MICHEI LOUTFALLAH THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA LOS ANGELES TESTS AND STUDIES OF THE OCULAR MUSCLES ERNEST E. MADDOX, M.D., F.R.C.S., Ed. Ophthalmic Surgeon to Royal Victoria Hospital, Bournemouth ; formerly Syme Surgical Fellow, Edinburgh University WITH 110 ILLUSTRATIONS THIRD EDITION Specially revised and enlarged by the author PUBLISHED BY THE KEYSTONE PUBLISHING CO. BOURSE BLDG., PHILADELPHIA, U.S.A. 1907 PREFACE TO THE THIRD EDITION That a second edition of this book should be asked for affords pleasant compensation for the time and thought it cost. In the choice of terminology, I have carefully refrained from the dislodgment of ancient landmarks. Thus the words "adduction" and "abduction" will be found retained in their time-honored sense a sense which is stamped on the very name of " abducens" muscle itself, and the dislocation of which would make the classics of Graefe and Bonders, Helmholtz and Mauthner less intelligible. The announcement, by one author and another, of supposed errors in the treatment of Listing's law by Bonders and by Helm- holtz have compelled me, even in these uncontroversial pages, to enter more fully into the laws of the parallel motions of the eyes than their clinical importance would perhaps otherwise warrant. The purely clinical reader can, if he prefer, pass over the section which shows these discoveries to be groundless. My thanks are due to Br. Asher, of Leipsig, tor several suggestions from the German edition, one of which has led me to add a chapter on Nystagmus. E. E. M. CONTENTS CHAPTER I PAOK THE GLOBE AND ITS SOCKET 15 The Ball of the Eye Ocular Muscles Normal Motions Orbits- Tenon's Fascia Check Ligaments Their Functions Internal Cap- sule Tenon's Space Bearing of Check Ligaments on Tenotomy. CHAPTER II THE OCULAR MOTIONS 34 Translations of the Globe Listing's Plane False Torsion Bonder's Law Listing's Law Helmholtz's Plane of Reference Donder's Plane of Reference Index of Torsion Use of False Torsion Azimuth and Altitude. CHAPTER III INDIVIDUAL OCULAR MUSCLES 52 Law of Innervation Description of the Recli Spiral of Insertions Description of Obliques Subsidiary Functions Medial Origin of Muscles Superductors and Subductors Lines of Force Muscular Planes Axes of Rotation Paralytic Equilibrium Con- secutive Deviation Arc of Contact Ophthalmotropes Landolt's Ball Tilted Axes. CHAPTER IV ASSOCIATED MUSCLES IN A SINGLE EYE 68 Isolated Contraction of some Muscles Unknown Superducting Muscles for Example Motion from Secondary Positions Compo- sition of Rotations Dynamics Moments Resolution of Rotations Co-ordination Vertical Purchase of Recti and Obliques altered by Adduction and Abduction Purchase of same Muscles about Vertical Axis Paralytic Exophthalmos Model with Tilted Axes Paralytic Semi-orbits. 7 S Tests and Studies of the Ocular Muscles CHAPTER V PAGB CONJUGATION OF THE Two EYES 79 True Associates Spasm of Single Muscles Conjugate Innerva- tions Conjugate Paralyses Convergence Confirmation of Rules of Conjugation Convergence and Accommodation Effort and Work Exophoria in Oblique Vision Latent Deviations. CHAPTER VI MOTOR FACULTIES g Point of Fixation Direct and Indirect Vision Fixation-reflex Persistent Fixation Binocular Fixation Projection Correspond- ing Points Physiological Diplopia Suppression of Images Origin of Projection Fusion Power of Overcoming Prisms Breadth of Fusion Power Monocular Perception of Distance Stereoscopic Vision of Relief Hering's Drop Test. CHAPTER VII STRABISMUS 115 Definition Chief Division Strabismus Convergens Accommo- dative Non-accommodative Predisposing Causes Congenital Amblyopia Nervous Element of Strabismus Imperfect Central Fixation Strabismus Incongruus Recovery of Lost Faculties Extension of Partially Preserved Faculties Strabismus Convergens Myopicus Alternating Squint Unilateral Squint Operations Strabismus Divergens Refractive After-treatment for Squints Javal's Course of Treatment Natural Cure of Squint Evidences of Squint Secondary Deviation Fallacy from Anisometropia Apparent Squint Intrinsic Aberrations of Eyeball Linear Stra- bismometry Hirschberg's Method Perimeter Method Charpen- tier's Method The Tangent Strabismometer Subjective Strabis- mometry Paralytic Strabismus. CHAPTER VIII OCULAR PARALYSES 151 Symptoms Order of Examination Diplopia Aphorisms, Incor- rect and Correct Complications Diagnostic Procedure Narrow- ing Circles Dextral and Laeval Torsional Purchase and Vertical Purchase Brief Summary Confirmation of the Diagnosis Measured Charts To Read a Simple Chart To Read a Multiple Chart Independent Diplopise Clinical Classification of Muscles. CHAPTER IX OCULAR PARALYSES {Continued) 165 Optical Illusion How to transfer Charts to Opposite Eyes Paraly- sis of Right External Rectus Paralysis of Left External Rectus Contents 9 Qualifications in Paralysis of the External Recti Paralysis of Right Internal Rectus Paralysis of Left Internal Rectus Qualifications in Paralysis of the Internal Recti Paralysis of Right Superior Rectus Paralysis of Left Superior Rectus Paralysis of Right Inferior Rectus Paralysis of Left Inferior Rectus Paralysis of Right Superior Oblique Paralysis of Left Superior Oblique Paralysis of Right Inferior Oblique Paralysis of Left Inferior Oblique Paralysis of the Third Nerve Test for Fourth Nerve Measurement of Ocular Paralyses Conjugate Paralyses Tests for them Tropometer Precise Tests for Convergence Mnemonics for Ocular Paralyses Dangers of Mnemonics Werner's Mnemonic. CHAPTER X NYSTAGMUS 192 Definition Rapidity Apparent Motion Examination of Etiology Nature of the Excursions Treatment of Nystagmus Curiosities. CHAPTER XI OPHTHALMOSCOPIC CORNEAL IMAGES 198 Precautions Fixation Position of the Corneal Reflection Apparent Squint Refraction Surmisable The Reflection in Cataract and Iridectomy Tests for Monocular Blindness Alternation Conco- mitancy Test for Binocular Fixation Hirschberg's Rule for Squint Priestley Smith's Mode of Strabismometry Different Points of View Error of Approximation Photography of Muscu- lar Anomalies Author's Camera Recording Reflections. CHAPTER XII HETEROPHORIA '. . , 213 Chief Divisions Dissociation of the Eyes Physiological Hetero- phoria Direction of Deviation Hyperphoria Objective and Sub- jective Tests Prism Tests Glass Rod Test and its Allies Two Species of Cylinder Manufacture Mode of Use Measurement of Latent Deviations Rare Anomaly Previous Tangent Scales Advantages of the Tangent Scale Which Eye Fixes ? Other Uses of the Scale Test for Concomitancy Trial Test for Reliability Heterophoria in Near Vision Mode of Use Its Meaning Physio- logical Exophoria Prism Diopters in Near-Vision Tests Inter- mediate Scales Tests for Breadth of Fusion Correction of Hyper- phoria Relative Importance of Correction Concomitancy vs. Paresis Horizontal Breadth of Fusion Orthoptic Training Simple Rules for Heterophoria Rules for Decentering : Prism Diopters Direction of Decentering Example Prism Diopters Operative Interference Graduated Tenotomy Marginal Tenotomy. io Tests and Studies of the Ocular Muscles CHAPTER XIII PAGE CVCLOPHORIA 236 Its Detection and Clinical Measurement Its Exact Measurement Paretic Cyclophoria Non-paretic Cyclophoria Explanation of Leaning Image Rule for Rod Test Cyclophoria in Near Vision Oblique Astigmatism Cyclophorometers Optomyometer Clino- scope Volkmann's Apparatus Meissner's Test Depression of the Visual Plane True Primary Position in Distant and in Near Vision Le Conte's Confirmation Savage's Test Compared Eaton's Apparatus Formula for Meissner's Test. CHAPTER XIV THE EYE IN DARKNESS 250 The Visual Camera Deviation in Near Vision Sense of Projec- tion Effect of Attention on the Desire for Fusion Speed of the Exophoria Laws of Conjugation Illustrated False Fusion Diluted Fusion. LIST OF ILLUSTRATIONS 1-2 Axes, Poles and Equator 17 3-4 Tenon's Fascia 19, 21 5-7 Check Ligaments 22, 23 8 Horizontal Section of the Globe and its Membranes .... 