENOMENA — PRISM — RECAPITULATION. / ■0 , IK n ^ r; •L - . CHAPTER II. LIGHT. ’Tis the sun that maketh all things shine.’’ What is light? What is that which we are told “was,” at the fiat of the Almighty, “the prime work of God,” the first of distinetly created things, and which has, ever since that fiat went forth, eeased not its shining? We know that it is light hy which, as our visual organs are con- stituted, we are enabled to take cognizance of the eolour, size, shape, and position of things around us. We know that this light is suhjeet to laws which it is in our power to trace ; hut what it is in itself, we know not certainly, how- ever beautiful, and apparently adaptable, are the theories we frame respecting its real nature. This, however, we do know, that the great source of light is the sun — that bright and glorious luminary, which may he taken as a type of the Deity Himself : its life-reviving rays, the 30 THE EYE. beams of His divine Love and Wisdom — their warmth, the Good — their light, the True. In order that we may properly understand and fully appreciate, the exquisite structure, and varied adaptation, by which the Creator has fitted the organs of sight to the habits and requirements both of man and of the lower animals, it is requisite that the properties and laws of light — at least so far as they affect vision — should be rightly comprehended. With the consideration of these it is proposed to occupy the present chapter. The actual nature of light, like sensation, must be classed amid those ultimate facts which encompass the domain of human know- ledge, which seem to say to the highest human intellect, “ Thus far shalt thou come, and no further.” [Fortunately, however, this necessity, which compels us, in the absence of certain knowledge, to search out and frame the theory apparently best adapted to explain the pheno- mena and laws of this wonderful agent of God’s purposes, does not interfere with or hinder the investigation. Two theories of the nature of light have been principally entertained in modern times. The first, that of Newton, which supposed light LIGHT. 31 % to consist of material emanations or atoms, continually thrown oif, with immense velocity, from the sun or other self-luminous body. The other, modified by various observers, and now most universally adopted, usually called the “ TJndulatory Theory,” supposes light to consist in, or to he the result of, rapid oscillations of the supposed spherical atoms, of the impon- derable, elastic, ethereal fluid, which is conjec- tured to fill up the interspaces of the atoms of all material things, including the air itself, and to he extended, not only beyond the narrow limits of our aerial atmosphere, hut through- out universal space. Thus it is thought that the sun, or even a simple candle, by first ex- citing this oscillation in the particles next to itself, occasions it to pass, or to he communi- cated, with immense rapidity from one particle to another, until at last reaching the eye, it occasions the sensation of light by its action on the nerve or retina of that organ. By this theory, light results from the excitement caused by oscillation or undulation,, ^darkness from its absence. It is calculated, however, that a certain rapidity of oscillation must be generated before the sensation of light can be excited in the eye. 32 THE EYE. Whatever the nature of light, it is found- to pass through air at the immense velocity Of 192,000 miles per second ; from whatever agent it may he projected, its waves or undulations, like those of sound, being transmitted in every direction from the luminous body with an in- tensity inversely as the square of the distance. It may here he well to caution the reader against confounding Avhat are called the “rays” of light with the light itself, the former being only imaginary straight lines, whether in language or diagram, taken to in- dicate the direction of the effect of the agent. All bodies may be regarded, in relation to light, as falling under the head of luminous, opaque, transparent. According as light comes in contact with one or other of those classes of bodies, its agency is modified. Thus if a ray, proceeding from one luminous body, meet with a ray from another, the intensity of each is in- terfered with. Some bodies may check its un- dulations altogether, others, by having fresh undulations excited in the ethereal particles occupying their interspaces, project rays from themselves which convey to the eye a correct idea of their colour and form. Some may reflect or refract the rays of light without LIGHT. 33 altering their character, whilst others have the property of dividing light, as it were, either giving rise to the double ray of polarization, or to the prismatic colours of the spectrum. It has already been remarked that black is merely the absence of all light or colour, consequent upon the power possessed by all bodies presenting a black appearance of check- ing luminous oscillation. Colour, on the other hand, is thought to be the result of differences in the rapidity of the oscillatory movements excited by various bodies, and not of any actual property of colour in the bodies them- selves, each colour requiring its distinct scale of undulations ; for instance, in order that the sensation of the colour red may he conveyed to the eye, it is computed that four hundred and seventy-seven millions of millions of oscil- lations in a second must he communicated to the particles of ether — an amount of velocity and action totally out of the power of our minds to comprehend. HEFRACTION OF LIGHT. Rays of light may he divergent, convergent, parallel, relatively, to each other ; each ray, c 34 THE EYE. however, maintains its original, individual, straight direction, so long as the transparent medium through which it passes continues of uniform density. Should, however, the ray of light pass from a rarer into a denser medium, as from air into glass or water, and he incident in any other direction than the perpendicular, the straight line of its original course is changed — it becomes refracted, or, as it were, broken; if it fall perpendicularly upon the surface of the medium, the direction remains ‘ unaltered. The simple hut illustrative experi- ment on this refraction, of the oblique ray passing from a rarer into a denser medium, is familiar to all. A coin placed in the bottom of a cup, so that it cannot he seen by a spec- tator standing at a certain distance, instantly becomes visible on filling the vessel with water, in consequence of the refraction, or bending, as it were, of the rays, which formerly passed in straight lines. The person may he said to see round a corner. Again, if A B (Diag. I.) he a plate of glass with parallel sides, a ray of light, P, falling upon it perpendicularly from the point L, will pass through its straight course unaltered ; on the other hand, the obhque rays, O 0, instead of continuing each LIGHT. 35 its original straight course to P, will be re- fracted towards the perpendicular, as soon as it enters the denser medium ; on leaving it, how- Diagram I. — Erfraction or Eays or Light traversing a Dioetric Medium rounded by Plane Surfaces. Diverging Days. — L, Common Point. A B, Plate of Glass. P, Per- pendicular Eay not refracted. 0 0, Oblique Pays refracted towards the perpendicular to the point at which they enter A B, away from the perpendicular to the point at which they leave A B. ever, and again passing into the rarer medium, another refraction takes place, now from the perpendicular, the rays assuming a direction 36 THE EYE. exactly parallel to their original one, and passing on to I 1. It is evident that the re- fracting medium, A B, without altering the relative directions of the rays, has yet exercised considerable influence over the amount of their divergence in a given space. No two laws of optics have a more im- portant hearing upon the physiology of vision than those just illustrated, viz. : — I. Light passing from a rarer into a denser medium, is refracted towards the per- pendicular of the surface on which it falls. II. Light passing from a denser into a rarer medium, is refracted from the perpendi- cular. Beversing the diagram, it is evident that a similar refracting agency acting upon the rays O O (Diag. II.), must, in like manner, alter their original direction, and cause them, instead of meeting at P, to delay their point of junction as far as P. There is considerable difference in the re- fracting power of various transparent, or, as they are called in optical science, “ dioptric” media : generally, the greater the density, the t Diagham II.— Refraction of Rays of Light traversing a Dioptric Medium bounded by Plane Surfaces Converging Rays . — Refractions similar to Diagram I. Effect reversed. 38 THE EYE. greater the power of refraction. To this law, however, there is an exception in the case of inflammable bodies, which are, comparatively, more highly refractive than any other ; a cir- cumstance which first led Sir Isaac Newton to hazard the assumption that the diamond, from its extreme power of refracting light, was in- flammable. The truth of the assumption was afterwards fully proved, when the diamond was found to consist of pure carbon. More- over, the refracting power of a transparent medium exerts considerable influence over the velocity of light transmitted through it. The greater the refraction, the less the velocity. It is not all the rays impinging upon the surface of a dioptric medium w^hich enter it ; a certain number always undergo reflection, in proportion to the obliquity of their inci- dence, until at last this becomes so great, that, instead of being permitted to enter, they are reflected from the surface on which they impinge, at what is called the “limiting angle” between refraction and reflection. Hitherto, the action of light in connection with refracting media, hounded by plane sur- faces only, has been considered, hut the sur- faces of the refracting media of the eye being LIGHT. 39 all of them curved, in order properly to un- derstand the physiology of that organ, it be- comes necessary to examine how the etfects already pointed out are modified hy the differ- ence in form. Refracting media with curved surfaces are called lenses, and may he of very different forms — spherical, double convex, piano convex, concave, meniscus, &c. As, however, the same law, differently applied, explains the action of the various kinds, it will he sufficient here to consider that of the commonest, and the one most nearly connected with the physiology of vision — the double convex lens. Diagram III. represents the section of a double convex lens through its centre, but of course the convex sides may have any degree of curvature, from nearly spherical to almost flat ; or, as in the lens of the eye, may he un- equally curved. ABC are supposed to he three parallel rays of light incidental upon one of the convex sides of the lens, L ; B, falling perpendicularly, will of course pass through the centre unaltered in direction ; A and C, however, being in the position of oblique rays as regards the surface of the lens, will he re- fracted towards o, the perpendicular to the Diagram III.— Refraction of Rays .of Light traversing a Diop- tric Medium bounded by Curved Surfaces. Tarallel Rays, — L, double convex lens. B, central ray not refracted. A C, eccentric rays refracted towards perpendicular, o, of point of in- cidence, away from perpendicular, n, of point of exit. P, focus of parallel rays. LIGHT. 41 plane of the point at which they meet the curve; this refraction throws them into con- vergence, and this convergence is rendered still greater, when, on leaving the lens, and passing into the rarer medium of the atmosphere, each ray is refracted away from n, the perpendicular, to the plane at which they part from the curve. The evident effect of this double refraction and double convergence is to cause the origi- nally parallel rays, A and C, to intersect. If these two rays have been equidistant from the axis ray of the lens, B, they will intersect it, and each other, at the same focal point, P ; continued beyond this they will become diverg- ent. This focal point, P, is the principal or focus of parallel rays, and is of course depend- ent for its position upon the density of the substance forming the lens, and upon the amount of curvature of its surfaces. The denser the lens, and the more convex, the greater will he the amount of refraction, the more marked the convergence of the rays, and the nearer their intersection, or focal point, to the refracting medium. The same rule which applies to the parallel rays, A C, is applicable to all parallel rays equally distant from B ; they will intersect at P. Vice versa, luminous rays 42 THE EYE. diverging from a point exactly in the focus of the lens will he rendered parallel by passing through it. Diverging rays, passed through a double convex lens, are necessarily refracted to a point beyond, and not to he confounded with the principal focus of the lens. The further distant the luminous point yielding the rays, that is, the more nearly they approach parallelism, the nearer of course does their point of intersection lie to the principal focus ; on the other hand, the nearer the luminous point to the lens, the more divergent the rays, the further from that focus will they meet. ^Example . — Two diverg- ing rays of light, o o (Diag. IV.), emanating from a point, M, and passing through the lens, L, would he refracted to the point, m, beyond the principal focus, whilst rays still more divergent, proceeding from N, would intersect only at a point, n, still further from that focus. By a little consideration, the foregoing observations, which have been made solely with reference to isolated rays, will explain the formation of an image, by lenses, as it is formed in the eye. An image of any body is but a collection of the various rays proceeding Diagram IV. — Kefraction of Rays of Light traversing a Diop- tric Medium bounded by Curved Surfaces. Diverging Rays. — L, double convex lens. M, distant point of diverg- ence of o o, which are refracted by the lens, and converged to m. N, nearer point of convergence ; the rays only converged at n. 44 /. THE EYE. from that body, and these, individually, are all snbjeet to the same laws. L (Diag. V.) represents a double convex lens, through which are transmitted rays from the body, D, with the effect of forming an image of that body on any receptive surface, placed near the proper focus. If the direction of the rays represented in the diagram he examined, it will he seen hoAV, in accordance with the laws of refraction, the image of the body, D, is necessarily repre- sented inverted at d. Hays which pass through the centre of a lens, even if they fall obliquely, do not emerge from it with much alteration in their direction, the two surfaces near the centre being so nearly parallel, as to approximate the effect of the refracting agent to that of the glass plate (Diag. I.). This is a point of some import- ance in connection with the physiology of vision. Moreover, the focus, or common point of convergence of all such central rays, Avill be tolerably well defined. As, however, the pencils of rays approach the circumference of the lens, that is, recede more and more from the central or axis ray, they necessarily become more and more oblique, relatively, to the curvature of the refracting surface; the consequence is, they DiAGAllM y. Formation of reversed image d of image D, by refraction of rays traversing double convex lens. 46 THE EYE. are more strongly refracted or converged than those which pass through or near the centre, and thus intersect at a point nearer to the lens than its principal focus, causing some degree of confusion in the definition of the image. This effect is known by the name of spherical aberration, and is inseparable from the refracting agency of lenses with spheri- cally curved surfaces. Let L (Liag. VI.) represent a fore-short- ened view of the surface of a double convex lens, and A its central or axis ray. “The focus for each circle of rays from the axis to the margin of the lens is different, becoming nearer to the lens the more remote the rays are from the centre.” All which fall within such a distance from it, as the imaginary circle C C includes, will converge so nearly to one point as to form a perfectly well defined image. On the other hand, rays such as o o, passing through the lens near its margin, will, as we have seen above, he converged more rapidly than the former, and tend to confuse w'hat would otherwise he distinct. This “ aberra- tion from spheiicity” is, indeed, capable of correction, by modification of the curvatures of the lens, or, as in the eye, by difference in Diagram YI. Foresliortened view of double convex lens, L, showing convergence of parallel rays from P, if traversing near the centre of lens. Spherical aberration of rays, o o, traversing margin. 48 THE EYE. the density of the central and marginal media. It is, however, more easily, and, for all practical purposes, sufficiently corrected hy the inter- position of an opaque ring, or, as it is called in optical science, a “ diaphragm,” between the luminous hody and the marginal portion of the lens, so as to intercept all rays sufficiently eccentric to cause confusion by spherical aber- ration. It is evident that the interposition of such a ring around the margins of the lens (Diag. VI.) will have the effect of interceptin all rays, such as o o, the refraction of which would interfere with the principal or image of the central rays. ' The diaphragm is much used in the con- struction of optical instruments, and in the eye we find it in the most perfect conceivable form. HEFLECTION OF LIGHT. Hitherto, the effects of transparent, or, as they are called, dioptric media, upon light, have only been considered; it is with these we chiefly have to do in the physiology of vision. When rays of light, instead of falling upon a transparent hody, impinge upon a plane LIGHT. 49 polished surface, or mirror, they are for the most part reflected, that is, thrown hack into the medium through which they reached the mirror. This reflection takes place according 1 . 2 . Diagram YII. — Eeflection of Light. 1. From plane surfaces. 2. From curved surfaces. to the fixed law, that the angle of incidence equals the angle of reflection. Thus a ray, R (Riag. YII., 1), falling upon a plane polished surface at the point F, will, on under- going reflection, pass off towards S, forming equal angles on each side of the supposed per- pendicular, P. Diagram VIII. Decomposition of light B, by prism P, and formation of prismatic spectrum, S. LIGHT. 51 ' Precisely the same law regulates the reflec- tion of rays impinging upon the surface of a curved mirror. In this case, each point of such a mirror on which luminous rays fall is regarded and acts as a small reflecting plane in itself. A ray, E- (Diag. VII., 2), falling upon the surface of a concave mirror, at the point P, the tangent, M, will represent the direction of the plane from which, in accord- ance with the above law, it must he reflected to S. It is unnecessary, for the purposes of this treatise, to pursue further the consideration of the laws and phenomena connected with reflec- tion of light. PRISMATIC OR CHROMATIC PHENOMENA OP LIGHT. According to the simple law, rays of light passing through a prism ought, whilst under- going refraction, to remain at the same time parallel. This, however, is found not to he the case. “ If a beam of the sun’s light, B (Diag. VIII.), fall obliquely upon the prism, P, it will he refracted twice, namely, by its anterior 52 THE EYE. and by its posterior surface ; but, instead of the rays continuing parallel in their new course^ the beam is spread out, and, when received upon an opaque surface, presents the colours of the rainbow.”* The image thus formed by the unequal refraction is the prismatic spec- trum S (Diag. VIII.), and is composed of seven colours, arranged in the following order : violet, indigo, blue, green, yellow, orange, red ; violet being the highest, or most refran- gible, the other rays range below it in the order given. The boundaries of the colours, how- ever, are not distinct ; they gradually melt into each other. Sir Isaac Newton, who was the first to per- form the experiment of separating light by the prism, inferred from it that white light is com- posed of seven primitive colours. It is now more generally thought that three colours only, - — ^blue, yellow, red — ^are primitive, and that the others are compounded by the intermix- ture of these at their edges in the prismatic spectrum. The fact elicited by the prismatic analysis, that white light is composed of the coloured rays, is susceptible also of synthetical illustra- * Miiller. LIGHT. 