24 9-10 Diagrams showing effect of Tenotomy 26, 27 ii Check Ligaments 29 12 Vertical longitudinal section of Eyeball and its adnexa . . 30 13 View of under-surface of the Eyeball 31 14 Author's Torsion Calculator .... 43 15 Author's Design for solving False Torsion 47 16 Diagram showing how the Eye reaches a new position by the shortest route 48 17-21 Diagrams illustrating Torsion 49, 50 22 Muscular Planes and Axes 59 23 Landolt's Ophthalmotrope 63 24 Anderson Stuart's Model of Ocular Muscles 64 25 Landolt's Ball 65 26 Tilting of the Axes 66 27 Helmholtz's Diagram for motion from one secondary posi- tion to another 69 28 Composition of Rotations 71 29 Composition of Muscular Forces 72 30 Planes of Corneal Orbits 73 31 Model with Tilted Axes . 76 32 Diagram illustrating Conjugation 87 33-36 Diagrams illustrating Convergence and Accommodation . . 88, 92 37-38 Figures for the Field of Fixation 101 39 Diagram illustrating Mis-projection 106 40 Proximal and Distal Diplopiae in 41-43 Stereoscopes HI, 112, 113 44 Home-made form of Hering's Drop Test 114 45 Ophthalmoscopic Corneal Reflections in Normal Eyes . . . 137 46-47 Diagrams illustrating Perimetric Methods of Measuring Squint 140 48 Diagram to illustrate "Spherical Aberration " 141 49-5 1 First and Second Steps in Tangent Strabismometry . . 142, 143, 144 52 Landolt's Dynamometer 154 53 Schweigger's Hand Perimeter 155 54 Dextral and Laeval Muscles 162 55 Mnemonic Attitude for Muscular Planes 162 ll 1 2 Tests and Studies of the Ocular Muscles 56 Method for Inscribing Charts 165 57-70 Charts of Ocular Paralyses . . 169, 170, 172, 173, 174, 175, 176, 177 178, 179, 181, 182 71 Tangent Scales 183 72 Specimen of Chart Entry 183 73 Kick's Motor Chart 186 74 Stevens' Tropometer 190 78 Corneal Reflections in Normal Eyes 199 79 Diagram showing Obliquity of Visual Axis with reference to Geometrical Axis 200 80-81 Diagrams showing Symmetry and Asymmetry of Normal Eyes 203 82 Unsymmetrical Angles Gamma 204 83 Priestley Smith's Tape Method 206 84 The Caustic Curve of a Convex Mirror 207 85 Analysis of Corneal Reflection 208 86 Photograph of Ascending Convergent Squint 209 87 Author's Camera for photographing Reflections 209 88 Photograph of over-correction, immediately after advance- ment 211 89 Chart of Congenital Defect of Right Superior Rectus . . 211 90-91 Author's Double Prism 217 92 Prisms of Stevens' Phorometer 218 93 First form of the Glass-rod Test 220 94 Diagram showing Use of Rod Test 221 95 Dioptric Tape Measure 222 96 Author's Tangent Scales 223 97-98 Trial-Frame for Use of Prisms 228, 229 99 Diagram showing Prismatic Effect of Decentering ... 232 100 Savage's Test . . 238 100% Illustrating Oblique Astigmatism 239 101 Optomyometer of the Geneva Optical Company .... 241 102 Stevens' Clinoscope 242 103-105 Haploscopic Figures 243, 244 106 Eaton's Apparatus for Meissner's Test 246 107-108 Author's Plan for solving Meissner-Torsion 248 109 The Visual Camera 251 no Camera for Testing Projection 254 TESTS AND STUDIES OF THE OCULAR MUSCLES TESTS AND STUDIES OF THE OCULAR MUSCLES CHAPTER I The Globe and Its Socket The motions of the eyes are notable for their combination of silence, swiftness and precision. The silence of the eye, or, at least, the absence of audible sound, is all the more remarkable because of the proximity of the organ of hearing and the ready conduction of sound by bone. The swiftness of the eyeball itself is not, perhaps, greater than that of adept fingers, nor is it desirable that it should be in the interest of its delicate contents; yet the act of winking or "twinkling of the eye" has always been accepted by com- mon consent as the briefest measure of time expressible by the human body. The precision of the ocular movements, together with the perfect co-ordination of the two eyes, is the most important virtue of the three, and is evidenced in thousands of ways every day. One example only need be given, namely, that in watching a small moving object in the distance, such as a bird a mile away, it is seen single instead of double, which could not be unless both eyes followed the object with the keenest exactness. Akin to this we may mention the steadiness of the eyeball in observing a fixed point ; a steadiness, however, which is not inherent in the ocular muscles, but which is maintained by an exquisite "visual reflex" mechanism. When the eyelids are closed the globes are in almost perpetual motion, as any reader may verify by laying the tips of his fore- fingers over the closed upper lids: moreover, if one eye be covered while the other is observing with comparative steadiness a fixed point, the covered eye does not share the steadiness of its fellow, but wavers slowly from side to side. This is easily demonstrated by the author's "visual camera" (Chapter XIV), which detects the movements of an eye placed in the dark. H 1 6 Tests and Studies of the Ocular Muscles Even in the light, an eye is unsteady unless occupied with a fixed object, as when, for instance, it only sees a false image of an object, the true image of which is seen by the other eye. The absolute steadiness of the eye during the study of minute objects, is entirely beyond our voluntary control, and I think we may fairly describe the parts played by volition and reflex action respectively, when we say that the former directs the eye, and the latter steadies it. It is true that in daily life the point of fixation is constantly on the move, but then it does not move in a wavering way, but purposefully, and in looking at an object it flits, as it were, from one salient point to another, dwelling upon each long enough to let the mind grasp the new picture presented each time. Lamare's ingenious plan of making the movements of the eye audible by a kind of binaural stethoscope attached by a point to the upper lid, showed that four or five slight movements take place during the reading of one line, and a greater movement when we begin to read a new line. Under ordinary conditions we can turn our eyes at pleasure from one object to another, but there is a peculiar pathological state in which this faculty fails, and in which this visual reflex appears to gain the upper hand, so that the eyes can with difficulty be made to look away from the object last looked at. To this subject we shall recur later on. The Ball Of the Eye- When we consider the spheroidal shape of the eyeball, and the character of its motions, we need not wonder that astronomical language has been so freely drawn upon for their descrip- tion. Thus we speak of the globe moving in its orbit (metaphorically like a planet), and distinguish its axis, poles, meridians and equator. The anterior pole is the mid-point of the cornea in front: the posterior pole the mid-point of the sclerotic behind (as in Fig. i). The axis of the eye, often called the "optic axis," extends between these poles. ' The equator is a circle or belt of the globe midway between the two poles.* (Fig- 2.) > The meridians are circles, each of which passes through both poles so as to have the axis of the eye for their common diameter. In the study of the ocular motions we assume the eyeball to be * This definition of the equator is an anatomical one. Physiologically, its axis coin- cides with the visual line, if we think of vision ; or with the fixation line if'we are occupied with the nrular motions. Since the eyeball is not a geometrically true body, it is customary t" disregard the little discrepancies between the position of the anatomical equator aud those of the visual and fixation lines. The Globe and Its Socket Fig. I spherical, though it is not strictly so, but flattened Irom before backwards, more like an "oblate spheroid," interrupted by the prominence of the cornea in front, which has a stronger curvature. Ocular Muscles. Each eyeball receives the inser- tions of six muscles, namely, four recti and two obliques. The recti have an almost common origin around the optic foramen (embracing the optic nerve at its en- trance into the orbit) and course forwards, diverging as they go to embrace the globe for a short distance before reaching their insertions. The superior and inferior oblique muscles act upon the eye from the upper and lower corners respectively of the inner v/all of the orbital outlet. The motions of the globe take place unerringly under the guidance of these six delicately-proportioned muscles, the importance of whose contribution to our daily comfort is not realized till one of them is disabled from any cause, and we see double. Normal Motions. The eye is so suspended in position that its ordinary movements are limited to those of rotation, no appreciable translation being possible. It is true that certain animals possess a "retractor muscle of the globe," capable of drawing the eye deeper into the recess of the orbit, but in man, exophthalmos and enophthalmos are only known as pathological conditions, due, in part, to such causes as variations in the size of the palpebral aperture, varying pres- sure ot the lids, varying tone of the extra- ocular muscles ; turgescence or spasm of the retro-ocular blood vessels, spasm and relaxation of the unstriped "orbital muscle'' of Muller, which spans the spheno-maxillary fissure ; and possibly also to contraction or relaxation of the unstriped muscular fibres described by Sappey as existing in the internal and external check ligaments near their orbital insertion. Fig. iS Tests and Studies of the Ocular Muscles It is probable that there occur, even in health, slight unnoticed physiological variations in the prominence of the eyes. Orbits. The orbits are two deep conical excavations in the skull, the anatomy of which is too well known to need description. At the apex of the cone are two apertures, the optic foramen and the sphenoidal fissure, the former transmitting the optic nerve and the ophthalmic artery, the latter all the other nerves but one, and the ophthalmic veins. The inner walls of the two orbits are almost parallel to each other, but the outer walls slope outwards so strongly that the axes of the two orbits (represented by imaginary lines from the apices to the centers of the orbital outlets) diverge from each other by from 24 to 30. The conical shape of the orbit is to accommodate the cone of muscles, and its apparently superabundant capacity is to permit the globe to be sufficiently packed in with orbital fat which plays a very important part in the formation of its socket. The orbital outlet is narrowed a little by the incurving of its upper and outer margins, and its outer margin is considerably posterior to its inner. From a series of measurements which. I have made, the outer margin of the bony orbit appears to be, on an average, about 22 mm. behind the root of the nose, 12 mm. behind the anterior ridge* of the lachrymal groove, and even 7 mm. behind the depression for the trochlea of the superior oblique. A needle run transversely inwards athwart the outer margin of the orbit would, it is said, pierce the center of an average eyeball. But great differences exist. The orbit is i^ inches deep, while its outlet is i)^ inches broad and i^ inches high. It is evident that the large size and conical shape of the orbit would make it by itself a most unsuitable socket for the eye to work in ; but, as a matter of fact, the eyeball comes nowhere in contact with it, and it may be regarded rather as a strong scaffolding for the real socket, as well as a storehouse for the orbital contents. The real socket is the capsule of Tenon, in conjunction with its supporting bed of orbital fat, supplemented by the concave surface of the eyelids in front. Tenon's Fascia. All the structures contained within the orbit are invested by sheaths derived from one and the same aponeurosis. The cornea, which, at first sight, appears to be an exception to this * The ridge referred to is the front edge of the groove formed by the superiol maxilla and lachrymal bone. The Globe and Its Socket 19 rule, is not, of course, strictly within the orbit. This "orbital fascia " is in some places or parts of it exquisitely differentiated to suit the requirements of the ocular motions, and where it surrounds the sclerotic forms the outer layer of Tenon's capsule, or, in other words, the external capsule of the eye. It is lined by a delicate internal capsule, which is of a different nature, being regarded by some as the serous membrane (/. e. , the pericardium or pleura) of the eyeball. The common aponeurosis of the orbit extends from one struc- ture to another, splitting to encapsule each, but it is convenient to commence its study by distinguishing that part of it which is specially in relation to the ocular muscles. We have already seen that the orbit contains a group of muscles which spring from the circumference of the optic foramen, and separate as they proceed forwards, so as to form a cone. Ensheathing this muscular cone there is a fascial cone, which extends from muscle to muscle, splitting to invest each with a fibrous sheath, and sending off layers here and there to enclose lobules of fat, vessels and nerves. This cone of fascia is attached at the apex of the orbit to the periosteum round the optic foramen, and widens as it advances till it gains the orbital outlet, to be rigidly attached to the periosteum all round the margin. There is, therefore (as shown at Fig- 3 ), a kind of cone of fascia Fig. 3 within a cone of bone, with this dif- ference between them, that while the bony cone contracts at its brim, the fascial cone expands at its brim, so that an interval exists between the two which is filled up with the peri-ocular fat, the extra- muscular fat, the lachrymal gland, etc. Fig. 3 shows very clearly how the eyeball is suspended in this cone, from above, and from all sides as well as from below ; so that the part beneath the eyeball, which partly supports it as on a hammock, receives too much credit by the name hitherto given to it, of "the suspensory ligament of the eyeball." The fascial cone lodges the eyeball in front, and the retro-bulbar fat behind. It is divided into two compartments an anterior one for the eyeball, and a posterior one for the retro-bulbar fat by a 2o Tests and Studies of the Ocular Muscles hemispherical aponeurotic septum {P. E. C. in Fig. 4), which adapts itself to the posterior hemisphere of the eyeball. This septum is given off from the fascial cone just opposite the equator of the globe all around, and from the same line of origin springs a companion membrane (A. E. C. ) which passes forward over the anterior hemisphere, investing it pretty closely as far as the margin of the cornea, where it becomes attached. These t\vo eye-investing membranes are regarded as forming one capsule, known as Tenon's external capsule. It sends pro- longations backwards in the form of a sheath for the optic nerve (separated from it by the supra-vaginal lymph space) and for the various vessels and nerves which enter the eye, and though nothing more than a part of the common aponeurosis of the orbit, is endowed with remarkable elasticity. Since on reaching the edge of each muscle the fascial cone splits into two layers, one to cover the orbital surface of the muscle and the other to cover the ocular surface, we find on studying a longitudinal section of a muscle that we have to take account of these two layers, as in Fig. 4. The deep layer (D. ) becomes con- tinuous at the equator of the eye (/ C. L.