53 tion ; the three kinds of coloured rays, by proper management or super-imposition, are capable of being combined to form white light. The colours of natural objects are supposed to result from the absorption or transmission of such of the component rays of white light as do not appear. Thus a flower, it is said, appears blue, from the fact of the surface of its petals absorbing the red and yellow rays of the light shining upon them. Although it is only by the prism that the separation of white light into its component coloured rays is perfectly brought out, any other transparent refracting medium, though not prismatic, exerts partially the same effect, causing what is called “ chromatic aberration.” The unequal refraction causes objects, seen through such refracting media, to appear surrounded by a fringe of prismatic colours. This long proved an obstacle to optical im- provement, but was at last removed by the dis- covery that different transparent media — even different qualities of glass — exert different re- fractive effects upon the component coloured rays. Taking advantage of this, opticians, by a combination of lenses, formed of different 54 THE EYE. kinds of glass, one to nentralize or correct the” chromatic aberration of another, sncceed in pro- ducing “achromatic” optical instruments, free from the confusion of colour which formerly- prevailed. In the human eye, it will he found that provision has been made for the same cor- rective purpose. To recapitulate, we find light taking its origin from the sun, or other luminous body, passing in straight lines in all directions, unless interrupted. Interrupted, becoming absorbed by opaque bodies of one description, or reflected more or less by others, imparting to the sense of sight, the idea of black, of white, or of colour. Heplected, under fixed laws. Transmitted by transparent media, under- going refraction according to the density of the medium, and the relative positions of the lumi- nous rays, and of the receptive surface ; or the ray split, as it were, by polarization, or divided by the prism into the rainbow colours of the spectrum. The consideration of the phenomena and laws of light has now been carried as far as requisite for the object of the present treatise — for the due understanding and proper appre- LIGHT. 66 ciatioH of the beautiful construetious and adap- tations by which the Creator has fitted the dif- ferent forms of visual organs, to act in unison with the light of the material universe — that “light from whose rays all beauty springs.” CHAPTEE III. ^natorng of ^umait ORGAN OF VISION GENERALLY — ORBITS — EX- TERNAL APPENDAGES — EYELIDS — ORBICULAR MUSCLE — ^CONJUNCTIV A— MEIBOMIAN GLANDS — LACHRYMAL CANALS — EYELASHES — LACH- RYMAL GLAND. 1 CHAPTER III. ANATOMY OP HUMAN EYE. — THE ORGAN OF VISION GENERALLY. That vegetables are in a degree sensible to the action of light over their entire surface, there can be no question. It is also probable that some of the lower tribes of the animal kingdom, such as the polyps and others, possess no higher endowment than this kind of diffused sensibility to the agent ; they do not, at least, present organ or formation to which a faculty resembling what we call sight, can be ascribed. The lowest and simplest appearances to which the name of eye can be given are the “ eye- dots” of the medusa and of the star-fishes, or the “ ocelli” of the annelida ; and these organs, probably, do little more, if as much, than con- vey to the animal an impression of the more marked differences in form and colour, destitute, as they are, of optical apparatus for the forma- 60 THE EYE. tion of a distinct image upon the nervous expansion, which, surrounded by a little black pigment, and shielded by a cornea, constitutes, with in some cases the addition of a lens, their simple eye. As the consideration of these simple eyes, as well as that of the second class of visual organs — the highly compound eyes of insects and crustaceous animals — will engage a future section, we pass at once to the third and highest class, that of which the human eye may he taken as a type ; in which accurate and per- fect vision results from the transmission of rays of light through refracting dioptric or transparent media, and the formation of exact images of external objects upon an ex- tended sheet of nervous substance placed in a dark receptacle — the interior of the eye — much in the way that images are thrown upon the white surface in the camera obscura. The eyes of the higher mammalia, and of vertebrated animals generally, very nearly — in essentials at least — resemble those of man ; and though, throughout the various tribes and species, we find modifications of the visual organs, suited to the habits aud requirements of the creature, and properties possessed which ANATOMY OF HUMAN EYE. 61 do not appertain to human sight, these are the result rather of appendages to the optie apparatus than of any greater perfection of the apparatus itself ; and may well he entered into, after the consideration of that heautiful construction and wonderful adaptation by which the Creator has fitted the human eye for its ofiices and powers : then shall we have seen how each and all of these contribute to the one great end of perfect vision, and how all are constituted in perfect harmony with the laws which are impressed upon and regulate the rest of material creation. A thorough under- standing of the structural beauty and optical perfections of the human eye will better pre- pare us to appreciate the superadditions ; hut whether it he the focal eye of the mammal or bird, the facetted organ of the butterfly, or the simple ocellus of the leech, we shall find each and all telling of Him who created them, and testifying, in language eloquent though soundless, “ the hand that made us is divine.” 62 THE EYE. THE HUMAN EYE — THE ORBITS OR EYE- SOCKETS. The orbits or sockets of the human eyes are two quadrilateral, hollow pyramids (Pig. I.), situated at the upper part of the face, and are formed by the union of portions of seyen diffe- rent hones ; they are pierced by nine foramina or holes, eight of which give passage to blood- vessels and nerves, the ninth to the duct (Pig. IV., B 4), by which the lachrymal fluid is con- ducted into the nose. In these bony cones, along with its mus- cles, nerves, blood-vessels, and other appen- dages, and suspended by a fascia of its own, is placed the eye-ball or globe, every space otherwise unoccupied being well fllled up and cushioned with fat, and all care taken for the perfect inward security, hut at the same time perfect mobility, of the important tenant. Violence from without is provided against by the strong hulwark-like projection formed by the margin of the orbit itself. To ward off lesser external irritations which might prove obnoxious, there exists that beautiful combination of protecting agents, the “tuta- Figure I. — View of Orbits or Eye-sockets, showing their Quadrilateral Conical Form. Figure 11 . — 1, Orbicular Muscle op the Eye -lid. E / . \ .1 ANATOMY OF HUMAN EYE. 67 mina”* of the eye. The pent-house of the eye-hrow above shields from excess of light, or from trickling perspiration; the fringed cur- tains of the lids are ever ready to close over, and mstinctively to protect their charge ; and the ever-flowing stream from the lachrymal gland ceases not to cleanse the surface of the hall from all that might mar or irritate. The sockets of the eyes do not, as many might suppose, look straight forward, hut out- ward ; that is, lines drawn through the axis of each must diverge from a common point pos- terior to the apex of the cones. The optic or sight nerves of the eye, each of which pierces the corresponding socket at the apex, exactly follow this course (Fig. III.), diverging from their commissure or point of junction within the skull. On the other hand, as a glance at the same Diagram will show, the axis line of each eye (Fig. III., 3) is parallel to that of its fellow, but intersects that of its own orbit. The consequence of this arrangement is, that each optic nerve enters the eye-hall rather at one side. Its importance we shall see hereafter. * Protectors. / 68 THE EYE. EXTEENAL APPENDAGES— EYE-LIDS. The general appearance and connections of the eye-lids in man are sufB.ciently well known, and any one Avho will he at the trouble of examining his own for a minute or two before a mirror, may easily verify many of the less apparent particulars here mentioned. The ex- ternal skin of these protective curtains, loose in itself to permit of free motion, derives its support and general form from a thin sheet of elastic but firm cartilage, which lies beneath it in each lid (Pig. IV,, B 1, 2). This cartilage may easily be felt by gently pinching the lid. In the upper it is considerably wider than in the lower, and in both its curvature is exactly adapted to that of the globe. The angles of the eye form, as it were, pivots for the movements of the lids. These are connected and strengthened, at the inner angle, by a strong horizontal tendon, which also affords attachment to the fibres of the orbicular muscle (Pig. II., 1). This muscle, the ofiice of which is to close the eye-lids, surrounds the margin of the orbit. Some of its fibres are situated between the skin and cartilage, more Figure III. — Optic Nerves and Eye-balls. 1, 1, Optic Nerves. 2, The Commissure of the Optic Nerves. 3, Line of Axis of Eyes. © 1 ANATOMY .OP HUMAN EYE. *71 particularly of the upper lid, which, in con- sequence of its greater curvature, is principally concerned and acted upon in closure of the eyes. As all are aware, the act of closing the eyes, or of winking, is involuntary. If violence is olfered, the movement is sudden, almost spasmodic ; hut generally it does not exceed a gentle sweep of the lid, by which the hall is continually kept clear from all impurity, the lachrymal fluid diffused over its surface, and at the same time assisted in its passage through the ducts hy the gentle pressure exerted. Should, as sometimes occurs, this act he in- terrupted, its importance is speedily attested hy the painfully inflamed state of the covering membrane of the eye-hall, which supervenes. CONJUNCTIVAL OR COVERING MEMBRANE OP LIDS AND GLOBE. If either, or both eye-lids he examined in- ternally, they will he found lined with a moist, very vascular membrane, the conjunctiva, which passes or is reflected from them upon the fore part of the eye-hall (Figs. IV. and V.), forming the outer covering of what is usually 72 THE EYE. called “the white.” That portion of the conjunctival ^membrane which lines the lid presents on its surface numerous minute microscopic papillae, which secrete the lubri- cating mucus of the eye, by means of which, whilst perfect apposition is preserved, the lid glides easily over the globe. Between the conjunctival lining and the cartilages, hut partly imbedded in the latter, lies a series of follicular glands, the “ Meibo- mian” (Fig. V., A B). Each of these glands is composed of a number of minute hags, having their mouths directed to one common excretory duct, which has its orifice on the edge of the eye-lid (Fig. V., A 2, C 2). There are about thirty of these glands in the upper lid ; rather fewer in the lower. Their office appears to he the secretion of an unctuous matter, by which all chance adhesions of the edges of the lids may he prevented. The conjunctiva, covering the entire front of the eye — including, in a modified degree, the cornea or transparent glass — is tlironm at the inner angle, into a semilunar fold (Fig. V., C 3), the plica semilunaris. This is the rudiment of, or eorresponds to, the third eye-lid, or nicti- tating membrane of many birds and animals. Figure IV., A,— Inner View of Eye-lids. A 1, Upper Eye-lid. 2, Lower Eye-lid. 3, Lining Membrane. 4, Semi- lunar Fold. 5, Ducts of Lachrymal Gland. 6, Lachrymal Gland. Figure IV., B. — Cautilages of Eye-lids, Lachrymal Sac, and Duct. 1, Upper Cartilage. 2, Lower Cartilage. 3, Lachrymal Sac. 4, Duct. 5, Lachrymal Puncta. 6, Semilunar Fold. 7, Caruncle. 8, Orifices of Meibomian Glands. 9, Lachrymal Gland. 10, Ducts. / ANATOMY OF HUMAN EYE. 75 Filling up the inner angle is the “ lachrymal caruncle” (Fig. V., C 4), a reddish body com- posed of a collection of small glands, similar in structure and function to the Meibomian. LACHRYMAL OR TEAR-CARRYING CANALS. If either eye-lid he slightly everted, there will he perceived at the inner angle a minute perforation (Fig. IV., B 5 ; Fig. V., C 5), which, in the natural position of the lids, is directed inwards. These two perforations are the com- mencement of the lachrymal canals, into which the watery secretion, which is continually flow- ing over the eye, is taken by capillary attrac- tion, conducted into the lachrymal sac (Fig. IV., B 3 ; Fig. V., Cl), and thence by the duct into the nose. Lastly, we have the eye-lashes, beautiful both in appearance and use : these, arranged in triple row on each lid, by cmwing upwards and downwards, avoid all chance of interlacement. Thus it is, from the. construction of the most important vital organ, to the arrangement of the most external adjunct to His creatures’ convenience or happiness, we find the same God of love and wisdom in all, through aU. 76 THE EYE. It lias been remarked that the conjunctiva is reflected from the lids upon the hall. The white of the eye, which owes its appearance to the outer coat of the latter, is, therefore, only seen through the covering membrane, which, further, as it passes over the transparent cornea, becomes still more transparent. The structure of this portion of the membrane will be alluded to in conjunction with the micro- scopic anatomy of the cornea generally. LACHRYMAL OR TEAR GLAND. At the upper and outer corner of the upper eye-lid, a little above the superior border of its cartilage, it is possible for the anatomist to distinguish, and to thrust hairs into, the minute ducts of the lachrymal gland (T'ig. IV., A 5, B 10). This body, which secretes the natural moisture of the eye at all times — the tears occasionally — is situated at the upper and outer angle of the orbit, and is about the size of a filbert (Big. IV., A 6). It discharges its secreted fluid through the small ducts or tubes, eight or ten in number. As all are aware, this fluid is frequently discharged in great abun- dance; continually laving the surfaces of the FiGuiiK y. — Cartilages and their Connections. A. 1, 1, Meibomian Glands on inside of upper and lov^er eye-lids . — 2, 2, Their orifices on the edges of Eye-lids. B. Meibomian Glands., enlarged. C. Eye. — 1, Lachrymal Sac. 2, Meibomian Orifices. 3, Semilunar Fold. 4, Lachrymal Caruncle. 5, Lachrymal Puncta. ANATOMY OF HUMAN EYE, 7^ lids and eye-ball, it assists their natural mucus to facilitate their easy gliding, and washes away whatever foreign matter may accidentally lodge upon or between them; being itself ab- sorbed and carried off into the nose, by the beautifully complete little system of drainage already described (page 75). Eew, perhaps, are aware of the considerable amount of fluid which is thus continually passed over the outer eye ; but any one unfortunate enough to become the subject of obstructed lachrymal ducts or sac, is soon rendered sensible of how much he owes to the integrity of their apparatus, by the copious flow of tears over the cheek, and its consequent excoriation, and by the dryness of the corresponding nostril. Although the lachrymal gland really lies behind, or is covered by the conjunctiva, it is so intimately connected with the outward and visible appendages of the eye — it is so com- pletely one of its “ tutamina,” that it has been classed with the external apparatus. How beautiful the combinations we have just examined ! The lids, with Arm, elastic, light, accurately-curved, and easily -moved car- tilages ; how obedient to the will ; how quickly, without waiting for the will, closing on the 80 THE EYE. / approacli of sudden danger; or how gently, without effort or fatigue, folding over their charge during the hours of sleep ; their edges fringed with those beautiful hairs whose curve is at once one of beauty and of use; and anointed from the elegant arrangement of the Meibomian glands ! How perfectly does the conjunctival membrane cover both lid and hall, bearing upon its surface its own microscopic antifriction apparatus, and becoming trans- parent as glass where light is to he transmitted ; and, lastly, how unfailing the ever-flowing foun- tain to keep all clear and bright by its constant stream, which its own tiny channels are suflB.- cient to carry off, except on those occasions when excess of joy or grief, or of violent emotion, causes the overflowing of that foun- tain to relieve the inward pressure, and its stream to break the hounds of the water- courses ! CHAPTER IV. anatorng of ^uman lEge* INTERNAL APPENDAGES — ELEVATOR MUSCLE OF UPPER EYE-LID — STRAIGHT MUSCLES OF EYE — OBLIQUE MUSCLES — FASCIA VESSELS — NERVES. CHAPTER IV. ANATOMY OF HUMAN EYE. It has already been observed, that the natural position of the human eye-balls in their sockets is looking straight forward; in other Avords, with their axes parallel (Eig. HI-), in contradistinction to the diverging axes of the sockets themselves. Where, as in the com- pound eyes of insects, the surfaces receptive of light and susceptible of vision extend, in many instances, over the larger segment of the sphere, the organ must afford a very AAude range of sight, and be amply sufficient for all the requirements of the creature, even without movement. On the other hand, the angle of sight of the refracting eyes of vertebrated ani- mals being comparatively limited, the exercise of the full utility of the organ requires for them the power of varied direction. It is amply provided for. 84 THE EYE. Seven muscles are contained within the hu- man orbit; of these, six are devoted to the service of the eye itself, and one to the upper eye-lid, of which it is the elevator or opener. This long thin muscle (Pigs. VI., VII. 1) takes its origin from the hone and optic sheath at the hack part or apex of the cone, and, as represented, runs close underneath the arched roof of the orbit, to he attached to the upper lid, its action in opening which must he suthciently obvious. Of the six muscles of the eye-hall, four are straight or “recti” muscles (Pigs. VI. 8, 9, VII. 3, 5, 6, 7) ; two are oblique (Pig. VII. 2, 4). The four straight muscles of the eye all take their origin from the bone and tendi- nous and fibrous tissues around it, at the apex of the conical cavity of the socket, and around the perforation which here exists for the pas- sage of the optic nerve. In Pig. V., which represents a vertical section of the orbit and its contents, the superior and inferior straight muscles are seen running forward, the one under the roof, the other on the floor of the cavity. In Pig. VII., the same pair are also seen, along with the .internal straight muscle — partly hid by the optic nerve — and along with the double origin, and the termination of the Tigure VI. — ^Vertical Section of Orbit and Eye-ball. 1, Upper bone of orbit. 2, Skin, &c., of forebead. 3, Eye-brow. 4, Upper lid. 5, Eye-ball. 6, Lower lid, 7, Elevator muscle of upper lid., 8, Superior straight muscle of eye-ball. 9, Inferior ditto ditto. 10, Optic nerve. 11, Central artery of optic nerve. 12, Fat in which the eye-ball is embedded. Figure YII. — Muscles of Eye-ball. 1, Elevator muscle of upper eye-lid. 2, Superior oblique muscle of eye-ball. 3, Superior straight muscle. 4, Inferior oblique muscle. 5, Inferior straight muscle. 6, Internal straight muscle. 7, 7, Origin and termination of external straight muscle. 8, Optic nerve. 9, Bone at apex of orbit. 10, Anterior white covering of eye-ball. 11, Pulley for the tendon of superior oblique muscle. ANATOMY OF HUMAN EYE. 89 external straight muscle, the body of which has been removed to permit a vieAv of the parts just enumerated. These four muscles bear the same relation to the globe of the eye, each of them being attached to the outer coat at about two lines from the margin of the cornea. The expansion of their tendons used formerly to be described as furnishing an additional covering (tunica albuginea) to the fore part of the organ ; that which, seen through the con- junctiva, constitutes the “ white of the eye.” This idea is now generally given up. A small “bursa” or elastic bag, containing a little fluid, is placed close to the eye-ball, under the attach- ment of each tendon, to prevent undue pres- sure in action. The continued tonic action of these four muscles is calculated to retain the globe in a state of steady retraction within the socket. Their contraction altogether will obviously effect actual retraction; the action of one, singly, must, of course, incline the eye inwards, out- wards, upwards, downwards, according to the muscle acting, whilst the diagonal of these directions must result from the simultaneous action of any contiguous pairs of muscles. The superior oblique muscle (]?ig. VII. 2) 90 THE EYE. presents, at least to the general observer, one of the most striking-ly obvious instanees of contrivance in the human form. Arising deep in the orbit, it passes, as represented, forward to its upper and inner angle, to a loop or pulley of fibrous gristle through which its tendon plays ; its direction being thus com- pletely changed, it passes downwards and back- wards, to he fixed to the globe underneath the superior straight muscle. The inferior oblique is the only muscle within the human orbit which does not take its origin from the apex of the cavity. It is short, arises from the inner portion of the orbital rim, and, passing outwards and back- wards, is fixed to the outer and posterior por- tion of the eye-hall. A moment’s reflection will show how beau- tifully and simply the arrangement of muscular apparatus within the orbit is calculated to originate the various direct motions of the eye, and, at the same time, to provide for those rotatory movements of the sphere upon its axis, which, we shall see hereafter, are so essen- tial for the perfection of vision. The antago- nistic power, too, of these six muscles, must do much towards steadying the eye-ball, both ANATOMY OF HUMAN EYE. 91 when at rest and in movement ; but still fur- ther to ensure steadiness, “ the globe, besides being embedded in fat (Fig. VI. 12), is sus- pended or slung in a capsule of fibrous tissue, with which it is in immediate contact.” Ac- cording to its describer, Mr. O. Ferral, this fascia or membrane extends from the cartilages of the eye-lids, back, over the ball and the optic nerve. It is perforated for the passage of the muscles, which, working through it as through pulleys, are thus prevented, when in action, from pressing injuriously either upon the ball itself, or upon the parts immediately around the nerve. The remaining contents of the orbit, ex- traneous to the globe itself, are the blood- vessels and nerves. The cavity and its contents are mainly supplied with blood by an artery which enters from within the skull close on the outer side of the optic nerve. This large nerve has already been noticed as entering at the apex of the orbit, and continuing its course directly in the orbital axis, till it pierces the inner and posterior side of the globe. “The marked manner in which the optic nerves terminate in the retina, the constant relation in size between them and the organ of 92 The eye. vision, the atrophy or wasting they suflFer when the visual apparatus has been destroyed, the impairment or loss of vision which follows a morbid state of them, place it beyond all question that they are the proper conductors of visual impressions to the sensorium.”* The optic nerve is pierced before it enters the hall, by a branch from the artery of the orbit (Tigs. VI. 2, VIII.), which, enclosed in the centre of the nerve, passes in to supply the retina or nerve lining of the eye with blood. In addition to the nerve of sight itself, three pairs of nerves entirely, and a fourth pair partially, are devoted to supply with nervous influence the contents of the orbit, including the eye-hall, which, deriving no other endow- ment from the optic nerve than the special sense of sight, requires the additional nervous supply for its general sensory, nutritive, and motor fulfilments. Accordingly, a small ganglion (the ciliary) is formed on the outer side of the optic nerve, by the union of a branch from a motor with one from a sensory nerve, and from this union proceed the ciliary nerves of the eye-globe. Todd and Bowman’s Physiology, Part III. ANATOMY OF HUMAN EYE, 93 These ueryes we shall learn more about as we proeeed. Even were all knowledge of that wonderful end absent, to which the perfect arrangements we have been examining are hut the subser- vient means, the most unreflecting mind must feel that, in them, wise adjustment and boun- tiful provision had been made for some good purpose. How well these means subserve that end, it is trusted the following chapters will fully show. CHAPTER V Anatoms of l^uman PORM OP EYE — SCLEROTIC COAT AND CORNEA — OPTIC NERVE STRUCTURE OP CORNEA — AN- TERIOR EPITHELIUM ANTERIOR ELASTIC LAYER LAMELLATED LAYER — POSTERIOR ELASTIC — POSTERIOR EPITHELIUM — CHOROID COAT — VESSELS PIGMENT CILIARY PRO- CESSES RETINA STRUCTURE — JACOB’s MEMBRANE — SENSIBLE SPOT — MACULA LUTEA — HYALOID MEMBRANE AND VITREOUS HU- MOUR — ^CRYSTALLINE LENS — SUSPENSORY LI- GAMENT STRUCTURE — DIVISIONS — PISHES — CILIARY MUSCLE AQUEOUS OR WATER CHAM- BER — IRIS AND PUPIL. 1 CHAPTER V. ANATOMY OF HUMAN EYE — THE GLOBE. The form of the eye-globe is spherical ; its general diameter, averaging about one inch, is increased from behind forwards by the promi- nence of the transparent cornea (Pig. VIII. 1), which occupies about one-fifth of the whole surface of the sphere. The eye-hall is usually described as possessing three principal tunics ; of these, however, the outer can only strictly be considered as an investment, the others being rather laminae of structure directly sub- servient to vision. THE SCLEROTIC, OR OUTER COAT. The sclerotic coat of the eye, as it is termed from its firm hard texture, is composed of in- elastic white fibres, crossing one another at right angles ; thus composed, it is endowed with great capability of maintaining its proper G 98 THE EYE. form — so much so, indeed, that even small pieces eut off retain that original curve of the tunic which is so essential to the perfeetion of vision. It is thickest posteriorly, and becoming thinner towards the fore part of the globe, again thickens slightly near the cornea. THE CORNEA, OR GLASS OF THE EYE. The cornea, or transparent glass of the eye, is generally described as being fitted or bevelled into the selerotic coat, as a watch-glass into its case. In some measure the comparison holds good ; but it is not to be imagined from this that the cornea is capable of being detached from the sclerotic around the bevelling : the two structures are intimately united. Some describe the cornea as a superaddition to fill up a gap, cut, as it were, in the sclerotic; others as the sclerotic itself, altered in struc- ture. Both belong to the fibrous tissues. Posteriorly, to the nasal side of the eye- globes, the sclerotic coat affords passage to the optic nerve, which here contracts considerably in diameter, and does not enter en mmse, but with its fasciculi separated as they pass through a cribriform or sieve-like tissue throuTi across the opening, its outer investing sheath becom- Figure VIII.— Horizontal Section of Left Eye-Ball. 1, Cornea, showing its LaminaB. 2, Sclerotic Coat. 3, Choroid Coat, prolonged over Ciliary Processes and Iris. 4, Retina, ceasing at (15 15) the Ora Serrata. 5, Hyaloid Membrane, inclosing Vitreous Humour and attached (13) to posterior portion of capsule of lens. 6, Ciliary Muscle, attached in front to the Cornea, exter- nally to the Sclerotic, internally to the Choroid, Ciliary Processes, and Iris. 7, Cili- ary Processes. 8, Iris. 9, 9, Posterior and Anterior Aqueous Chamber. 10, Crys- talline Lens inclosed in its Capsule. 11, Suspensory Ligament of Lens commencing at (15) the Ora Serrata. 12, Canal of Petit, formed by Hyaloid Membrane posteriorly. Suspensory Ligament anteriorly, and by rim of Lens. 13, Hyaloid Membrane. 14, Vitreous Humour. 15, Ora Serrata, the termination of the Retina, the commence- ment of the Ciliary Processes, and of the Suspensory Ligament, corresponding also to attachment of Ciliary Muscles. 16, Cribriform Fascia of the Sclerotic through which the Optic Nerve enters the Eye. 17, Optic Nerve with its central artery, ’ Ti- ANATOMY OF HUMAN EYE. 101 ing at the same time continuous with the scle- rotic. “ Though the nerve is moveable on its entrance, it is alv^ays fixed firmly at the inner surface of that aperture, where the retina com- mences.” Eurther, the sclerotic is pierced in various parts, more especially posteriorly, by smaller nerves, and by vessels passing to the interior of the globe. The transparent cornea, looked at superfi- cially, appears as one homogeneous glassy body; examined carefully, and with the aid of the microscope, it is found to he remarkably com- pound — so much so, indeed, that although al- ready examined by many and careful observers, recent researches have elucidated interesting and beautiful points of structure, previously unknovm. The cornea may now he considered to he made up of five different layers of structure (Eigs. XV. A, XVI. A) — the outer, or epi- thelial, the anterior elastic, the laminated, the posterior elastic, and the posterior or aqueous epithelial. The outer epithelial coat may he considered as a continuation of the conjunctiva or membranous eye- covering, which here be- comes remarkable for its assumed transparency. Eigs. XV. A, XVI. A, represent the appear- 102 THE EYE. ance of thin sections of the cornea under high magnifying powers ; and 1, 1, the appearance and structure of the outer and conjunctival epithelium, which consists of three layers of remarkably transparent cells, the outer im- bricated, that is, arranged like tiles on a roof, the inner cylindrical or tapering (Pig. XV. B). The continuity of the conjunctival membrane covering the cornea, with that covering other portions of the eye, and indeed with the skin generally, is demonstrated in the case of snakes, which, in casting their skins, cast with them this outer case of the cornea structure of the eyes. Figs. XV. A 2, XVI. A 2, represent the anterior elastic lamina, or layer, of the cornea, of Mr. Bowman. “ Its thickness in the human eye is from TvVoth to Wo^th of an inch it extends over the whole surface. “ It is a con- tinuous sheet of homogeneous membrane,” .perfectly transparent and glassy, without ap- pearance of internal structure ; “ it is very elastic.” Not the least interesting feature of this layer, is its mode of fixature to the cornea proper, or lamellated cornea, immediately be- neath. This, as represented (Figs. XV. A 3, XVI. A 3), and as described by Mr. Bowman, Figure IX. — Enlarged View of the Connections of the Ciliary Body and Parts adjacent. The Conjunctiva is shown (C 1) covering the Cornea. The Anterior Elastic ^Lamina (C 2) braced down to Middle Lamina, and passing into •dbrous attachment with Sclerotic. The Middle Lamina (C 3) conti- nuous with Sclerotic. The Posterior Elastic (C 4) giving commence- ment to Ciliary Muscle (11) and Fibres to form the Pillars (9) of the Iris. The connections of the Ciliary Muscle shown, anteriorly to Cor- nea, outwardly to Sclerotic, inwardly to Choroid and Ciliary Processes (10), and by these to Suspensory Ligament of Lens, connected also to the Iris. ■I /r. " ■ ( ‘ • '-.T, ■ ■ - ■ UK 7 U" ANATOMY OF HUMAN EYE. 105 is effected by means of fibres passing from its inner surface down into the substance and among the lamellae of the cornea proper ; these fibres cross one another at right angles. At the edge of the sclerotic, the lamina appears to resolve itself entirely into such fibres, which here take a more oblique direction, and passing into the former tissue — the sclerotic — -become continuous with it. The evident purpose of this anterior elastic lamina, is the preservation of the proper degree of curvature of the cornea so necessary for distinct vision. It is impos- sible to conceive a structure more especially adapted, more exquisitely arranged, for its offices than this, till lately unknown, tissue. The cornea proper, lamellated or middle layer of the cornea (I'igs. XV. A 3, XVI. A 3), constitutes the great mass of the structure ; it is tough, transparent, very unyielding, and composed of numerous layers or lamellae, not extending in one sheet throughout, but run- ning one into another as it were. About sixty of these layers may be counted in any single section. The interesting discovery was made by Mr. Bowman, that the interstices between these layers, instead of being simply, as might at first appear, irregular spaces, were entirely 106 THE EYE. filled with a series of regular tubes (Eigs. XVI. B, XVIII. B). Mr. Bowman eonsiders the use of these tubes to he conneeted with the permeation of the thick mass of the cornea, “ by the more fluid portions of the blood which alone have access to it;” no blood-vessels, even of the smallest size, passing beyond a short distance from its margin in the healthy state (Pig. XVII. A). At the point of junction with the sclerotic, the lamellated cornea becomes continuous with that tissue (Pigs. VIII. 3, IX. 3). The change in the disposition of the tissue is represented in Pig. XVIII. A, which shows the sclerotic running into the cornea, the tubular interstices cut across. Behind the cornea proper is placed the pos- terior elastic layer (Pig. XV. A 5), closely resembling the anterior elastic, but differing from it in being much thinner and not braced down to the middle layer by those fibrous attachments which, highly requisite in the for- mer, would here he evidently superfluous, thus affording one other instance of the universal rule evident throughout creation, that its Au- thor, omnipotent in resource, never wastes. The office of this layer is evidently the same as that of the anterior, to assist in preserving the PiGmE X. — Choroid Coat and Arteries. 1, Pupil. 2, Iris, appearing convex from lying upon the Lens. 3, Ciliary Muscle. 4, 6, Ciliary Arteries. 5, Sclerotic Coat turned back. 7, Choroid Coat and its Arteries. ANATOMY OF HUMAN EYE. 109 curvature of the whole ; at its junction with the sclerotic, this lamina resolves itself into fibres, the course and destination of which will he adverted to hereafter. The posterior elastic lamina is lined on its inner side hy the fifth eorneal layer, the thin posterior epithelium (Fig. XV. A 6, E E). It w^ould evidently interfere with distinct vision, did vessels large enough to convey red blood traverse the cornea throughout ; conse- quently we find that, in the healthy state, no vessels of such size pass beyond half a line from its margin, thus avoiding all chance of interference with visual rays entering tlie pu- pil. The vessels which do advance over the margin of the cornea (Eig. XVII. A) are all derived from the conjunctiva and are super- ficial ; others, derived from the deeper or cho- roid lamina, turn hack just before reaching the margin. The cornea is dependent for its nu- trition upon the colourless and more fluid portions of the blood, which probably transude the tube system of Mr. Bowman. This system is all-suflicient in the healthy state. In certain conditions, however, of disease or of reparative action, arteries and veins conveying coloured blood are projected across the transparent me- 110 THE EYE. dium in a wonderfully short space of time, and as speedily withdraw themselves when the eanse of their formation has ceased and their mission been accomplished. How great the variety of structure, how evident the careful design, how exquisite the adaptations in this small portion of our frame, this atom of creation. The natural philosopher employs the sense of sight to explore the height and depth of structure in the very organ through which that sense is conveyed; he sees upon one portion of its components — the tiny structures of the cornea — care the most unwearied, skill the most magnificent, have been lavished without superfluity, and all in keeping, from the least to the greatest. Must he not feel that the care which made, will also sustain in perfection and beauty as Iona: as that Divine Providence deems well ? Will he not raise the mind from the things of earth ? Will he not feel in the exploration of these his Creator’s works, in the examination of the “ fearfully and wonderfully” constructed human frame, that He who made so beautiful a dwelling for the living tenant within, and en- dowed that tenant with the heaven-sent gift, the immortal powers of using that habitation Figure XL — Internal View of Choroid Coat, &c. A — 1, Cornea. 2, Anterior Aqueous Chamber. 3, Iris. 4, Ciliary- Muscle. 5, Ciliary Processes. 6, Sclerotic Coat. 7, Choroid. 8, Optic Nerve. 9, Central Artery. B — 1, Pupil. 2, Posterior Lamina of Iris, usually called the Uvea, showing its converging folds. 3, Ciliary Processes. 4, Choroid. 5, Sclerotic. 6, Ora Serrata. ■ fr ) 'V* ■ ■ 1 ANATOMY OF HUMAN EYE. 113 for his rational, his intellectual, and his spi- ritual weal, is but preparing him — if he will be prepared — ^for that higher state of existence, compared with u'^hich, this, with all its elabo- rate organization and harmonious working, mth our “ perfect vision,” is hut seeing ^‘through a glass darkly.” THE CHOROID, OB COLOUR COAT. If the outer coat of the eye, consisting of the sclerotic and cornea, he now removed or turned aside, the second lamina, usually called the choroid coat, is exposed. It appears exter- nally of a deep chocolate colour, and flocculent ; its pigmentary matter being easily detached, clings to the fingers. Near the optic nerve, this coat adheres to the sclerotic; but except at this point, the connection is extremely slight, only by a sort of web of remarkably fine cellular tissue. Anteriorly, about a line from the margin of the cornea, the choroid appears to he covered exteriorly by a gray, softish-looking structure (Figs. X. 3, XII. 6), the ciliary body or muscle, which, anteriorly, forms a common point of union for the sclerotic and cornea, the ehoroid H 114 THE EYE. and iris. The choroid is very vascular; its blood-vessels pass in and out, through the sclerotic, at various points, hut chiefly poste- riorly (Pig’. XIII. 3). The distribution of the arteries is shown in Pig. X. ; that of the veins is peculiar : their courses, directed to certain common points of union, form beautiful curves over the choroidal surface (Pigs. XII. 4, XIII. 5) ; within the larger vessels, the finely divided capillaries form a vascular network (Pig. XIX. P). The black pigment so characteristic of the choroid coat of the eye is produced by, and contained in, a series of minute cells. These form on the inner surface a distinct layer, are hexagonal in form (Pig. XIX. A D E), and arranged like a tesselated pavement. They are, however, likewise distributed among the blood-vessels and their branches, and then be- come prolonged in various irregular forms (Pig. XIX. B C). A little in front of the optic nerve, the sclerotic is pierced by the nerves which are to supply the iris and surrounding parts ; they run between it and the choroid, and become flattened so as to lie compactly (Pig. XII.) If the inside of the choroid be now ex- Figure XIT. -Choroid Coat. Nerves and Vorticose Veins. 1, Sclerotic turned back. 2, Pupil. 3, Iris. 4, Posterior Veins of Choroid. 5, Ciliary Nerves. 6, Ciliary Muscle. ANATOMY OF HUMAN EYE. 117 amined, it will be found perfectly black (Fig. XI.), and, anteriorly, at a line corresponding to the commencement of the ciliary body, assum- ing a remarkably beautiful appearance (Fig. XI, A 6, B 3), being thrown, as it were into a regular series of triangular folds, the ciliary processes. These commence at a wared line, the “ ora serrata,” at first merely as streaks, but gradually become marked into distinct plications, the black pigment lying in greatest abundance between the folds. The illustration will convey a better idea of the position and appearance of this beautiful ring than any de- scription. The ciliary bodies are extremely vascular ; their points or anterior portion just overlap, without touching, the lens, and pro- ject slightly into the aqueous chamber of the eye, behind the iris, with which they are con- nected (Figs. VIII., IX., XI.) THE RETINA, OR NERVE LINING. The retina, or inner coat of the eye, is the expanded sheet of nervous matter derived from the optic nerve ; it is the essential portion of the visual organ, the mysterious link between material appearances and mental impressions. 