} with the posterior hemi- sphere of Tenon's capsule (P. E. C. ), so that from thence forwards the deep surface of the muscle and its tendon have no fascial investment. When we consider the orbital layer of each muscle-sheath we find the case is not so simple. As it approaches the neighborhood of the globe it thickens, and becomes more closely attached to the muscle itself, till opposite the equator the attachment reaches its maximum ; after that it quits the muscle altogether, though not until it has sent off a prolongation forward over the tendon to contribute to the anterior portion of the globe's investment (A. E. C. ), and proceeds in the form of a thick band (Ext. C. L. ) to the orbital margin. Check Ligaments. The thick band, just spoken of, is not a separate structure, but only a greatly-thickened strip of the anterior part of the fascial cone which we described first of all and which has a continuous attachment all around the circumference of the orbital outlet. Its principal thickened bands are, to use Tenon's words, "singularly supple and elastic," and are called the internal and external "check ligaments." Sappey has described smooth muscular fibers in them close to their orbital attachment. The Globe and Its Socket 21 Anteriorily they are, through the periosteum, rigidly fixed to the orbit : by their posterior extremities they are attached to (a) The outer layer of the sheath of the muscle, which, it will be remembered, is part of the fascial cone described first of all ; (6) The muscle itself through both fibrous and muscular attachments to the belly of the muscle in that region ; (c) To the posterior hemisphere of the fascial investment of the globe (P. E. C. ) by means of a crescentic thickening of the Int. C.L. Int. R.-*^ ^aaBOfao^ ' .Ext. R. D, Fig. 4 Tenon's Fascia (from Motais). Int. C. L. and Eit. C. L. Internal and External Check Liga- ments. Int. R: and Eil. R. Internal and External Kecti. A. E. C. and /'. E. C. Anterior and Posterior External Capsule. /. C. L.- Ifltra-capoular Ligament, or "Collarette." /. C. Internal Capsule. D. Deep Layer of Muscular Sheath. deep layer of the muscular sheath just where it is reflected (/ C. L. ) back on the globe. The check ligament draws on the horns of this fibrous crescent (called by Lockwood the " intra-capsular ligament" and by Motais the " collarette "), past the edges ot the muscle, so that some suppose it to act as a kind of pulley or stirrup over which the muscle works and which keeps it from exert- ing injurious pressure on the globe during its contraction. Uses Of the Check Ligaments. Besides this action, it is evi- dent that the check ligaments, by acting on the posterior hemi- sphere of Tenon's capsule, help to draw the eye forward (like, e. g., the strings of a night cap) against the backward traction of the recti, and in this they are aided by the entire anterior portion of the fascial cone, of which they are only thickenings. 22 Tests and Studies of the Ocular Muscles By their direct attachments to the recti, too, they moderate the power of these muscles on the eye even with respect to the muscular tone in the absence of voluntary contraction. When a muscle con- tracts they act very beautifully, extending in length and acting, no doubt, according to Hooke's law, ' ' ut tensio, sic vis," so as to oppose greater and greater resistance to the further contraction of the muscle, like an elastic "brake." (Compare Figs. 5, 6 and 7.) Merkel* pointed out that when a check ligament is divided, an excessive rotation of the eye is permitted, and to this Motais f has added proof that at every stage of rotation less muscular power is required to produce the same effect on the eye after division of the ligament than when it is intact, so that it is not only a check ligament but also a "moderating agent of the move- ments of the globe during the whole duration of muscular contraction." My thought is that they help to slow off the movements of the eye towards their limits, so as to avoid shock to its contents by sudden arrest or by change of direction of motion. For when a continuous force acts on a moving body (unless the resistance (Motais) The Check Ligaments during . , N ., the primary position of the eye. increases proportionately) there is a constant acceleration of speed. Were the muscles to act in an unrestrained way on the eye, its motion would be far more rapid at the end than at the beginning of each movement. Owing to the provision made against this, I think that the motion rather slows off as the limits of mobility are reached, a greater and greater resistance being interposed. Indeed, we may think of the internal and external recti as each possessing two tendons of insertion one inextensible tendon attached to the globe, and another highly extensible tendon attached to the orbit, like the two limbs of a letter Y. When the stem of the Y, which represents the belly of the muscle, contracts, let us see what happens. At the commencement of a muscular contrac- tion almost all the force acts on the globe, through the inextensible limb of the Y, namely, the tendon, since the extensible one (the *Graefe und Saemisch, Band I., p. 59. t Motais : " Anotomie de 1'appareil nioteur de 1'ceil." The Globe and Its Socket 23 check ligament) offers but little resistance, and thus the early part of the movement of the eye takes place with that velocity which is so valuable for the requirements of life ; but as the motion con- tinues, more and more of the force is transferred from one limb of the Y to the other, till at last it nearly all acts on the rigid bone of the orbit. The eye is thus preserved from the development of excessive kinetic energy, which, it will be remembered, varies not as the speed merely, but as the square of the speed. It is also preserved from excessive traction on its coats, which might distort it ; and the limitation of its arc of mobility is determined rot so much by Fig. 6 (Mot a is) The Check Ligaments dur- ing partial contraction of the Ext. Rectus muscle, the Int. Check Lig- ament being in a state of maxi- mum relaxation, and the Ext. one being somewhat stretched. Fig. 7 (Motais.) To show how, during full contraction of the Ext. Rectus, the Ext. Check Ligament is stretched to its maximum length, and the Int. is slightly stretched also. impediments acting against the eyeball itself as by the restraint imposed on the acting force. This is a perfect arrangement. The maximum extensibility of a check ligament is 10 to 12 mm. (Motais), and this exactly agrees with the known shortening of a muscle which would require to produce a maximum excursion of the eye, say, of 45 or 50. The excursion does not cease because the eyeball itself is not capable of a more extreme rotation, nor yet because the muscle has attained its maximum contraction. Of these two statements, the first is proved by the fact that division of the check ligament allows a super-physiological effect on the eyeball* (Merkel), and the second by the fact that the con- traction of the rectus required for a maximum physiological excur- sion of the eye is scarcely more than a quarter of its length, while * Graefe und Saeruisch, Band I., 1874. 