118 THE EYE. without which, or in a diseased state of whieh, all other arrangements for vision, however per- fect, must he futile. The eye may seem as lustrous as ever, its depths as clear, hut if the retina or its nerve fail, all is dark, “ and know- ledge from one entrance quite shut out.” This nerve blindness, “ amaurosis ” of physicians, is the “ gutta serena” of the blind Milton. These eyes, though clear To outward view of blemish or of spot, Eereft of light, their seeing have forgot; ^^or to their idle orbs doth sight appear.’^ The retina lines the entire back portion of the eye, terminating anteriorly at the “ ora serrata,” or ciliary commencement (Fig. XIV. B) ; from this point, a non-nervous lamina, marked by the plaitings of the ciliary processes, extends forward to the edge of the iris. In a living, or perfectly fresh eye, the retina is transparent, hut soon after death acquires an opacity somewhat resembling that of ground glass. Its composition is complex. Big. XX. exhibits its various layers of nervous matter, as they appear in a magnified seetion; the special object of these arrangements is not un- derstood. The remarkable outer layer (Big. Figure XIII. — Vorticose Veins of Choroid. 1, Iris. 2, Ciliary Muscle. 3, Portion of Sclerotic, pierced by small Vessels and Nerves. 4, Long .Ciliary Nerve and Vein. 5, Vorticose Veins of Choroid. ANATOMY OF HUMAN EYE. 121 XIX. B 7), frequently described separately as the membrane of Dr. Jacob (Big. XIX. A 2), lies between the colouring pigment of the choroid and the other portions of the nervous sheet. Its structure is, at the same time, sin- gular and mysterious, being made up of innu- merable “rod-like” bodies (Big. XIX. B 8), hooked or clubbed at the extremity ; seen with the naked eye, this membrane looks like the finest film. The inner layers of the retina ap- pear composed of arrangements of nerve, vesi- cles, and tubes, along with — most internally — the extremely fine vascular expansion of its blood-vessels (Big. XIX. B 3). Posteriorly, the retina presents two spots (Big. XIII. Cl 2). The entrance of the optic nerve, with its central artery (2), lies of course to the nasal side of the axis, and is quite insensible as regards vision. Brom this point, both the nervous and vascular expansions may be said to have their origin. The other spot of the retina (Big. XIV. Cl), or “ spot of Soemmerring,” found only in man and some animals which have the axes of the eyes paral- lel, lies exactly in the centre of vision. It is of a yellow colour, and about ^Vth of an inch in diameter; it is the only part of the retina on 122 THE EYE. . / which distinct vision can exist, hut its exact use is unknown. The interestingly complex structure of this little spot has been described from a minute examination of the human eye very shortly after death — a fact of great con- sequence — by Dr.Gruhe, of Konigsherg. “With the naked eye, it was at once easily seen that the macula lutea — yellow spot — was not a little raised above the surface of the retina. On placing it and the part around it under a microscope, magnifying 300 times, and com- pressing it hut slightly, the macula lutea pre- sented exactly the appearance of shagreen; elegant rounded particles gradually tapering towards the middle, which were smaller the nearer they were to the centre, and there not more than one-fourth or one-fifth of the size of the medullary corpuscles on the surface of the rest of the retina, were arranged close together and with great regularity, like rays passing from the centre to the circumference of the spot,”* in the form of a star. It is remarkable that the gray fibrous layer of the retina imme- diately in connection with the optic nerve, and apparently a continuation and modification of Microscopic Journal, 1841. Figure XIV. — IIetina. A, Exterior View of Eetina and its Arteries — 1, The Grooves of the Ciliary Processes. B, Interior View of Eetina, anteriorly — 1, Iris. 2, Ciliary Folds. 3, Ora Serrata, where the Eetina terminates. 4, Eetina. 5, Choroid, 6, Sclerotic. C, Interior View of Eetina, posteriorly — 1, Macula Lutea, Yellow Spot or Foramen of Soemmerring. 2, Insensible Spot of Eetina, the entrance of the Optic Nerve and Artery of the Eetina. ANATOMY OP HUMAN EYE, 125 its nerve tubules, is the only nervous element present at the insensible spot of the retina. It may he justly concluded from this, that the vesicular layers are necessary for the sensation of sight. “The incapacity of vision at the entrance of the optic nerve seems to he essen- tial to the mode of junction of the retina with the nerve, since it appears to he the chief rea- son why the nerve was not made to enter in the axis of the eye.” Had it done so, of course vision would have been completely spoiled. HYALOID MEMBEANE, AND VITEEOUS OE GLASS-CLEAE HUMOUE. The inner surface of the retina is covered by the transparent membrane, the Hyaloid, the only separation being a layer of translucent cells. This membrane contains within it the vitreous humour of the eye, that exquisitely transparent, jelly-like substance which makes up about four-fifths of the contents of the hall. The structure of the vitreous humour is still undecided ; being composed for the most part of water, the fluid, according to some, is kept in situ, by laminae connected with the hyaloid membrane, and radiating from the centre, like 126 THE EYE. the divisions of an orange ; by others the layers are said to he concentric. IVhatever the structure, the effect is a comparatively firm, most transparent substance, in the fore part of which lies imbedded the crystalline lens. THE CRYSTALLINE LENS. The crystalline lens, of firmer, denser con- sistence than the vitreous humour, appears perfectly transparent. It is situated just behind the pupil, and is imbedded in the anterior part of the vitreous, which slightly overlaps its rim. It is closely enveloped in a transparent, elastic capsule, which is thickest anteriorly. In ad- dition to being imbedded in the vitreous, the lens is sustained in position by its own suspen- sory ligament (l^igs. VIII. 11, IX.), described by Mr. Bowman as commencing at the ora serrata, lying underneath the ciliary body, between it and the hyaloid, and leaving the former in front, the latter behind, passing to its attachment on the fore part of the lens, a little below its rim. The same author repre- sents the hyaloid membrane as passing under- neath the ciliary body, between it and the vitreous, and turning hack as represented (Tigs. Figure XY.— Cornea, A, Appearance of Magnified Section of Cornea — 1, Conjunctival Epi- thelium. 2, Anterior Elastic Lamina, with, 3, its Fibres passing into 4, Middle or Laminated Cornea, or Cornea proper. 5, Posterior Elastic Lamina. 6, Posterior Epithelium. E, The same Posterior Epithelium, magnified in section. F, in front. B, Cells of Section of Anterior or Conjunctival Epithelium magnified. C, the same in front. D, Elastic Fibres of Posterior Elastic Lamina. ANATOMY OF HUMAN EYE. 129 VIII., IX.), to 1)6 affixed to the back of the lens capsule. The elfect of this arrangement is the formation of a small canal — that of Petit— all round the rim of the lens. The consequence of this, as regards vision, will he considered in a future page. The crystal lens, though apparently homo- geneous in structure, is, like the cornea, when minutely examined, found to he most heautiful, most elaborate. Its shape is oval, a double con- vex lens ; its width about one-third of an inch’ its thickness about one-sixth (Pig. XXIII. E) ; its sides unequally curved, the posterior being the more convex. The substance of the lens is comparatively soft towards the outside, but gradually increases in density and firmness as the centre is approached. Between the body of the lens and its capsule a layer of trans- parent cells is interposed (Pig. XXIII. A). The body of the lens is capable of being separated into layers or lamellae, like the coats of an onion (Pigs. XXII. C, XXIII. P); these layers are, however, limited by certain deter- minate lines, which radiate from the centre. The primary number of these lines seems to be three (Pig. XXII. B). They occur on both sides of the lens ; those, however, on the pos- I 130 THE eye ..i._ terior surface, instead of being opposite to those on the anterior, lie between them, as it were (Fig. XXII. B) ; thus, if the lines 3 were prolonged, they would reach the circum- ference fit 1, the corresponding lines on the other side would come upon it at 2. The di- rection of these primary lines in the lens would seem to differ in different animals; in man, though the primary division of three is recog- nisable, they quickly branch out (Fig. XXII. A), and thus render the other arrangements more complicated. Moreover, the laminae of the body of the lens are beautifully constructed of flat, toothed fibres (Fig. XXIII. C G), laid side to side (Fig. XXIII. B). This arrangement is remarkably evident in the lens of the cod-fish, in which it was first discovered (Fig. XXIII. B C). It is not so strongly marked in man, but is sufiiciently distinguishable. The toothing is deepest on the superficies, and becomes less as the centre of the lens is approached. These toothed fibres are all arranged with reference to the primary divisions radiating from them (Fig. XXII. B). The necessity, the object, for this elaborate structure is beyond our ken ; we only know that it is, and we can feel that it does not exist in vain. Man’s words can Figure XVI. — Cornea. A, Highly Magnified View of Section of Cornea — 1, Anterior Epithe- lium Cells of Cornea. 2, Anterior Elastic Lamina. 3, Laminated Cornea and Fibres from interior Elastic Lamina. B, Cornea Tubes of Mr. Bowman. ANATOMY or HUMAN EYE. 133 add but little to heighten the reverential admi- ration with which the mind contemplates all that is cared for in the perfect construction of this little body, which, to the superficial ob- server, presents no more trace of structure than a fragment of common glass. CILIARY BODY OR MUSCLE. It has been observed that the choroid coat of the eye appears, externally, about a line from the sclerotic and corneal junction, to lose its black colour, and to become of a gray, semi- transparent tint ; this is the result of its be- coming covered by the ciliary body or muscle. Deriving fibres from, and attached to the pos- terior elastic lamina of the cornea, this body — now without doubt proved to be muscular — extends backwards, between the sclerotic and the choroid and ciliary processes, till it gradu- ally thins off to its termination, at a line cor- responding to the ora serrata (Pigs. VIII. 6, IX.) Thus we have the keystone of the eye, as it may be called, connected anteriorly with the cornea, outvmrdly with the sclerotic, inwardly with the ciliary bodies, aud their lining choroidal epithelium, and by means of 134 THE EYE. them with the vitreous humour, its hyaloid containing membrane, and, necessarily, with the suspensory ligament of the lens ; lastly, with the iris. These manifold connections must give the ciliary muscle most important influence over certain of the finer adjustments of the optical apparatus. AQUEOUS OR WATER CHAMBER. Between the fore part of the lens and the posterior layer of the cornea is the chamber of the aqueous humour, containing, as its name implies, a few grains’ weight of colourless fluid, little more than water. This fluid, in con- junction with the cornea, in the first place, exerts a powerful refractive effect upon all rays which penetrate the latter ; and, in the second, forms a fit medium in which the iris, which divides this chamber into anterior and poste- rior, may float and easily move (Bigs. VIII. 9 9, IX. A A). THE IRIS. The iris, the most inimitable of optical con- trivances, is named from its appearance and Figure *XYII. — Corxea. B 2 3 A, Magnified Yiew of Portion of Cornea — 1, Cornea ; 2 2, Superficial Yessels from Conjunctiva, advancing a short way over its margin. 3 3, Deeper Yessels of Sclerotic, turning back before reaching margin of Cornea., B, Front Yiew of Conjunctival Epithelium magnified. ANATOMY OF HUMAN EYE. 137 varied hue. Its form and colours are familiar to most, or may soon become so by ten minutes’ study before a mirror. Suspended like a cur- tain, and floating in the aqueous fluid, it forms the coloured moveable background to the cor- nea ; varying in size and appearance according as it contracts or enlarges the pupillary aper- ture (Pig. XXI. B 1), which is formed in it, a little to the nasal side of its exact centre. The iris may be regarded as a prolongation or pro- ceeding from the choroid, which not only sup- plies its vascularity, but is at the same time continued over it posteriorly from the ciliary bodies. The posterior coloured layer of the iris is frequently named the uvea; and Dr. Jacob describes this as covered, and the colour- ing pigment separated from the aqueous fluid by an extremely fine membrane, also continued from the ciliary bodies. The remaining por- tions of iris structures are peculiar to itself. The circular iris has its outside or peripheral attachment to the fore part of that common bond of union, the ciliary body (Pigs. VIII., IX., XI., XXI. B). Posteriorly, it has the epithelial connection with the choroid, already mentioned; anteriorly, with fibres, the pillars of the iris, which proceed from the posterior 138 THE EYE. lamina of the cornea, and curve round the rim of the aqueous chamber (Pigs. VIII. and IX. 9). The posterior layer of the iris has the ap- pearance, when the pupil is contracted, of being thrown into folds, converging to the edge of the pupillary aperture. These converg- ing folds (Pig. XI. B 2) have been considered* as a provision for allowing a more abundant deposit of pigment. The anterior surface of the iris beitig coloured and mottled, obscures in a great measure the disposition of its mus- cular fibres. Prom the periphery, or outer margin of the iris, a number of slightly ele- vated lines converge towards the pupillary aperture, and, coalescing, form a peculiar knotted-like ring, about one-tenth of an inch from its margin (Pig. XXI. B 2). Between this ring and the edge of the aperture are a number of converging striae, which communi- cate with each other. In addition to the con- verging fibres, others, lying in front of them, but circular, or encircling the pupil, are also to be detected. Observers ascribe to these the power of contracting the pupil, as to the Dr. Jacob- Figure XVIII. — Cornea. A, Magnified View of Continuation of Sclerotic Fibrous Tissue into Cornea proper. Vertical Section, 1, Sclerotic. 2, Cornea. B, Corneal Tubes of the Ox. C, Elastic Fibres, commencing in Posterior Elastic Lamina at 1, running to Iris, 2. ANATOMY OF HUMAN EYE. 141 former that of dilating it ; hy some, however, the contraction is partly attributed to turges- cenee of the vessels, caused and maintained by contraction of the muscular fibres of the iris itself, or of the ciliary muscle. The iris is vascular, and is supplied by the ciliary arteries (Fig. X.), which form a kind of branched ring around it. The nerves of the iris (Fig. XII.) are abundant ; piercing the sclerotic, as already described, they become flattened as they pass between it and the cho- roid, to be distributed to the iris and ciliary body. The most remarkable and wonderful attri- bute of this beautiful structure is its immense power of contraction and dilatation. Under some circumstances, and in some conditions of the system, as exposure to strong light, or narcotism from opium, the piipil is diminished to an aperture scarcely larger than the head of a pin, by the spreading of the whole curtain over its dark expanse; under other circum- stances, more especially under the influence of certain drugs, as belladonna or henbane, the iris curtain is drawn up, almost to disappearing, and the full size of the black-looking pupil displayed. The variations of the pupil are 142 THE EYE. from one-third to one-twentieth of an inch in diameter. Commencing with the sclerotic coat of the eye, we have seen the adaptation of its in- elastic, firm tissue to he the investing, form- preserving, and protecting tunic of the delicate structures mthin. The cornea, with its various layers, all telling of wise purpose, has been examined; it yet remains for us to consider how its combination with the other transpa- rent, refracting media, the aqueous, the crys- talline, the vitreous, constitutes a beautiful op- tical instrument, of which the aberrations are corrected by the moveable iris, the adjustments regulated by the ciliary structures, and the superfluous light absorbed by the dark lining of the choroid. Thus far we can go — we can follow the rays of light till, directed in obedi- ence to known laws, they image upon, in, the complex substance of the nervous sheet of the retina the things of the external world ; hut here we stop. Our microscopes can reveal the complex and curious organization of this nervous expansion, hut they cannot carry our mortal vision over the great gulf which lies between the things of matter and the existences of spirit. We know that we see ; hut how we. Figure XIX. — Pigment Cells of Choroid. A, Hexagonal Cells of Choroidal Epithelium. I), the same, more highly magnified. B C, Irregular Pigment Cells, lying among Vessels of Choroid. E, Arrnagement of Pigment Cells on the Coat of a Vein in Eye of Ox. F, Capillary Vessels of Choroid. ANATOMY or HUMAN EYE. 145 our real selves, our conscious spirits, look through the windows of the soul upon the face of creation, the microcosm upon the macro- cosm, we cannot tell. How much is contained within that little “ one-inch” globe ! How much that tells of wisdom, infinite to design ; of power, omnipotent to execute ; of love, un- bounded to bestow ! K Figure XX. — Retina. A 1, Retina. 2, Jacob’s Membrane, partly detached. B, Magnified Section of Retina and Hyaloid Membrane — 1, Hyaloid Membrane. 2, Layer of Transparent Cells between it and Retina. 2', Single Cell. 3, Vascular and Fibrous Lamina of Retina. 4, 5, 6, Granular and Vesicular Layers. 7, Jacob’s Membrane. 7', as seen in front. 8, Detached Rods of Jacob’s Membrane. Figure XXL — Iris. A, Corona Ciliaris. B 1, Pupil. 2, Knotted Circle running round Iris. 3, Iris. 4, Ciliary Muscle or Ligament. Figure XXII.— Crystalline Lens. A, The Primary Divisions, branching in Human Lens. B, Lens of Sheep ; three Primary Divisions of Kadiating Fibres. C, Division of Lens into Laminse. D, Lens and Capsule enlarged. Figure XXIII.