24 Tests and Studies of the Ocular Muscles it is known that striped muscles are generally in their maximum contraction shortened by half their length. Motais pointed this out. Internal Capsule Of the Globe. This thin, transparent mem- brane, which is represented in Figs. 8 and 12, is lined on its sclerotic side by endothelial cells, and envelopes the whole eye as far forward as the insertions of the tendons. By its outer surface, it is attached to the more resistant outer capsule. Underneath the tendons of the muscles, however, it runs A.E.C. LL .(*, .L-* .....:. _, '..-^i.-L j&SS Fig. 8 A horizontal section of the Globe and its Membranes (after Molais). E. C. L. and /. C. L. External aud Internal Check Ligaments. A. E. C. and P. E. C. Anterior and Posterior portions of the External Capsule. /., /'., /. C., /. C'. Reflection of Internal Capsule on the tendon. ti. B. Serous Bursa. forwards to the insertion of the tendon, and is then reflected back- wards on to the under surface (/ C. ) of the tendon and muscle, as far as the collarette, and mounting round its edges to gain its upper sur- face, it there forms a serous bursa (S. B. ) which is elongated antero- posteriorly, and partitioned inside by filaments of cellular tissue. Between the inner capsule and the sclerotic is Tenon's space, which is really a finely-divided multilocular lymph sac. It is in principle the subarachnoid space of the eye, since the inner capsule represents the arachnoid mater, and the outer capsule the dura mater, of the globe. These peri-ocular membranes are indeed continuous with the arachnoid and dural envelopes of the brain, though the continuity cannot always be demonstrated. There are 25 thus two sheaths on the optic nerve, of which the inner is con- tinuous anteriorly with the internal capsule of the globe and poster- iorly with the arachnoid of the brain, and this sheath can be distended by injection from the sub-arachnoid space ; the outer sheath of the optic nerve is continuous anteriorly with Tenon's outer capsule of the globe, and posteriorly with the dura mater of the brain. Injections practiced into Tenon's space, by Schwalbe and others, show that it communicates with the supra-chorioidal space, through the openings of the sclerotic which transmit the vasa vor- ticosa.* In the operation for strabismus, it is customary to pick up Tenon's capsule in the forceps, together with the conjunctiva, and as soon as the scissors have entered within the capsule their points move freely within the space, for the fine filamentary and areolar tissue which partially occupies it (" tunica adventitia oculi ") offers almost no resistance to their movement. Hemorrhage may take place into this space from wounding one of the muscular arteries and may sometimes envelope and protrude the whole of the posterior two- thirds of the globe. In certain conditions of health the presence of liquid in Tenon's space can be demonstrated by pressing the eyelid against the globe from the equator forwards. Tenon' s space contains scarcely any free lymph, or its pressure would be noticed during the operation for strabismus. The numerous delicate fibrous connections which exist between the inner capsule and the sclerotic, remind one, though much finer, of the loose cellular tissue between the occipito-frontalis tendon and the peri-cranium, and no doubt play a part in the motions of the eyes, which is very similar to that which the sub-tendinous tissue plays during contraction of the occipito-frontalis. Movements of Tenon's Capsule. From what we have now studied of Tenon's capsule, we may see at once that it differs entirely from such a bony socket as that of the acetabulutn, since being fixed to the globe near the cornea, and loosely so at the optic nerve, it accompanies the motions of the globe to a large extent. Not entirely, however, except just in front, for its elasticity allows it to " give " in some places more than in others. Motais has shown by careful experiments that the fatty tissue which immediately surrounds the globe, also to a large extent accompanies its movements, and every succeeding layer moves less * One observer has denied that Tenon's space is a lymph space, on the ground that he could not find an open space at all ; but this is no proof, since the same might be said of the lymph spaces in ordinary areolar tissue. 26 Tests and Studies of the Ocular Muscles than the one within it. He suggests that the real socket is formed by the inside of the eyelids, since they move least in accordance with its motions ; but the fact of the matter is that the eye is an organ sui generis, and must not too closely be compared to a bony joint. When we remember the elastic nature of its connections, it strikes us as exceedingly well poised that its center of motion should remain so stationary while acted on by such various muscles. Why the External Check Ligament Should be Thicker and More Powerful than any other, is not at first sight evident, but it may possibly be explained by the fact that the ocular muscles all rise nearer the median plane than they are inserted, and unless some special provision existed, the eyeball as a whole might be drawn too much inwards. As it is, however, it is poised in the aponeurotic funnel, or fascial cone which extends from the margin to the apex of the orbit, and the outer part of this funnel between the globe and the orbit is endowed with greater strength than any other portion. Bearing of the Check Ligaments on Tenotomy. Motais called attention to the way in which the check ligaments affect the result obtained by a tenotomy, and the following are almost his own words : " The tendon, say, of the internal rectus, is cut. Immediately, by its tonicity alone, the muscle retires backwards, drawing the tendon with it, it may be, let us say, 5 mm. "The check ligament adher- ing on one hand to the muscle, and fastened on the other hand to the orbit, can only lend itself to the retreat of the muscle by elongating 5 millimeters (compare Fig 9)- "Henceforth, therefore, in consequence of the new anatomical conditions introduced by the tenot- omy, the check ligament, during muscular repose, \v\\\ already be ex- periencing an elongation of 5 mm. But we know that its maximum elongation is not greater than 10 to 12 mm. It has, consequently, only 5 to 7 mm. of further lengthening at its disposal, during muscular contraction. There Fig. 9 (Molais.) To show the effect of Tenot- omv, M being the Muscle before, and M' after, the operation. O-Lis the Check Ligament before opera- tion, and C L' the same elongated after operation by recession of the muscle. The