— Structure oe Lens. A, Cells between Capsule and Body of Lens, magnified. B, Adapta- tion of Toothed Fibres of Lens. C, Deeply-toothed Fibres from Lens of Cod-fish. D, Fibres of Lens with merely Sinuous edges. E, Human Lens. F, Human Lens separated into Laminae. G, Toothed Fibres from Lens of Ox. CIIAPTEE VI. of penman 'iTtisitott* ESSENTIALS OP AN ORGAN OP VISION — COURSE OP RAYS ENTERING EYE — CORNEA AND AQUEOUS — LENS — VITREOUS — RETINA — CHOROID — IRIS, ITS ACTIONS ; PERPECTION AND BEAUTY — LENS — REPRACTING POWERS — VITREOUS — CHOROID— ABSORPTION OP LIGHT — VISION — POCAL CONVERGENCE — ADAPTATION — DIS- TANT VISION — MYOPIA — NEAR VISION — PRES- BYOPIA — RANGE OP VISION — VISION WITH TWO EYES OPTIC COMMISSURE — DOUBLE VISION — PERSPECTIVE DISCRIMINATION — AP- PRECIATION OP DISTANCE — CHROMATIC COR- RECTION REPLECTIONS ON EVIDENCE OP CREATIVE POWER AND WISDOM. CHAPTEE VI. ON THE PHYSIOLOGY OF HUMAN VISION. The essential component of an organ of vision is the termination, more or less enlarged or expanded, of an optic nerve, specially endowed with the faculty of conveying to the sehtient creature the sensation of sight. Along with the nerve two supplementary additions are perhaps invariably present, a transparent cover- ing or cornea, partly intended for protection and partly for refraction, and a colouring matter, generally black, for the absorption of superfluous light. In the human eye these component parts occur (Diag. IX.) — a nervous expansion, lining a dark chamber, glazed an- teriorly by the cornea. This simple arrange- ment would, however, be insufficient to admit of the clear and accurate vision requisite for man; accordingly, those exquisite adaptations 158 THE EYE. of structure which have already engaged our attention are associated with it. It remains to consider how that structure and adaptation conduce to the one great end of perfect sight. Rays proceeding from external objects, and entering the human eye, have to traverse four, or, considering the cornea and aqueous as one, three transparent media, before reaching the retina. The cornea and aqueous, the crystalline lens, the vitreous humour (Diags. IX. and X.) As these media are all of diiferent density, and present curved surfaces to the impinging rays, they must necessarily exert considerable refrac- tive power upon their course and direction. A certain proportion of the luminous rays which impinge upon the cornea are reflected hack ; the greater number, however, enter the eye, and between the curved surface of the transparent covering and the lens, traversing in the interval the fluid of the aqueous cham- ber, suffer their first and greatest refraction (Diags. IX. and X.) By this they are made to converge considerably towards the axis of the eye-glohe. Beaching the anterior convex sur- face of the denser lens, this . convergence is increased by the additional refraction. Lastly, emerging from the lens into the comparatively Diagram IX.— Optical Apparatus of Human Eye. A A, Cornea and Aqueous ; first Refracting Media. B, Lens ; second Refracting Medium. C, Vitreous ; third Refracting Medium. D, Posi- tion of Receptive Surfaces of Retina and Choroid. E, Diaphragm formed by Iris ; interior lined black, Ray reflected from Iris. PHYSIOLOGY OP HUMAN VISION. 161 rarer vitreous, the refraction, now from the perpendicular, continues the inclination in the assumed direction, and preserves it, until the rays are absorbed by the black pigment of the choroid, after traversing the transparent retina. In accordance with the optical laws which re- gulate refraction, and as a glance at Diag. X. will show, the necessary effect of these altera- tions in the course of the rays is to form in- verted images of external objects upon the retina. ^ The considerable refraction’which rays un- dergo on entering the cornea and aqueous, tends to converge them strongly throi|gh the pupillary aperture, the amount admitted de- pending, of course, upon its state of contrac- tion or dilatation, according to which more or fewer are intercepted, absorbed, or reflected, by the curtain of the iris. There can be no question that one princi- pal object of this beautifully contrived “ dia- phragm ” is to counteract the confusion which otherwise must result from spherical aberra- tion, in consequence of rays entering the eye through the marginal portions of the crystal lens, whilst at the same time, in conjunction / with, and assisted by, the eyelids and eyebrows, L 162 THE EYE. it regulates the amount of light admitted to act upon the delicate retina. In ordinary daylight, whilst objects at a moderate distance are viewed, the iris main- tains the pupil in a state of moderate contrac- tion, sufficient light being admitted to render the images of objects, formed by the central rays, perfectly distinct. In obscure light, on the other hand, retraction or drawing up, as it were, of the iris, and consequent enlargement of the pupillary aperture, is requisite, that such additional light may he introduced into the interior of the eye as will compensate for its diminished intensity, even at the expense of some distinctness of definition by the admis- sion of marginal rays. The effect of this re- gulating power of the iris is often practically exemplified in the case of those who are be- coming the subjects of cataract, and in whom the opaque spot, formed in the centre of the lens, often completely obscures vision, as long as the pupil is contracted under the influence of bright daylight, but are enabled to see Avith tolerable distinctness when, in the dusk of the evening, it expands sufficiently to admit rays t through the clear marginal portions of the lens. PHYSIOLOGY OP HUMAN VISION. 163 Some have imagined that the rapidly sen- sitive movement of the iris must result from the action of light directly upon its own sur- face, and not, as it assuredly does, from sym- pathy with the retina. The error of this opinion has not only been shown hy experi- ment, but is evident from the fact that when, either in consequence of disease or accident, the retina of one eye has become insensible to luminous impressions, light directed upon that of the other will excite action in both irides at the same moment, notwithstanding that of the injured eye had previously remained immove- able under the influence of strong light im- pinging directly upon itself. Doubtless, then, the movement of the iris is regulated by, and takes place in obedience to, effects exerted by luminous rays upon the retina expansion of the optic nerve ; its message of sensation con- veyed to the brain, being there instantaneously converted into one of motion, and telegraphed back through its own peculiar nerve, testifies the fact of its delivery by the alteration of the pupil. Thus, the iris of the injured eye par- taking of the influence exerted upon the retina of the sound one, sufficiently evidences the community and interchange of influences 164 THE EYE. which must take place at the central station, as it may be called, in the brain. Looking at the objects fulfilled by the iris — its perfect adaptation to the ends for which it has been created, as it floats free in the aqueous humour, ever ready, like a watchful sentinel, to guard, or as a faithful attendant to serve, its charge within— it is impossible to imagine a structure more fitted for the purposes in view. None, perhaps, in the vast range of the world’s creation, exhibits more strongly, in its simple perfection, the wide gulf which inter- venes between the living creations of the Almighty and the dead imitations of man. Combining both use and beauty, the rainbow curtain of the “ sun-bright eye” confers much of their expression upon those organs whose “ heavenly rhetoric” will sometimes persuade u'here the tongue would fail — -which, by their “bend,” speaking the silent superiority of mind over mind, compel the obedience of the irresolute to the stronger will, or “awe” the fiercest animal, till it shrinks away from the look of man, and tacitly yields him the do- minion assigned him by his Maker. PHYSIOLOGY OF HUMAN VISION. 165 LENS. All rays which penetrate the pupillary aperture necessarily impinge upon the lens (Diags. IX. and X.), and in doing so again become refracted. Central rays, and those falling near the centre, even though obliquely, do not of course undergo the same change of direc- tion as those further removed. What is the object — and it is certain there is one — attained by the elaborate workmanship displayed in the structure of the crystal lens, is not at present fully known. It is, however, easy to understand why it should he denser in the centre than at the margin; the provision against spherical aberration furnished by the iris diaphragm is, by this, further assisted, and the increased re- fraetion of the circumference being in a mea- sure compensated for by the diminished density of structure, the w^hole of the transmitted ravs are more nearly collected into one foeal point. Passing from the lens. into the vitreous, the rarer body, the rays, again suffering a slight refraction as they traverse the latter, enter the retina, their final destination,, ere they are absorbed by the dark choroid behind. 166 THE eye: Rays, of course, form their image directly in the axis of the eye, and exactly upon the yellow central spot of the retina — we only know the fact (Diags. IX. and X). The image, moreover, is inverted, and must he curved, in accommodation to the curve of the eye itself, being hy this, at the same time, rendered more distinct. How it is that one image, formed in- verted upon the retina of the bodily eye, is seen by the mental eye erect, we cannot say. Many learned and ingenious explanations have been attempted. The fact is not more wonder- ful than that we see at all, and probably its explanation is equally hid from human ken. Without some special provision for their removal, the rays of light which enter the eye to form the sight-picture, must inevitably he refieeted hack in various directions within the globe, with the effect of occasioning great visual confusion. This special provision we find in the dark light-absorbing choroid, which lines the whole interior, even over the hack of the iris to the verge of the pupil. Diagram X, Refraction of Rays entering Human Eye, and forming a Reversed Image. 168 THE EYE. VISION. Distinct and perfect vision of any ordinary object requires it to be so placed before the eye, as that the rays from it being converged by the refracting media of the organ, may come to a focus exactly in the retina. ABO (Diag. X.) represent three points of an object, from which rays proceed to enter the eye. The central ray of B suffers, of course, no refrac- tion, that of A and C hut slight ; these cen- tral rays, however, determine the line of direc- tion to which the more diverging rays from the same points, which pass through the cir- cumference, are deflected. It is evident, that if the ohjeet viewed be placed exactly at the proper focus suited to the particular eye, that is, so that the refracted marginal rays shall interseet their perpendiculars at the same points, mutually and relatively, the result will he a perfectly defined image of the ohjeet viewed, formed in the retina. It is no less evident, that should the eye remain in statu quo, and the image he moved to any extent in a straight line, either nearer to it or further away, perfect definition must he lost : in the PHYSIOLOGY OF HUMAN VISION. 169 former case, the focal mtersection would he thrown to a point behind or beyond the retina (Diag. IV.); in the latter, it must take place in front of it. In either.' case, the image being indistinct, “ on the supposition that the eye admitted of no adjustment to distance, there could he only one distance at which objects could he seen perfectly.” Every individual has his own focal distance at which he can see more clearly than at any other. The general average may be reckoned at ten inches. As all know, however, the eye can be adapted to view, perfectly, objects much further removed ; we can raise the eyes from a book to the distant landscape, and, almost instantaneously, from seeing the letters or engraved lines of the one distinctly, view the distant objects of the other with perfect clear- ness. The nature of the power of adaptation, thus evinced, has long exercised the thoughts and investigations of physiologists. It would here be out of place to enumerate all the ex- planations offered; suflB.ee it, that the most probable is that which ascribes the adaptation to the muscular power of the ciliary body, by which, when near objects are to be viewed, the lens is slightly advanced towards the cornea ; the 170 THE EYE. latter, probably being rendered somewhat more convex by the same power (acting through the attachment to the posterior elastic layer), not only converges the rays more quickly, but also must slightly increase the length of the eye’s axis. A glance at Tigs. ^ VIII. and IX. will show how, as the suspensory ligament of the lens adheres to the ciliary processes, these being advanced by the ciliary muscle, the liga- ment and lens must necessarily be also ad- vanced to the cornea. Were it not for the canal of Petit, the strain would be exerted upon the whole vitreous body, instead of, as it is, upon the lens only. When distant objects are looked at, the relaxation of the muscular power permit- ting the lens to be retracted by the natural elasticity of the parts, and the cornea to be- come flatter, refraction will be diminished. If, when an object is placed in the exact focus of the eye, light be sufficiently abundant to permit such contraction of pupil as will ex- clude all possibility of confusion from spherical aberration, vision will be perfect and accurate ; nevertheless, large objects at a distance may be seen with perfect clearness, the pupil being dilated, and even if the amount of light be less. In the former, vision will embrace the PHYSIOLOGY OP HUMAN VISION. 171 distinct definition of detail in the object viewed; in the latter, only the more marked outline. Between the two there must he every gradation. At the extremes, however, the size of the pupil is markedly diflFerent, and is evi- dently conneeted with visual adaptation of the eye. This adaptation of the eye in man is for the most part involuntary; some individuals, however, acquire the power of exercising it at will,* and in these it is said the adaptation is accompanied hy voluntary contraction or dila- tation of the pupil. “It is probable, how- ever, that we may have pretty distinct vision, when the foci of the ‘pencils’ of rays are at some distance beyond or before the retina, and that the larger the object the greater the lati- tude of aberration, before we are sensible of any indistinctness.” Although most eyes may he enabled to view distant objects with tolerable clearness, the range of distinct vision nearer than ten inches is very limited; at half the distance confusion of outline occurs, and nearer still, a minute object becopies invisible. The focal * Dr. Eoget lias stated, in his “ Physiology,” his possession of tliis power. 172 THE EYE. distance for distinct vision, of course, varies somewhat in different persons ; when, however, it is much diminished, the individual is con- sidered to have “ short sight,” a condition of eye depending partly on undue convexity of cornea, by which the rays are converged so rapidly, that the image is formed anterior to, instead of in, the retina. The defect is recti- fied, naturally, hy diminishing the distance between the eye and the object of sight, hut as this is neither convenient nor possible at all times, artificial correction, by glasses which slightly disperse the rays before they reach the eye, is more generally resorted to. Short sight may, however, in some measure he due to want of adapting power in the eye, as well as to convex cornea; this is more probable, from the fact that children’s eyes are more convex than those of adults, and yet vision is perfect. In the eyes of aged persons, the opposite condition to the above more generally prevails ; the cornea becomes comparatively flat, its re- fractive power diminished^ and the image is thrown posterior to the retina. Convex glasses, which assist the convergence of the rays, are, of course, the appropriate remedy. PHYSIOLOGY OP HUMAN VISION. 173 If observation be directed to diiferent objects around, the eye naturally inclines towards them, moved by its own muscular apparatus, so as to throw the image upon the central portion of the retina, where it will be most clearly and distinctly defined. The one point of sight is distinct, other objects are seen, but it isundefinedly; being thrown out of the visual axis, they are formed upon the less sensitive portion of the nervous sheet. The range of vision conferred upon the eye hy the action of its own muscular appendages has been calculated at fifty-five degrees, on an average, in every direction, more extensive outwardly than inwardly ; it is, of course, in- creased by the use of hoth eyes. It is matter of experience with all, that whilst the body is conveyed rapidly along, as in a carriage, and the eye kept fixed in one position in the socket, various objects appear to pass by in more or less confused sequence according to the rapidity of the motion ; hut that if any one object attract the attention particularly, it is possible to “follow” it with the eye. Further, that the same may be done in movement of the head simply. In the first place, the confused succession is due to a 174 THE EYE; mi m her of different images being formed con- secutively upon the retina, and, if with great rapidity, one impression not having subsided before another succeeds it ; the more vivid im- pression, moreover, frequently obseuring the weaker one, or even annihilating it altogether. The counteraction to this effect, hy which the eye is made to follow the moving object, and to keep it fixed on one spot of the retina, is ef- fected by the aetion of museles of the eyeball, more partieularly hy that of the oblique. The provision is one of the most important as regards vision, and is constantly being called into action during the various changes of po- sition our bodies are momentarily subject to in waking life. Hitherto, attention has been given to vision with one eye only. When that with both visual organs comes to be considered, the ques- tion presents itself, how is it we see only one image when two must be formed, one in each eye ? This question is probably involved in that respeeting the mysterious connection which links the material retina with the sen- tient mind; it affords, however, considerable scope for reasoning and hypothesis, founded upon the peculiar arrangements of the optic PHYSIOLOGY OP HUMAN VISION. 175 nerves, more particularly at their commissure or partial junction within the skull. At this point (Fig. III.) the principal and central mass of fibres from each side of the brain crosses over to the opposite side and eye, com- paratively few of the outer fibres of the nerve being continued on to the eye of their own side. In consequence of this arrangement, the retina of the inner side of one eye, that of the outer side of the other, derives its nerve tissue from the same side of the brain ; and as images formed eccentrically in the retina must neces- sarily fall on the outer side of one retina, on the inner side of the other, we recognise a beautiful provision for conveying to the same nervous root, or sentient origin in the brain, the double impression blended into one whilst traversing the common conductor. The efiect produced by images of external objects being formed in non-corresponding points of retina is easily shown by pressing one eyeball gently, so as to throw its axis out of correspondence with that of the other ; objects are seen double. Internal causes which disturb the consensual action of the visual organs have a similar effect. The double vision of intoxication is proverbial ; the poet represents Orestes in his 176 THE EYE. anger as “ seeing a double Thebes, and two suns blazing in the firmament.” Mr. Wheatstone has shown that the pro- vision for eombining two material images into one sensation gives also the power of perspec- tive perception. When objects are viewed at such a distance that the eye-axes are parallel, the power is not exercised ; but when a solid object, such as a cube, is looked at near, in a certain position, it requires the use of both eyes to see that it is solid. Eor instance, a square box, placed at a little distance in front of one eye, the other being closed, will look like a flat surface until the other eye is opened, when it at once appears in relief. This fact renders it impossible, in drawing, to represent an object — such as a house — to appear otherwise than as a flat surface when the view is taken from a short distance in front. A person with only one eye remedies the loss of perspective discrimination by the muscular movements and change of position of the head, and probably also by experience and mental association. Our appreciation of distance in vision is evidently the result of education and experi- ence. The infant will frequently grasp at objects far removed from reach, and adults PHYSIOLOGY OP HUMAN VISION. 177 who have had their sight opened by operation, after being blind from birth, require some time before they learn the relative distances between themselves and external objects ; and even those whose sight is usually perfect are apt greatly to miscalculate distance, where, as at sea, or on extensive plains, there are no inter- mediate objects to assist the discrimination. Lastly, there has to he overcome in the eye the tendency to “chromatic aberration,” found in all dioptric media. Some observers maintain that this is not entirely corrected in the eye, and that the images formed in it are actually surrounded by coloured fringes, hut to so small an extent as not to be observable. However this may he, there certainly exists no inconve- nience from it in the exercise of the sense of \ sight ; and it is probably nearly, if not entirely corrected by the combinations of different lenses contained ivithin the eye. Valee considers that the vitreous humour “ is composed of layers of different density, and that the rays of different colours are thus brought into one line.”