Globe and Its Socket 27 results a proportionate insufficiency of adduction, a diminution in the arc of rotation. ' ' But that is not all. The tension of the ligament, feeble at the commencement of elongation, gradually increases. The more it elon- gates, the greater becomes its tension, the more energetic becomes its resistance to muscular action. ' ' If the tenotomy has already produced a lengthening of the liga- ment 5 mm., the muscle from the beginning of its contraction will be restrained by a ligament already considerably stretched. Its con- tractile power will, by just so much, be lessened. ' ' Therefore, we shall have at once from the ligament, diminu- tion of the extent and of the energy of the muscular action. In advancements, the same phe- nomena occur in the opposite sense. "The ligament is advanced at the same time as the muscle. In its new position its two points of orbital and muscular insertion be- ing brought nearer to each other, it is of course relaxed (Fig. 10). "If the advancement be 3 mm. the ligament will not reach its maximum extension till 3 mm. later, if I may so express it. "Further, the ligament, being completely relaxed at the beginning of contraction, during the first three millimeters will be slack to resist the muscular contraction. We shall have, therefore, at the same time, an increase of the extent and of the energy of the muscular action."* All the muscles of the globe seem to be provided with some- thing answering to check ligaments. The following account of them is taken chiefly from Motais : External Check Ligament (Aileron ligamenteux externe). This is shown as Ext. C. L. in Fig. 4, and E. C. L. in Fig. 8, and is a thick, grayish-w r hite band, which leaves the external rectus *" L'appareil moteur de 1'oeil " (1887), pp. 147 to 150. Kig. 1O (Motau. ) To show the effect of Ad- vancement, M being the Muscle before, and M ' after, the operation. CL is the Check Ligament before the operation, and C L' the same relaxed after operation. 28 Tests and Studies of the Ocular Muscles muscle near its anterior extremity, proceeding forwards and slightly outwards, continuing in very nearly the same direction as the belly of the muscle, to the outer margin of the orbit. Its mean breadth is 7 or 8 millimeters ; its length from the farthest back point of its adherence to the muscle is from 1 8 to 20 millimeters. Its greatest thickness, which varies between 3 and 6 millimeters, is at its orbital insertion. These are the figures given by Motais. He adds that it is not formed of a compact bundle, but of a great number of compact fascicles, some of which are very thin. In its posterior two-thirds it is composed of a mixture of fibrous and elastic tissue : in its anterior third M. Sappy discovered numerous smooth muscular fibres. The sheath of the external rectus muscle, thin and cellular at the back of the orbit, becomes more and more compact as we trace it forwards along its belly. In its posterior two-thirds it is loosely attached to the muscle. But, all of a sudden, about 20 millimeters from the sclerotic insertion of the muscle, it thickens considerably and plants itself on the muscle so firmly that in detaching it we always tear some of the muscular fibres. These adhesions extend forwards 5 or 6 millimeters. The muscle then changes its direction to incline inwards towards its sclerotic inser- tion. The check ligament, instead of following the curve of the muscle, abandons it, at an angle which varies according to the position of the globe, to reach the margin of the orbit, where its insertion has a breadth of 7 or 8 millimeters, and a depth of 3 to 6 millimeters. The upper border of the ligament is reinforced by a band from the superior check ligament.* Internal Check Ligament (Aileron ligamenteux interne). This is shown as Int. C. L. in Fig. 4, and / C. L. in Fig. 8, and is broader, but thinner, than the external check ligament. It has no interstices like the latter. Its color is a yellowish gray, and near the orbital margin a pale red. Though the prominence which forms it is much less differen- tiated from the neighboring parts of the aponeurosis than that of the external ligament, it can easily be distinguished when it is put on the stretch by drawing on the internal rectus muscle behind. Its breadth is from 8 to 10 millimeters. , Its length from the pos- terior extremity of its attachment to the muscle to its bony insertion * Panas says that between this check lipament and the corresponding palpebral ligament, and the suh-conjuiictival fascia, there exists a space containing fat, and the small accessory lachrymal gland, which, he says, we constantly discover ill the operation of cauthopla^ty. The Globe and Its Socket 29 " along the line of the crista lacrimalis posterior and the wall of the orbit just posterior to this line" (Howe), is from 15 to 18 millime- ters. Its thickness is from i to 1^2 millimeters, near to its bony inser- tion. In very fatty orbits, if the muscles are atrophied, it is the least distinct of all the check ligaments. Panas says that it is fused (by an expansion which covers Horner's muscle) with the internal palpebral L.P. LSt#. 0. R. Ext. R S.R. Inf. R. (After Muiii MM ligament, so that when the right internal rectus contracts, it draws back the inner commissure of the lids, the semilunar membrane and the caruncle, at the same time that it compresses the lachrymal sac. Superior Check Ligaments. These are two. Owing to the broad tendon of the levator palpebrae being interposed between the superior rectus and the orbital margin, the superior check ligaments from the superior rectus cannot reach their orbital insertions except by passing each border of the levator palpebrse tendon. Were there a single median check ligament, it would have to pierce this intervening tendon to reach the bone (see Fig. n). 30 Tests and Studies of the Ocular Muscles That this affords an explanation for there being two superioj check ligaments is shown by the fact, noted by Motais, that those vertebrata which possess a levator palpebrae have these ligaments double, whereas those which possess no levator have a single median ligament. 1.0. l.L. Fig. 13 I.E. (After Motate). Vertical longitudinal section of Eyeball and adnexa. R. Reflexion of fascia from the under surface of the Levator Palpebne on to the upper surface of the Superior Kectus. C.- Part of the funnel-shaped expansion proceeding to the margin of the orbit. /".Expansion to contribute to the anterior part of the capsule. /. R. Inferior Kectus. 1. O. Inferior Oblique. /. L. Check Ligament of the Inferior Rectus, embracing the Inferior Oblique. He describes the internal superior check ligament (/ S.) as a fibrous cord which leaves the inner border of the superior rectus muscle, applies itself to the tendon sheath of the superior oblique muscle, and is inserted with it at the trochlea. Sometimes a few muscular fibres run into it, and in any case it is intimately adherent to the muscle, just as the internal and external check ligaments are to theirs. The external superior check ligament (E. S.~) is a more flattened band than the preceding cord, and divides into two, one L.I.R.. portion joining the upper border of the external check ligament and the other portion reaching the orbital margin midway between this and the outer extremity of the tendon of the levator palpebrae. The Inferior Check Ligaments are also two, but are a little difficult to understand. That of the inferior rectus leaves the sheath and belly of the inferior rectus at the point where that muscle begins to curve round the globe, adhering intimately to the muscle and to its thickened shealh, for a length of 5 or 6 millimeters (see / L. in Fig. 12). It proceeds to the middle part of the inferior oblique muscle (/ O. ), splitting into two so as to embrace it, as shown in the figure, establishing thus a strong connection between these two muscles. Its appearance is whiter and its structure more ex- clusively fibrous than that of the other check liga- ments, so that, with a moderate thick- ness, it is as resis- tant as the external check ligament itself (Motais). It obtains no direct Int. insertion into the margin of the orbit, but only through a loop, of which one (Motais.) View of the under surface of the Eyeball, the floor of , . the orbit being removed to show the Check Ligament (L.O. ) limb IS formed by of the Inferior Oblique muscle (/. O.). These form two . . limbsof a Y, the stem of which isthe Check Ligament (L.I.R.) the inferior Oblique of the Inferior Rectus {Inf. R.), seen to embrace the Inferior , ? Oblique. Int. R. Internal Rectus. S. R. Superior Rectus. muscle and the other limb by the check ligament of that same muscle. This is shown in Fig. 13. For this reason it. is that Fig. 12 looks as if the check ligament of the inferior rectus had no insertion except its slender offshoot to the eyelid, which, however, accounts for the eyelid being drawn down during contraction of the inferior rectus. We have seen that it embraces the inferior oblique muscle, in Figs. 12 and 13. From the point where it does so, springs the structure we have next to consider. 32 Tests and Studies of the Ocular Muscles The Check Ligament of the Inferior Oblique. This fibrous bundle (L. O. in Fig. n), derived in part from the fibres of the check ligament of the inferior rectus, in part from the sheath of the inferior oblique muscle, leaves the anterior border of the inferior oblique about 8 or 10 millimeters from its orbital origin, and from thence courses obliquely outwards and forwards. It forms an obtuse angle of about 120 with the check ligament of the inferior rectus muscle. With the inferior oblique it forms an angle of about 110. Its length is from 10 to 12 millimeters, and it is inserted into the lower outer angle of the orbit, 4 or 5 millimeters behind the orbital margin,- about midway between the external check ligament and the origin of the inferior oblique. This bundle is the most pearly looking, the most purely fibrous of all the aponeurotic lamellae of the orbit. Its breadth varies at different parts of its course : in the middle, 2 or 3 millimeters ; at its muscular insertion, 7 or 8 millimeters ; at its bony insertion, 5 or 6 millimeters. It presents, therefore, the shape of two triangles united by their apices. Motais thinks it acts not only as a moderator for the inferior oblique, but also as a "pulley of reflection."* Together with the inferior oblique itself, it forms a kind of musculo-aponeurotic loop, the two ends of which are inserted near the orbital margin, one at the outer angle, the other at the inner angle. And the check liga- ment of the inferior rectus muscle embraces the middle portion of this loop, so that when the inferior rectus begins to contract, its check ligament stresses the loop. The check ligament of the inferior rectus has therefore for its orbital insertions the tendon of the inferior oblique muscle, and the check ligament of the same muscle like the two limbs of a Y. The Connection of the Levator Palpebrae Muscle with the Superior RectUS deserves a passing notice, since these muscles work so uniformly together. From the upper surface of the sheath of the superior rectus, near its inner border, and along its whole length from the apex of the orbit to the equator of the eyeball, is given off a sheet of fibro-cellular tissue, which reaches the under surface of the levator. and splits into two to enclose that muscle, thus providing it with a sheath. * By this he must mean that when the muscle contracts, the ligament slightly bends the muscle by drawing its middle part outwards, so as to make its traction on the eye a little less oblique. The Globe and Its Socket 33 On reaching the equator, however, the upper surface of the sheath of the superior rectus is reflected {R in Fig. 12) as a whole on to the under surface of the levator, describing a strong curve, with concavity backwards, in its passage from one muscle to the other. A prolongation (P), however, still continues to cover the outer or upper surface of the tendon of the superior rectus, and forms, in fact, part of the anterior hemisphere of the external capsule of the eye continuous with A E C in Fig. 4. From the upper surface of the levator is given off a facial layer (C), which goes to the orbital margin, and forms part of that facial cone in the orbit which we commenced this whole subject by describing. Notice, too, that the nerve for the levator penetrates the superior rectus. CHAPTER II The Ocular Motions A universally mobile body is capable of no fewer than six inde- pendent motions, which are called " degrees of freedom." It can be translated as a whole in any three directions at right angles to each other, or be rotated about any three axes at right angles to each other. Translations Of the Globe. If we regard the head as fixed, and confine ourselves to the study of the voluntary motions of the eye- ball, we shall find it approximately true that translation of the globe is forbidden in virtue of its attachments to the orbit. Were we to investigate this statement very strictly, we should not, however, find it rigidly true, since the center of motion lies a little farther back than the geometrical center of the eyeball, in con- sequence of which the globe is slightly translated in whatever direc- tion the eye is made to turn. On looking to the right, the globe is translated slightly to the right ; on looking to the left, to the left, and so on. In the maximum excursions of the eye, this translation is probably not less than I, or greater than 2 millimeters. Center Of Motion. The distance between the mid-point of the optic axis and the center of rotation is given by Bonders and Mauthner as follows : R EFR ACTION DONDERS MAUTHNER In Krnmetropia i 77 mm. 1.24 mm. " Myopia 1.75 mm. 1.82 mm. " Hypermetropia 2.17 mm. 1.47 mm. Since, except to the trifling extent just noticed, translation is denied to the eye, we may now turn our attention to its rotations. Rotations. A body deprived of translation might still be able to rotate, and that about three axes at right angles to each other. Rotations about all other axes are resolvable into rotations about The Ocular Motions 35 two or more of these, from which it follows that a body which enjoys three degrees of rotational freedom can rotate about as many diameters as are conceivable. We have, therefore, next to inquire whether the eyeball retains this full rotational freedom. One Voluntary Rotation Denied. Actual experiment has shown, what we could not have otherwise proved, that one degree of free- dom is lost in all voluntary parallel movements of the eyes which start from the straight forward position.