* AVe have reviewed the requisite optical laws, have examined with sufficient minuteness the Brit, and Foreign Medico- Chirwrg., vol. xi., p. 250. M 178 THE EYE. arrangements and structure, general and micro- scopic, of the human eye, and have considered how these arrangements and structures, exactly adapted to the requirements of optical pheno- mena, conduce to the formation of the perfect vision we enjoy. One conviction forces itself upon the mind — a conviction that only the most determined opposition to all argument, the most wilful hlinding of the eye of rational intelligence can shut out — that if there is a God who made the sun to shine, who said that light should he, that He made also the eye. Upon such as resist the evidence — the revela- tion written by the finger of the Almighty, in his handiwork — argument is wasted; with such it is indeed arare littus ; the thankless t . . sandy soil yields no return, and the first wave of pride obliterates the loose half-formed furrow which unwilling conviction had drawn. In vain do we point to the laws of general refrac- tion, and to their fulfilment in the focal refrac- tion of the eye ; to the iris, the regulator of light and corrector of aberration ; to the dense centre and less dense margins of the lens arranged for a similar purpose ; to the absorb- ing power of the clioroid, and to the delicate expansion of the retina, witli its various layers ; PHYSIOLOGY OP HUMAN VISION. 179 to the focal adaptation to distance, and to the correction of chromatic aberration ; to the con- sensual action of both sight-globes ; all is nought. Let those who doubt, ponder upon the provision for acting, and also for prevent- ing ; look, and look again — examine, think, and then ask themselves what improvement they can suggest in this one little work of creative power and wisdom : will not the answer of every rightly constituted mind he — None would attempt it hut “ the fool,” who “hath said in his heart there is no God ?” It is evident that structure and arrange- ment the most complete must be insufficient to confer clear vision, without perfect transparency of the dioptric media; at the same time, we know that all organized structures require a constant supply of nutrient elements from the blood. The admission of vessels carrying red blood being, however, impossible in any way across the line of vision, the difficulty is obvi- ated by the transudation of the transparent tissues, by the more liquid and colourless por- tions of the vital fluid ; in the cornea, probably by the tube system of Mr. Bowman ; in the lens and vitreous, by filtration from the ex- tremely vascular ring of ciliary processes. CHAPTER VII ©omparatibj ^natoma anb i^bastolosa of tbt INVERTEBRATE ANIMALS — SENSIBILITY TO LIGHT OE LOWEST ANIMAL TRIBE OCELLI OE MEDUSA AND STAR-EISH POLYGASTRIC AND WHEEL ANIMALCULES — PARASITES — BARNACLES — ANNELIDA, OR WORM TRIBE — CENTIPEDES — • EYES OE INSECTS — COMPOUND AND SIMPLE MODE OE VISION SPIDER TRIBES — CRUSTA- CEA, OR CRAB TRIBE — SIMPLE — AGGLOME- RATED COMPOUND — MOLLUSCA, OR SOET- BODIED TRIBE. CHAPTER VII. THE EYE. ITS COMPARATIVE ANATOMY. INVERTEBRATE ANIMALS. The lowest tribes of the animal kingdom, the sponges and polyps, appear to possess simply such general sensibility to the agency of light as we may conclude is sufficient for the re- quirements of their humble state of existence. In a species of locomotive medusa, organs to which the sense of vision may be attributed first make their appearance, and similar simple “ ocelli” are observable in some of the star- fishes, albeit, in the majority of the species belonging to these classes of animals, nothing resembling an eye has been discovered. If, however, these creatures do not possess the sense of sight, its absence is, to them, fully compensated for by the extreme delicacy and acuteness of their general sensation, which, by making them aware of the slightest vibra- 184 THE EYE. tions occurring’ in the surrounding media, must, we can conceive, afford cognizance of all external relations, sufficient for the limited and peculiar wants of the being. Indeed, Cuvier considered that the sense of touch alone is so acute in zoophytes as to be sensible to light. In the extremely minute hut more highly organized polygastric and wheel animacules, the red spots, or “ eye-dots,” first described by Ehrenberg, are now generally considered to be organs of vision, some are even described as containing a lens. In the parasitic animals, the visual ocelli are more highly developed, and in the still higher class, the barnacles, are possessed as long as an organ of vision can be useful ; that is, whilst the young animal is capable of swimming about in the water, but become obliterated shortly after it fixes its habitation, and when the loss is supplied by the development of the exquisitely sensible ciliated tentacula. In the annelida, or worm class, eyes become sufficiently developed to be capable of definite demonstration. Muller describes them as ex- ceedingly simple in construction (Eig. XXIV. A B), merely “the expansion of the terminal Figure XXIV. A B, Simple Ocelli of Annelida. C, Tentaculaof Snail, showing — Eye ; 2, Ketraction Muscle ; 3, Optic Xerve ; 4, Optic Nerve thrown into folds by retraction of tentaculum ; 5, Cerebral Ganglion. D, Enlarged view of Eye of one of Snail tribe. E, Enlarged view of Eye of Scorpion ; 1, Cornea; 2, Spherical Lens ; 3, Aqueous Chamber ; 4, Vitreous; 5, Choroid ; G, Optic Nerve and Retina. F, Eye of one of the Crustacea, on a peduncle ; 2, Orbital Sheath. AK ATOMY AND PHYSIOLOGY OF THE EYE. 187 extremity of a nerve, spread out beneath a kind of cornea formed by the delicate and transparent cuticle, and behind this a layer of black pigment.” In the common leech, which belongs to this class of animals, the eyes, ten in number, “ are ranged in a circle at the anterior part of the head, above the mouth, raised above the surface, like warts, and pro- longed as cylinders into the interior of the animal;” according to Professor Grant, con- taining a lens. Probably the sense of sight enjoyed by these creatures is of an order as low as its range is limited, but sufficient for them, “ they creep, yet see.” The eyes of the centipedes resemble either those of the inferior classes already adverted to, or partake of the compound character of the insect-eye. In some, eyes have not been detected. Insects, properly so called, possess two forms of eye, the simple and the compound, or aggregated. Some have the simple eye merely, others, as the butterfly, only the com- pound ; generally, both kinds exist together in the same insect. The simple insect-eye may be described as a low form of that construction of which the human eye is a type, resembling 188 THE EYE. / mucli the eyes of the spider tribe (Fig. XXIV. E) 5 to he described hereafter. The compound eyes (Eig. XXV.) have ever excited the asto- nishment and admiration of observers, and are regarded as presenting one of the most magnificent and elaborate specimens of minute structure with which we are acquainted in creation. As, in the vegetable kingdom, the composite flower of the dandelion or hawk- weed is constructed of an aggregation of simple florets, each, hotanically speaking, in itself a perfect flower, so is the compound visual organ composed of an aggregation, greater or smaller, of simple eyes, each in itself capable of conveying the sensation of a certain limited portion of light, hut, acting all together, conferring upon the creature an ex- tensive field of vision. The surface of these eyes, examined under the microscope, is found to he divided into minute hexagonal spaces or facets (Eig. XXV. C). In the ant the divi- sions do not exceed fifty, hut generally their number is much greater ; in the dragon-fly they are computed to count nearly twelve thousand, and in butterflies greatly to ex- ceed this almost inconceivable amount. "Will has computed the facets on the eye of the Figure XXV. u CM A, Eye of Cockchafer, enlarged section : 1, Ocelli; 2, Optic Nerve- Filaments ; 3, Common Choroid ; 4, General Eetina ; 5, Nervous Pillars proceeding from, 6, Enlarged Extremity of Optic Nerve. B, Single Ocellus, enlarged view: 1, Hexagonal Facet, or Cornea ; 2,^ Lens 3, Vitreous; 4, Retina Expansion of Nerve -Filament ; 5, Choroid Pigment. C, Anterior view of Facets. ANATOMY AND PHYSIOLOGY OP THE EYE, 191 common house fly at four thousand nine hundred. Every facet, or cornea, is “ found to belong to a distinct eye, provided with a perfect ner- vous apparatus, and exhibiting its peculiar lens, iris, and pupil.” The nature of the com- pound organ is most usually exhibited in the well-known figure of the magnified section of the eye of the common cockchafer (Eig. XXV. A) , In this, from the bulbous extremity of the optic nerve, 6, there project a number of short nervous pillars, 5, whieli again unite to form a nervous expansion, 4, or general retina, which in this insect is covered by a bright red pigment, or choroid, 3. Erom this general retina numerous nervous filaments, one for each cornea or facet, pass through the com- mon choroid, or pigment, each to supply its separate ocellus, 2, 1. According to recent description, each sepa- rate eye (Eig. XXV. B) is composed of an hexagonal cornea, 1, coveidng a crystalline lens, 2, behind which is a vitreous humour, 3, and hyaloid surrounded by the -retina expan- sion of the nerve-filament, 4, and this again hacked by its layer of coloured pigment, 5, which rises between the conical ocelli to the 192 THE EYE. surface of the eye. In some insects, it covers a portion of the rim of each cornea, so as to leave merely a minute pupil for the transmis- sion of light. Although the principle on which the compound eyes of insects are con- structed remains the same, the details are of course liable to variation, and the propor- tions to he altered ; the choroid, moreover, differs much in hue, and may he black, violet, blue, purple, yellow, brown, green. Minute hairs are, as in the bee, frequently observable between the facets, and the divisions are de- scribed as at times being projected like the cell- walls of a honeycomb. Of the kind of sight which the insect derives through these compound eyes it is impossible to speak with certainty ; it has been supposed that all objects within the range of vision at the same moment might be aggregated into one definite image, apparently continuous, but actually made up of numerous discrete por- tions, like a mosaic picture ; more probably the creature has the power of directing atten- tion through - one ocellus at a time, which thus corresponds to the small central cone of rays, by which the defined but limited image of one object is formed in the refracting eye of ANATOMY AND PHYSIOLOGY OF THE EYE. 193 man, tlie rest of the picture being indistinct. If so, it is easy to see that this multiplication of minute eyes in the insect, and their aggre- gation into a fixed mass of more or less spheri- cal form, is a beautiful compensation for the want of that power of movement which is con- ferred upon the eyes of the higher animal. The latter has the power of so turning and directing the organ, as to throw the image it wishes to view into the line of the centre of vision; the former has the organ so disposed and constructed, that, though fixed, it may collect at the same time distinct pencils of rays, each corresponding to this visual centre, from all surrounding objects, and in all pro- bability has the power of giving the special attention of the moment to the impression con- veyed by one ocellus only. Such we can easily conceive to be the case, from many of our own experiences as regards vision. Lastly, it is very possible that these com- pound eyes are incapable of much adaptation to varied distance, and that, as Muller con- cludes, being fitted only for distant vision, the simple eyes have been provided for viewing near objects. The above high authority has cited many instances, to prove that the latter 194 THE EYE. are so placed as to be suited for this purpose only. However the fact may he, any one who has noted the rapid flight and instantaneous rectangular turns of a dragon-fly when feeding, must feel convinced that, in it, sight exists in perfection, quick, and well defined. In the Arachnida, or spider tribes, the principle on which the eye is constructed much resembles those appertaining to the highest type. Professor Muller has shown in the eye of the scorpion (Pig. XXIV. E), which, from its size, admits of comparatively facile demon- stration, that the cornea, the lens, the aqueous and vitreous humours, the retina, and choroid coat, are all in the same relative positions as in man, and that the sight of these creatures must be extremely perfect, more so, indeed, than that of any of the other articulated classes. The number of eyes in one individual varies from two to ten. Spiders, generally, have eight, arranged in two rows, on the back. In crustaceous animals, of which the crah, lobster, and cray-fish are characteristic and best-known representatives, the eyes are either simple, agglomerated, or compound. The simple eyes, or stemmata, resemble those of the spider class just described, and never ex- ANATOMY AND PHYSIOLOGY OP THE EYE. 195 ceed three in number ; the cornea, which is in immediate contact with the lens, being simply a modification of the common integument. The agglomerated organs consist of a number of simple eyes collected under one common cor- nea. The compound eyes resemble those of insects, and, like them, usually have hexagonal, hut sometimes, as in the lobster, square facets, or cornea. A few species have a supplement- ary lens, inserted as it were in the cornea. Although the visual organs of insects are fixed and sessile, the immobility is compen- sated for, partly by the spherical form of the eye, partly by the mobility of the head itself. Crustaceous animals being deprived of the latter advantage, in consequence of the union of the head and thorax into one, in many of the higher genera, in which the habits and re- quirements call for more extended powers of sight, the eyes are placed upon the extremities of pedicles, or stems, which being jointed at their junction with the skull, are capable of movement in various directions, by means of the appropriate muscles attached to them. Some species are provided with a grooved cavity, or “ orbit,” into which they can draw the eye for protection (Tig. XXIV. T). 196 THE EYE. Evidently, the lower tribes of molluscous animals, such as tbe mussel, oyster, eockle, &c., fixed to one spot, or very slow in move- ment, do not require any visual organ at all, or one of much inferior powers and capabilities to that of the active Articulata. Accordingly, in many, eyes are not apparent, and where they do occur, are numerous, simple, and not con- fined to any one part of the body ; forming, as it were, an “intermediate link between the diffused sensibility of the lower tribes and the localized eyes of the higher.” In the snail tribe the eyes assume a higher development (Eig. XXI Y. D C), and resemble the simple eyes of insects and spiders, being, like those of the former, adapted apparently only for near vision. They appear like black spots, and are situated either at the extremity, middle, or base of the superior tentacula, or horns, as they are usually called (Eig. XXIV. C 1). The animal possessing the power of in- drawing or inverting these tentaeula, along with the aceompanying eye, and the optic nerve being long enough to reach the entire length of the organ when extended, provision is made for its being thrown into folds (Eig. XXIV. C 4) during the contrary state. ANATOMY AND PHYSIOLOGY OF THE EYE. 197 In the highest class of Mollusca, that to which belong the nautilus and cuttle-fish, the eyes contain all the essential parts of the organ of vision found in man, and in vertehrated animals generally, modified, of course, to suit the aqueous medium in which the creatures live and move. They are of comparatively large size, placed symmetrically on each side of the head, and sometimes on peduncles. The aqueous humour, nearly of equal density with the surrounding sea-water, is deficient, hut the crystalline lens is of short focus and great power (hig. XXXI. 11). It is described by Professor Jones as “having the form of a simple magnifier, most approved of by opticians as being best adapted to secure a large field of view.” Whoever is conversant with the principles on which the well-known “ Coddington lens ” is constructed, will have little difficulty in appreciating the advantages derived by intro- ducing a precisely similar instrument in the eye of the cuttle-fish. The Coddington lens is a sphere of glass, divided into two portions by a deeply cut circular groove, which is filled up with opaque matter. The lens of the cuttle- fish is in like manner divided into two parts, of 198 THE EYE. unequal size, by a circular indentation, wherein the post-pigmental retina, with its coat of dark varnish, is fixed, and thus a picture of the most perfect character is ensured. Another striking peculiarity connected with the eye of the cuttle- fish is the absence of a proper cornea, there being merely a transparent covering continued from the proper integument of the front of the orbit (Kg. XXXI. 2), on raising which, the lens is found naked and exposed beneath (Pig. XXXI. 1). One step more brings us to the eye of the vertebrated fish, complete, as in man. CHAPTER VIII. ©omparatibe ^natorng of lEge* EYES GENERALLY PISHES ADAPTATION TO WATERY MEDIUM PLAT PORM — SPHERICAL LENS — CARTILAGINOUS PLATES — PLAITED NERVE AMPHIBIA — REPTILES — BIRDS — ADAPTATION OP PORM OP EYE — OSSEOUS PLATES PECTEN ADAPTATION IN RAPID MOTION — OWLS AQUATIC BIRDS — EYELIDS AND MUSCLES — MAMMALIA — EYES GENE- RALLY WHALE TRIBE — AMPHIBIOUS MAM- MALIA — MOLE — ORBITS — PUPILS — TAPETUM LUCIDUM. .4 ' % M , { CHAPTEH VIII. COMPARATIVE ANATOMY OF EYE. VERTEBRATE ANIMALS — FISHES. In the -iiautilus and cuttle-fish, we find the representatives at once of the uppermost grade of. molluscous animals and of the most highly organized class, the most largely brain-endowed of the lowest of the two great divisions of the Animal Kingdom,* the Invertefirata. Did the object of the present essay lead to' the inquiry, these beings would be found to possess general structure and special senses, so highly deve- loped as to make the step but a short one to the lowest class of the Vertebrata, that higher division of animated beings, characterized by the possession of an internal skeleton, more or less osseous, or bony ; of a brain-mass collected into and defended by a hard bony case; and of structures, senses, and capabilities very greatly exceeding those of the inferior division.- 202 THE EYE. The eyes of vertehrated animals, two in number, and disposed symmetrically on each side of the head, are constructed precisely on the same principle, and with the same parts similarly disposed, to those of man himself. Behind a transparent cornea, an aqueous fluid more or less in quantity, floats the iris, and covers the crystal lens, behind which a vitreous body, inclosed in a hyaloid, and surrounded by a nervous retina, a coloured or lustrous choroid, and a fibrous, cartilaginous, or bony sclerotic, completes the visual instrument. How the principle of construction is modi- fied, by alteration of form or by apt addition, and yet, in conformity to the great laws of light, is made subservient to the every-day requirement of the class, it will be the object of the present chapter to explain. In fishes, which constitute the lowest class of vertehrated animals, the eyes, though con- structed after the human type, are found to present modifications adapting them for the fulfilment of their office, in the dense medium in which the creatures live and move. A re- ference to Big. XXVI., A B, Avill show that a comparatively flat cornea covers a spherical crystal lens, and that the capacity of the aque- FiGuiiE XXVI.— Eyes of Fish. A, Enlarged Section : 1, Cornea; 2, Aqueous Chamber; 3, Lens ; 4, Iris ; 5, Vitreous ; 6, Retina ; 7, Sclerotic Coat, with imbedded plates of cartilage’; 8, Choroid ; 9, Falciform Process. B, Section of Eye on a peduncle, 1 . C, Posterior View of Fish Eye, showing position of muscles, and optic nerve opened out to show its plaited structure. J COMPARATIVE ANATOMY OF EYE. 205 ous chamber is extremely limited, eontaining just sufficient fluid to float the iris ; further, that instead of the spherical form of the human eye, the organ in the fish is diminished or flattened in its antero-posterior diameter, that is, from before backward, so as to throve the posterior surface of the lens very near the retina. The slightest reflection in connection with optical laws must at once show how admirably these modifications in the form of the fish eye are adapted to meet the altered conditions conse- quent upon the density of the medium through which rays of light are conducted to it, and to give clear vision, which could not possibly have resulted from eyes constructed after the same proportion as those of air-breathing animals. An aqueous fluid, of density scarcely differ- ing from that of the surrounding v^ater, could have hut little effect in refracting rays of light towards a sight focus ; accordingly, we find a dense lens brought close up to the flattened cornea, a lens, too, of which the form gives such a degree of refracting power as may com- pensate for the dense nature of the medium through Avhich the rays are received. The focus of this spherical refractor being necessa- rily very short, the alteration in the shape of 206 THE EYE. the eye eonforms the position of the receptive retina to the diminished focal distance, by bringing it relatively nearer to the lens than it is in terrestrial animals. The flattened form of the eye of the fish, which is the necessary result of these modifications, is preserved by additional firmness imparted to its sclerotic coat, most frequently by two strong plates of carti- lage (Tig. XXVI. A 7 7) imbedded in its sub- stance, at times by the cartilaginous hardness of the entire tunic itself, or by its conversion, in some of the larger fishes, into a complete cup of hone. The iris in the fish (Tig. XXVI. A 4), usu- ally bright coloured externally, dark, like the choroid, within, and surrounding a pupil of wide circumference, for the admission of much light, is possessed of little, if any, of that mobi- lity and power of contraction requisite for the visual perfections of beings subject to the varied intensities of atmospheric luminosity. In some fish, its upper edge is continued over the pupil, in the form of a palmated or vine leaf-shaped process. Visual adaptation is most probably effected by a vascular organ of a deep red colour, the choroid srland, situated between the coats of COMPARATIVE ANATOMY OP EYE. 207 the choroid membrane, and which, hy its dis- tension or relaxation, probably confers upon fish the power of adapting the eye to varied distance. That fish do possess such a power, and in considerable perfection, is evident from the fact familiar to every one who has handled a fly-rod, that a fish will rise with great rapidity from a considerable depth, and accurately seize, or sagaciously refuse, the artificial insect. Besides the choroid gland, the eyes of fishes present another addition not found in other animals, in the form of a delicate sickle-shaped projection from the retina — according to some, from the choroid gland — which passes in the vitreous humour along the cavity of the globe, and is fixed to the capsule of the lens (Fig. XXVI. A 9). Its object is probably to main- tain the refracting media of the eye in situ, and its necessity, perhaps, arises from the na- ture and pressure of the surrounding medium. To the eyes of fish, constantly exposed to the action of water, lachrymal apparatus would evidently he a superfluity ; it is consequently absent, and, doubtless for the same reason, eye- lids are unprovided. The eyeballs, although comparatively little moveable, are, like those of man, provided with six muscles, four straight 208 THE EYE. and two oblique ; neither of the latter, how- ever, arise from the hack of the cavity, as in the human orbit, both are situated entirely in front. In some fishes, as in the sharks and rays, the eyes are supported on moveable car- tilaginous pedicles (I’ig. XXVI. B 1), articu- lated to the bottom of the orbit, and allowing of greater extent of motion ; their position in the head, moreover, is different in different species; “in some they are placed high up near the top, more frequently on the flattened side of the head, but always so situated as best to suit the exigencies of the particular fish.” In fishes which bore in sand or mud, the eyes are comparatively smaller than in other spe- cies; and in the eel they are covered by a dense transparent membrane, continuous with, and separating along with, the general integu- ment. Although the inner layer of the choroid is generally dark, in some fishes it possesses the brilliant metallic lustre observed in species of land animals ; externally it has a beautiful pearly, shining appearance. The composition of the lens, of transparent, serrated fibres (Big. XXII.) has already been alluded to m the description of that of man. The structure of the ’ COMPARATIVE ANATOMY OP EYE. 209 optic nerve in many fishes is peculiar and beau- tiful. When its investing membrane is re- moved, the nerve tissue is found to be disposed in plaitings, which, opened out, closely resemble a fan (Pig. XXVI.) The exact object of this arrangement is unknown; hut doubtless it is requisite for the perfection of the visual organs, with which He who created them all in wisdom has endowed and fitted the “ tenants of the deep,” to make their way through the “ paths” of the great waters, fitted them in myriads for the enjoyment of the gift of life, of existence in their own peculiar element — in that “ great and wide sea wherein are things creeping innu- merable, both small and great beasts,” and who, when He had created, saw that all was “ good, and blessed them,” saying — ‘‘Ee fruitful, multiply, and in the seas And lakes and running streams the waters fill. Each creek and hay With fry innumerable swarm, and shoals Of fish, that with their fins and shining scales Glide under the green wave. Or, sporting with quick glance. Show to the sun their waved coats dropt with gold.’^ O 210 ” ■ THE UYE. : : \ : AMPHIBIA AND HEPTILES, ^ In amphibia, of which the frog is the most familiar example, the eyes, as we might perhaps expect, hold a place somewhat intermediate between the same organs in fish and those of air-breathing animals in general— the cornea remaining flat, the aqueons deficient, the lens^ more or less spherical, the iris nearly without motion, and, undoubtedly for the same reason as in fish, no lachrymal apparatus provided. On the other hand, not only are upper and lower eyelids developed, hut, as in birds, the third or nictitating membrane. In those reptiles, as serpents, lizards, &c., which exist entirely on land, the eyes approach more nearly the optical proportions displayed in those of man and of the higher animals, the cornea becoming more convex (I'ig. XXVII.), the aqueous more abundant, and the lens less spherical ; at the same time, considerable varia- tion is observed in the arrangement of external appendages, and in some species, as the tor- toise, turtle, &c., the sclerotic coat of the eye is surrounded anteriorly by a circle (Fig. XXVII., B) of bony plates, as in birds. The motor apparatus, of four straight and two Figure XXYII. — Eye of Reptiles. A, Eye of Tortoise. B, Osseous Plates of Eye of Tortoise. C, En- larged Section of the same Eye. J), Diagram of Eye of Serpent, showing : 1, The Continuation of the common Integument over the Cornea. COMPARATIVE ANATOMY OP EYE, 213 oblique muscles, is similar to that of the fish; lachrymal organs are developed, there exists a nictitating membrane, and upper and lower eye- lids are more or les^ moveable ; the latter are sometimes united, and extend over the eye, being either pierced merely by a fissure for the admis- sion of light, as in the chameleon, or, as in ser- pents, forming over the cornea a perfectly con- tinuous transparent covering (Pig. XXYII., D), which is merely a prolongation of the common integument, and is regularly exuviated, or cast off, with it. The lachrymal fluid is described as flowing beneath this outer covering of the eye, and being, as in man, carried off by a lachrymal duct. BIRDS. In our inquiries hitherto, amid the com- paratively low grades of creation, the visual organs have been found exactly suited to the position, the habits, and reqnirements of the creature, neither exceeding nor falling short. With such premises any reflecting mind, reason- ing from the known habits and varied endow- ments of birds, would expect to find in them vision, regarded as vision merely, in its most perfect state and most capable adaptation ; and 214 THE EYE. sHch it is, suited alike to the swift darting turns of the swallow, to the rapid swoop of the hawk, or to the noeturnal predations of the owl — ^in each perfect. , The eyes of birds, generally, are compara- tively of large size, commensurate with their importance to the well-heing and existence of creatures to which they must he indispensable ; we cannot conceive the possible existence of a class of blind birds, and none such has ever been discovered. Less moveable in their sockets than the eyes either of man or quadruped, the deficiency is compensated for by the greater prominence of the cornea, the lateral position of the organs, and the great mobility of head and neck. The eye of the fish exhibits an organ pro- portioned in strict accordance with the laws of optics, to confer as perfect vision as possible upon creatures living and moving in a watery world ; how opposite must he the structure of the same organ, fulfilling all the requirements of their life, to the dwellers in “ the mid aerial sky,” the far-seeing birds of the air ? Exactly opposite. Instead of flat, the cornea is ex- tremely convex, the axis of the eye from behind forward lengthened, the aqueous and vitreous Figure XXYIII. — Eye of Owl. A, External view : 1, Osseous plates. B, Section : 1 Marsupium Pecten. COMPARATiyE ANATOMY OP EYE. 217 abundant, the lens comparatively flat, and its density diminished (i'ig. XXVIII.)* In rapa- cious birds more especially, in which, the tubular form of eye and rapid adjustment are specially called for, the former is preserved, and the sphericity, which the combined fluids of the eye would normally tend to give the organ, obviated by a circle of imbricated osseous plates (Vig. XXVIII., A 1) as in reptiles, enclosed between two layers of the outer sclerotic coat anteriorly, and extending as far as the margin of the cornea. These plates, obviously, must not only resist pressure of the fluids from within when the whole eyeball is pressed upon by its ex- ternal muscles, but from their capability of overlapping, materially assist the rapid adjust- ment of the eye to distance, by throwing the whole weight of the pressed fluid to increase the convexity of the cornea. Of course, these plates do not extend so far in birds generally, as they are represented to do in the night-feed- ing, predatory owl. The contrary state to that * In this figure, which represents the eye of an owl, the lens is spherical, belonging to a creature which exerts its sight in dim light. The same exceptional form occurs in the cormorant, and other birds which seek their prey in water by the eye. 218 THE EYE. alluded to above, flattening of the eornea, will of course result from muscular relaxation generally ; but the muscular flbres of the ciliary body demonstrated by Sir P, Crampton, must undoubtedly assist materially the adjustments of the eye, drawing back the cornea by their attachment to its internal layer. The iris in birds is frequently bright coloured. It is susceptible of much movement, and, more particularly in night-feeding birds, is capable of affording great breadth of pupil. In some classes, moreover, as in the parrots, its move- ments appear subject to the voluntary control of the individual. The choroid coat of the eye in birds is, like the human, covered with dark pigment, and forms, anteriorly, ciliary folds or processes. The optic nerve, instead of piercing the eye of the bird like a round cord, as in man, becomes flattened out vertically ; it still, how- ever, contains in its substance arterial vessels, which pass through the slit formed in the retina by the entrance of the nerve, and are devoted to form one of the most remarkable means of optical adjustment met with in organs of vision — the marsupium, or pecten, a body composed of folds of vascular membrane, and COMPAKATIVE ANATOMY OP EYE. 219 supplied by these central arterial branches which enter in the substance of the nerve. This vascular organ (Kg. XXVIII., B 1), which varies much in the number of folds com- posing it, is inserted, or wedged, into the sub- stance of the vitreous humour, passing forward to a greater or less extent, in some birds even as far as the lens. It is covered with the black pigment of the choroid, but is quite unconnected with the tunic itself. Whatever other uses this remarkable addition to the ordinary struc- tures of the eye may subserve, its evident capa- bility of being distended by additional blood poured into it — as in similar textures — or of being relaxed, points it out as admirably adapted for the ofldce most generally assigned to it, that of assisting the optical adjustment of the eye ; either by, in its state of distension, pushing forward the lens, and convexing the cornea for near vision, or, in its relaxed condi- tion, allowing the textures to fall back to their usual state by their own natural resiliency. Thus we can imagine the bird of prey “ loosely wing” the air, and scan, calmly but attentively, the far -beneath, the eye comparatively at rest, and adapted, by its lessened convexity, to dis- tant vision ; suddenly the looked-for victim is 220 THE EYE. seen, passion is aroused, and as with, lightning- like swoop the bird descends on his prey, the blood of excitement pressing the distensible organ of the pecten, adapts the eye more and more to the rapidly- diminished distance, and by thus enabling the object seen from high in the air to he kept in view through the long, hut fast descent, makes even the nimhle-moving mouse an easy prey. In addition, however, to this great power of sudden adjustment which must necessarily be possessed by predacious birds, the class generally have a keenness and length of sight superior to that of any other division of the animal kingdom. Buffon states that a hawk can distinguish a lark on the ground, of a similar colour to the clod of earth it sits upon, at twenty times the distance a man can. Un- questionably this power must be greatly owing to increased nervous sensibility of the special sense ; it is, however, assisted in many species of rapacious birds, in consequence of the eye being screened from light by superciliary ridges of feathers, so that the organ is enabled to be concentrated without distraction upon things beneath. An arrangement somewhat analogous is said COMPARATIVE ANATOMY OE EYE. 221 to be found in the male glow-worm, by which he is enabled, while winging his flight in the air, better to descry his creeping mate on the ground. The quick perceptive sight of the swallow, and of other birds which make rapid, short darts and turns, affords yet another instance of the remarkable sensibility and quick accurate adjustment of sight in this class of created beings. In birds of prey, again, which feed by nights more especially the owls, the eyes, in addition to being very large, with a large pupil, are surrounded by a disk of shining white feathers, which answer the purposes of collecting and reflecting all available light which can assist vision ; this “ facial disk,” however, is but feebly developed in owls, which hunt by day.* In contradistinction to the large eyes of the night-feeders, humming-birds, which seek their food under the mid-day glare of a tropical sun, have eyes, actually and comparatively, most minute. In such aquatic birds as ducks, finding their food under water, rather by the sensibility of the beak than by sight, the eyes are com- paratively small in size, and of flattened curve. Swainson. 222 THE EYE. / Probably the most remarkable adduced instance of the perfection of sight in the feathered tribes, is that of the condor of South America. It has been fully proved* that this carniverous bird cannot discover carrion by the sense of smell alone, even when very close to it; it is not likely, therefore, to do so at the immense dis- tances from which condors discover the dead body of an animal ; to the sense of sight, therefore, has been attributed the faculty they possess, but so incredible does it seem, that naturalists have endeavoured in other ways to account for the phenomenon. There yet, per- haps, remains to be discovered a sense, an instiuct which guides the creatures of instinct in their far journey ings — the dog to the home of its master, over trackless hUls or across rapid rivers — the “birds of the wandering wing” “ through the wastes of the trackless air.” Sight may not do all this, and though we cannot, as in the case of a special sense, lay our finger upon it and say here lies the instru- ment, there is every good reason to attribute the wonderful faculty, thus displayed, to some special instinct with which the Almighty has gifted these. His lower creatures, by which He Darwin’s Yoyage of a Naturalist.” i--- 4 f t s Figuee XXIX. A, Muscles of Eye of Bird : 1, 2, 3, 4, Straight or Recti Muscles ; 5, 6, Oblique Muscles ; 7, 8, Muscles of Nictitating Membrane winding round ; 9, Optic Nerve. B, Bird’s Eye, eyelids turned back to show : 1, Nictitating Membrane. COMPARATIVE ANATOMY OF EYE. 225 guides the “ranged figure” of the “a’ery cara- van high over seas,” till, “ desert and deep” passed over, “ each worn Aving hath regained its home.” The eyes of birds are protected hy upper and lower eyelids, and hy a third or nictitating membrane. The lower eyelid differs from that of man in being the more moveable and ex- tensive, and hy the possession of a depressor muscle, similar in action to that described as elevating the upper human lid. Eyelashes are present in some species, more particularly in such as seek food among close thickets or dense foliage they are thick and strong. A lachrymal gland and apparatus exist, and another gland, situated underneath the con- junctiva, at the inner angle, supplies a lubri- cating fluid. The third eyelid, or nictitating membrane (Eig. XXIX., B 1), is sometimes sufficiently transparent for the bird to look through it when draAAm down; more generally, however, it is drawn, with a momentary sweeping motion, rapidly over the surface of the eye, hy a strik- ingly beautiful arrangement of muscular me- chanism. In addition to the four straight and two oblique muscles of the eyeball (Eig. XXIX., p 226 THE EYE, A, 1, 2, 3, 4, 5, 6) in the bird, two other mus- cles (7, 8) are provided. The square-shaped muscle (8), arising from the upper and anterior part of the globe, descends backwards towards the optic nerve (9), where its fibres form a long pulley or sheath, through which passes the tendon of the second muscle (7), as it winds round the optic nerve before passing through a fibrous sheath under the eyeball, to he attached to the lower angle of the memhrana nictitans. A moment’s reflection will at once make plain how the simultaneous action of these two mus- cles is exquisitely adapted to fulfil the object of their construction. MAMMALIA — QUADRUPEDS. Our inquiries have at length conducted us to the highest class of vertehrated animals — the loftiest reach of the scale of animated nature — the Mammalia, which, deriving their name from the characteristic provision for the nourishment of their offspring, by virtue of the endowment, extend to man himself, the last link of the great chain of physical nature, and in the love of that offspring exhibit the nearest approach which “ the brutes that perish ” make to Figure XXX. Section of Eye of Whale, showing-1, greatly thickened Sclerotic Coat. COMPARATIVE ANATOMY OP EYE. 229 man, who has life and immortality before him. The “masterwork” — Who not prone And brute as other creatures, but endued With sanctity of reason, might erect His stature, and upright, with front serene Govern the rest.” In mammalia generally, living in the same medium, with habits and requirements approxi- mating those of man, the proportional arrange- ments of the eyes do not differ essentially from those of the human organ, the description of which — already given — is sufficiently applicable to the class at large to render any repetition superfluous. At the same time, there are found among the lower animals certain modifications by which the eye is adapted to the peculiar necessities of the creature, too striking not to be adverted to. The greater proportion of the mammalia are quadruped animals, living con- stantly on the surface of the land; but there are classes belonging to the order whose life is essentially different, the most remarkable, per- haps, being the warm-blooded whale tribe, in- habiting the depths of the ocean, which, amid other modifications, have the spherical lens and flattened form of the fishes’ eye (Fig. XXX.) 230 THE EYE. The eye of the whale, indeed, appears nearly spherical externally ; hut this appearance is caused by the extreme thickness — at least one inch — of the cartilaginous^ sclerotic (I’ig. XXX., 1), which encloses the compressed globe, and is, doubtless, required to preserve the form of the organ under the immense pressure to which it must be subject in deep water. In whales, as in fish, lachrymal organs are absent, although eyelids, and a gland for the secre- tion of lubricating matter, exist; moreover, the tribe resemble fish, but differ from other mammalia in having both the oblique muscles of the eye arising from the fore part of the orbit. As some mammal animals are amphibious in their habits, living generally in or under water, but visiting land occasionally, the eyes of course require the power of adaptation to the varied circumstance. This is well seen in the Greenland seal, described by Blumenbach as having the cornea thin and yielding, the anterior segment of the sclerotic thick and firm, its middle circle thin and flexible, and, again, the posterior segment thick, almost cartilagi- nous, the whole eyeball being surrounded by strong muscles, capable either of shortening, or Figure XXXI. — Section of the Eye of the Cuttle-fish. COMPARATIVE ANATOMY OE EYE. 233 permitting the elongation of the visual axis, according to external requirement. In burrowing animals, as in the mole, the orbit is very superficial, and the extremely small eye protected from injury by a covering of fur. The bony orbits, so deeply capacious in man,' exist in less proportion in mammalia generally, and the relative axes, both of orbit and eye, differ much in different species, being more frequently directed outwards, so as to afford a considerable extent of lateral, and, when very prominent, as in the hare, even of posterior vision. In nocturnal-hunting qua- drupeds, and in man, the eyes being directed forward, and their axes parallel, or nearly so, a greater precision of sight is thereby attained. All the land-dwelling mammalia have the superior oblique muscle, as in man, passing through a pulley ; a few possess a gland similar to that in birds, at the inner corner of the eye, for the secretion of lubricating fluid. In a few animals the form of the pupil varies from round ; in the cat tribe the opening is vertical, and its enlargement and contraction appear to he under the voluntary control of the animal, the arrangement being doubtless con- 234 THE EYE. nected with its peculiar night-hunting habits. In many of the mammalia the posterior portion of the choroid is deprived of pigment cells, and instead possesses a shining, metallic lustrous surface, the tapetum lucidum, which, instead of absorbing, reflects the smallest portions of light that may enter the eye. The well-known glare from the eye of the cat is caused by the reflec- tion of rays of light from the tapetum lucidum ; other animals exhibit the same phenomenon. It may be observed that this reflection cannot happen — is not developed — in absolute dark- ness, but requires some amount of light ; very little, however, suffices. In our investigations of the general anatomy and microscopic structure of the human eye, how admirable did all appear — how far tran- scending, beyond hope of approach, the highest productions of human art. In our further in- vestigation of the physiological fulfilments of the same organ, how exact the keeping, how exquisite the adaptation to the one great end, causing the most perfect human instrument to sink into insigniflcance in the comparison. Trom the contemplation of this high type of optical arrangement, we descended at once to the low- est manifestations of a visual organ, the simple COMPARATIVE ANATOMY OF EYE. 235 ocellus, or eye-dot, of the humbler classes of animated nature, thence ascending, and having passed in brief review the modifications of the organ in the various classes of the animal king- dom, we have arrived at that stage where the next step is once more to man himself. How* many different forms of animated existence are comprised within the vast limits of that one division of ereation, the animal kingdom — how all but infinite in variety the habits, require- ments, and conditional existences of its com- ponent creatures ; and yet, in all and each, every structure is perfect. In the one we have found it so ; in the creeping things of earth — in the tenants of the mighty deep — in the birds of the air — in beast, in man, the eye is the work of God. CHAPTER IX. 0tt SHtStfom of ®0i3 DISPLAYED IN THE STRUCTURE OP THE EYE. ■ f. -sv CHAPTER IX. THE WISDOM OF GOD AS DISPLAYED IN THE STRUCTURE OF THE EYE. “A SINGLE example seems altogether as con- clusive as a thousand ; and he that cannot dis- cover any trace of contrivance in the formation of an eye will probably retain his atheism at the end of a whole system of physiology.”* To the man who, in contemplating the works of creation around him, whether the “ curiously wrought” structures that tax the highest powers of the microscope to unfold their beauty, or the wide expanse of the “ uni- versal frame,” so “ wondrous fair,” which the most far-searching glass of the astronomer can- not fathom — to him, knowing, feeling the irrepressible conviction that all is the work of an Almighty hand, nothing is more puzzling than the state of mind which requires to argue Edinburgh Review^ vol. i., p. 289. 240 THE EYE. itself into the belief of the existenee of one omnipotent and all- wise Author of all, nothing more fearful than that wilful blindness whieh closes the eye of a fool who says “ there is no God,” and who, wrapping himself up in the pride of his own fancied intelligence, seeks only for what he imagines flaws and errors in the grand cosmorama, gives ear only that he may catch some jarring note in the “ music of the harp of universal nature,” and, led by his own imperfect or disordered impressions, mistakes for discord the variation which he cannot un- derstand. For such a state of mind it is use- less to multiply instances ; structure, arrange- ment, adaptation, may he presented to the eye of that rational mind which in this world man holds by the free gift of the God who made him, hut into that higher region of his spiritual being — that lofty, real intelligence which raises him above the brutes, and fits him for immor- tality — that inward temple where the things of God and heaven meet those of earth, the “ won- derful works” of creation carry no conviction ; the door is shut by the free agency of that will which overrules the understanding ; the whole being is compassed about with the fitful sparks and fancied shining of the fire itself hath THE WISDOM OE GOD. 241 kindled, and knows not that it “ walketh in darkness and hath no light.” Gladly we turn to the mind that thinks it no degradation to acknowledge that all is from God — that the ever-flowing life from Him sustains all in per- fection and harmonious working, even the power of man’s physical and mental being ; to the mind which, exploring the deep arcana of Nature, accepts her great book as a second reve- lation — the scripture of material creation — the mighty tablet on which the hand of Omnipo- tence has written in the effects of wisdom the ends of love. The greatest intellects have ever been the most humble, and “ those who have been dis- coverers in science have generally had minds the disposition of which was to believe in an intelligent Maker of the Universe.”* It is matter of familiar history that Neivton, at the close of a life devoted, all know how success- fully, to the elucidation of truth, compared himself to a child who had been playing on the shores of a mighty sea, picking up here and there a beautiful shell or shining pebble, but with the great ocean of truth still unexplored before him, so well did he feel the inadequate * Whewell, “Indications of the Creator.” Q 242 THE EYE. power of the human mind — and that mind a Newton’s — to search the infinite things of the truth of God. What says a philosopher,* who has been among the foremost to travel and push forward the path cleared hy the great author of “ The Principia ?” “ The further man inquires, and the wider his sphere of observation extends,” instances of unlimited power and intelligence “ continually open upon him in increasing abundance “ the study of one prepares him to understand and appreciate another, refinement follows on refinement, wonder on wonder, till his faculties become bewildered in admiration, and his intellect falls back on itself in utter hopelessness of arriving at an end.” . 264 THE EYE. Although every year of the world’s age sees new contrivances, adapted hy philanthropy to mitigate the loss, and to increase the comfort of the blind, every year renders man’s eyesight more precious to him ; never were the “ ample” pages of the hook of knowledge more widely spread, never richer with “ the spoils” of ancient time, with the triumphant results of our modern era. From the press there flows ever, night and day, an unceasing stream of know- ledge. Surely, it needs not here to point to the great and abundant blessing which every man in this realm, from the peer to the peasant, may, unfettered by the censor or the priest, draw from the printed page, that common treasury in which are laid up the best thoughts of the best men. If the sound of their voices has long ceased upon earth— if our senses can- not hear their words, our eyes can read the silent language of their spirits. But not the printed page of man’s works is it which alone yields knowledge ; his manifold inventions, his splendid triumphs of engineer- ing science or of architectural magnificence, his nicely calculated mechanisms, the beauti- ful textures of his looms, the teeming conveni- ences of social life, all speak to us_ words of THE PHILOSOPHY OP VISION. 265 instruction through the sense of sight. As well might we try to count the rain-drops which swell the fountain, as to pass in review all those things which God has given to man, through man, which feed the strearns of human knowledge and power. What thinking man could walk through the “ long-drawn aisles” of the Crystal Palace of 1851, surrounded by the trophies of man’s industry, the conceptions of his brain embodied by the labour of his hands, and not feel reason to bless God for all his good gifts “in' wisdom, in understanding, and in knowledge, and in all manner of workman- ship ” — “for He alone fiUeth the heart with these things”— to bless Him for the bodily eye to see, for the mental eye, the capacity to understand. Again it is repeated, “the eye to see, the capacity to understand.” The mere seeing — the child-like, wondering, undirected gaze of the unthinking or uneducated — can scarce give new or true ideas, or originate those higher thoughts which elevate the mind above the fascinations of sensual indulgence. If the eyes of men are to perform their part in the great upward movement, in the mighty upheaving of human intellect, which is now begun, man must 266 THE EYE. be taught how to look through these “ windows of the soul” — must know how, through the sense of sight, as well as through that of hear- ing, to supply unto themselves that stimulus which their nature ever craves, which it must have, and will have, and which, unsupplied to it through the higher, will he sought through the medium of the lower senses. On those Avho feel and know the unnumbered blessings which spring from an enlightened use of those higher senses devolves the duty of guiding their less favoured brethren on the same path, of giving them eye-drink and ear-drink — giving, where they can, cheerful sights and sounds at home, objects of interest and for thought abroad. Thus will they most surely assist in establishing the cause of order and temper- ance, of social and moral elevation, of enlight- ened religious feeling, and thought, and action. It is interesting to remark how much more importance the ancients attached to the mode of imparting knowledge by the sight than we do; the symbolical meaning attached to colours is a fact testified not only in the heathen mythology, but even in the Bible itself. In all probability, in the painting in the ancient Egyptian temples and tombs, in THE PHILOSOPHY OP VISION. 267 that .of the curious monuments of Etruria, of Copan and Palenque, in Central America, &c., colours were used, not by chance, or to suit some mere external idea of harmonious combi- nation, or pleasing contrast, hut for the express purposes of conveying to the mind, especially in religious worship, definite ideas connected therewith. Nor is it the useful only which man’s eye drinks in for the nourishment of his spirit ; the beautiful also exists — a means of elevation above things grovelling and sensual, given by God to man. Nor can we doubt what all cre- ation proclaims in its fair face of attractive loveliness: “Behold the lilies of the field,” why were they made so fair ? Why all the beautiful forms and colours of the flower world ? The perfecting of the seed, we know well, needs not the beauty of the flower, and for this we can trace no other object than to delight man’s sight, telling of God’s love, to Comfort man, to whisper hope Whene’er his faith grows dim ; Tor whoso careth for the flowers Will care much more for Him.” But in vain for its highest ends would the brightest scenes of this world of matter be 268 THE EYE. represented to the bodily eye, were that inward power nngiven by which man feels the influ- ence not simply of material beauty, hut of the presence of God in all that is beautiful in nature. “ In the uniform plain hounded only by a distant horizon, where the lowly heather, the cistus, or waving grasses deck the soil ; on the ocean shore, where the waves, softly rippling over the beach, leave a track green with the weeds of the sea — every- where the mind is penetrated by the same sense of the grandeur and vast expanse of nature.” “ Everywhere, in every region of the globe, in every stage of intellectual culture, the same sources of enjoyment are alike vouchsafed to man. The earnest and solemn thoughts awakened by a communion with nature intui- tively arise from a presentment of the order and harmony pervading the whole universe, and from the contrast we draw between the narrow limits of our own existence and the image of inflnity revealed on every side, whe- ther we look upwards to the starry vault of heaven, scan the far-stretching plain before us, or seek to trace the dim horizon across the vast expanse of ocean.”* Humboldt. THE PHILOSOPHY OP VISION. 269 We. gaze upon tlie face of nature, and our minds mirror her ever-changing features ; whe- ther brightening in the rosy light of morning, or glowing in the meridian beam, they “ shed sunshine through the heart,” or, shadowed by the dark cloud, infect our spirits with their own gloom. We gaze upon the face of a friend, and feel a stronger influence still ; varied ex- pression in every feature tells of the conscious spirit within ; hut, like the sun, the eye lights up the whole — through it the silent spirit - language passes with double impress — through it reaches our minds, full oft our hearts; all know it, all have felt its power, compelling in its fascination as that of the Ancient Mariner — “ teaching beauty,” as nothing else can teach it, from the bright eyes of woman, or in the calm, clear look of the loved or revered, impart- ing ‘‘ access in every virtue.” hull oft may the eye more eloquently reveal what the tongue cannot, would not, speak ; tell in its clear depths of innocence and truth, or downcast, shaded from another’s gaze, express in natural language the consciousness of guilt, or the acknowledgment of shame. Scarcely is there hope that to the confirmed sceptic, who blinds himself to the light of THE EYE. 270 /. truth, anything’ which has heen said in the foregoing pages will bring conviction; whilst wilful pride obscures the mental vision, JSTot worlds on worlds in phalanx deep” will speak of God to the darkened mind ; hut remove the veil which the evil will, or pride of intellect, has drawn down, and the lowliest weed will carry evidence to the understanding which it cannot refuse. Many there are carelessly indifferent, with- out formed opinion, possessed of abundant leisure, opportunity, and mental capability, who waste, in frivolity or idleness, time and powers which might open up to them the trea- sure of God’s creation, give them new life, new interests, true satisfaction, give elevation to the mind and thought, too apt to sink into sensual indulgence. Perhaps, to some of this class, the exposition, however imperfect, of one little work of God may awaken desire to know more of the beauty which He has given to the things of matter, which day by day are stared at without thought, or at best with childish wonder, knowing not, nor caring to know, not even the simplest arrangements and building up of the house they live in. Nature is Truth, THE PHELOSOPHT OP VISION. 271 because it is of Grod, and Truth is instructive beauty. Not one particle of her wide domain is without its interest ; it matters not which. Take the broader distinctive features of the animal or vegetable world — the field is inex- haustible ; investigate with the microscope the minutest atoms of structure, or direct the tele- scope to worlds and systems far, far removed beyond our power of calculation — all is full of interest without end. Lastly, let it not be forgot that, however perfect the structures — however exquisite the adaptations of the material organ which it has been the object of the foregoing pages to explain — however beautiful the scenes and things of this beautiful world, the works of God, the works of man — ^however extensive the knowledge to be acquired, these things are but means to an end — arte but the fleeting things of time leading to eternity ; that, real as they appear to us here, there are far brighter reali- ties — those of the eternal world, near th6 throne of God. Let it not be forgot that there are other things than those we can touch with our bodily hands, or see with our bodily eyes, real and true ; all cannot, perhaps, see them alike ; some cannot see them at all, because they will 2.72 THE EYE. not, for they “ love darkness rather than light still, the truth of God — immutable, un- ehangeahle, eternal as his own nature — is, whether man sees it or not; its light ever shines in the believing, loving heart, bringing comfort and peace here, and hope for hereafter ; hope, that when the bodily eyes are glazing in death, and “those that look out at the win- dows” are darkening — when the world, its bright scenes and familiar objects, its hopes, its joys, its sorrows, and — best loved, last looked upon — the faces of the near and dear, are fading fast from the sight, the mental, spiritual eye of faith and of love may open clearer and clearer upon the brighter things of its eternal home, and accustomed to the light of God’s truth here, find itself in its real element when, freed from the gross covering of earthly matter, it wakes in that heaveU of which God is the true light — ^the true and only source of truth, of love, of all things. THE END. London : Thomas Harrild, Printer, 11, Salisbury Square, Fleet Street. V ♦ ^