* The degree of freedom lost is that of rotating about the fore- and-aft axis (considered as fixed in the head), while the two free- doms retained are those of rotation about the vertical axis, and about the transverse axis (both considered as fixed in the head). Listing's Plane. Simultaneous rotations about the vertical and transverse axes can be variously compounded into rotations about any intermediate axis. This is equivalent to saying that they are limited to rotations about all conceivable diameters in one plane, namely, that plane in which the vertical and transverse axes lie, and which it is convenient to call " Listing's plane," since this degree of constraint was discovered by Listing. Listing' s plane passes through the center of motion of the eyes, and is a vertical transverse plane (corresponding to a coronal section) fixed in the head, and perpendicular to the fore-and-aft axis, about which rotation is denied, f When the head is held erect and the eyes look straight forward at a very distant object on the horizon, they are gener- ally said to be in their "primary position," and though we shall have to quote a truer definition later on, we may for the present accept this simple one, in order to say that however many and complex the motions of an eye may be in glancing from point to point, the ultimate result of them all is equivalent to a single rota- tion of the globe about some one axis in Listing's plane, provided the eye has started from the primary position. J Torsion. By torsion we mean rotation of the eyeball about its own fixation line. * Latent Torsion, discussed in Chapter XIII., is not voluntary. tin the "primary position" of the eye, Listing's plane is practically identical with the "equatorial plane" of the eye, but it must not be identified with it, since the latter moves with the eye, whereas Listing's plane does not. Jit will he seen that I have guarded myself from stating that rotations from one secondary position to another are about axes in Listing's plane. They are not. Helmholtz has correctly shown in what plane they lie. " We will call torsions rotations of the eye about the line of fixation " ( Helmholtz) . 36 Tests and Studies of the Ocnlar Muscles Let us remember that there are two fore-and-aft axes we have to consider, one of which is fixed in the head and which we have already treated, and another proper to the eyeball itself and moving with it, so as, indeed, to be for all practical purposes regarded as identical with the fixation line. Secondary Torsion. When the eyeball (starting from the pri- mary position) rotates either vertically upwards or downwards, or horizontally to either side, its motions are called "cardinal motions," and are not accompanied by torsion. But when the eyeball looks obliquely, in any intermediate direction, two cardinal motions are compounded together. Every motion of an eye from the primary into an oblique position is accompanied by torsion as an essential component of the motion. Bonders' Law. Bonders' observed that whatever position the eyeball may take, there belongs to that position a definite amount of torsion which remains the same no matter how often the eye may return to that position, and however many motions it may make in arriving at it. To quote his own words : "For any determinate position of the line of fixation with respect to the head, thereto corresponds a determinate and invariable angle of torsion, a value independent of the volition of the observer, independent also of the manner in which the line of fixation has been brought into the considered position. ' ' The same law has been put more concisely by Helmholtz (and at the same time amplified) in the words : " The wheel-movement of each eye is, with parallel fixation lines, a function only of the elevation angle, and of the lateral deflection angle."* Listing's Law. The law of Listing goes a step further than that of Bonders, and is as follows : " When the line of fixation passes from its primary to any other position, the angle of torsion of the eye in /his second position is the same as if the eye had arrived at this position by turning about a fixed axis perpendicular to the first and second positions of the line of fixation.'' 1 This simply means that when the eye starts from the primary position and glances towards an object situated obliquely (^. g. , up and to the right), the line of fixation takes the shortest possible cut to its new direction, and in so doing must necessarily sweep along a plane common to its original and its new position. To "Helmholtz's " Optique Physiologique," page 602. The Ocular ^fotions 37 permit it to do this the eyeball must rotate around an axis perpen- dicular to this plane and, therefore, perpendicular to the line of fixation throughout the whole of its motion. Since the shortest cut requires the briefest time to traverse, it is manifest that this law is essential to the perfection of the ocular movements where rapidity is so advantageous. The exquisiteness of this design is apparent when one considers that no fewer than three muscles are concerned in every oblique motion of the globe, not one of which, acting individually, would rotate the eye about the required diameter. Reasons for Listing's Law. I think a little consideration will show that the arrangement on which Listing's law is based is that which entails the absolute minimum of motion (calculated as the sum of the motions of all the particles of the eye), so that (#) the momentum of the ocular contents is the least possible ; (<5) the time is the shortest ; (V) the work done the least, and (^) the lowest amount of dangerous "kinetic energy" is developed. A second, more important, reason is, that by no other equally efficient arrange- ment could the law of Donders be possible, since the torsion belong- ing to each position of the eye would not be a constant quantity, and this would throw the brain out in its calculations. Proof Of Listing's Law. The truth of this law has been con- firmed (within the sphere in which it holds good, namely, that of the parallel motions of the eyes) by every observer that has under- taken to test it by actual observation. It is desirable to study the motions of the eyes before com- mencing to consider the muscles by means of which they are brought about. This will save us from falling into errors from failure to distinguish between motions actually observed and those which our preconceived notions of the oblique muscles might make us think ought to take place. A most delicate means of following the para/lei movements of the eyes, attributed to Rente, is afforded by the experimental use of ' ' after-images. ' ' The following mode of inquiry thereby is that most to be recommended : Let the experimenter affix vertically a scarlet ribbon, two or three feet long, to a gray wall, with the center of the ribbon at the same height as his own eye, when seated at some distance there- from, and with his head erect and squarely facing the wall let him gaze at the center of the ribbon for a minute or two. On now raising his gaze directly upwards, while keeping his head 38 Tests and Studies of the Ocular Muscles immovable, a faint after-image will move upwards with the eyes, but will remain strictly vertical. On lowering his gaze, the after-image will sink simultaneously but still remain vertical. If, however, after raising his gaze he were to turn his eyes to the right, the after- image would no longer remain vertical, but would slope to the right ; on looking up to the left, it would slope to the left ; on looking down to the right its upper end would again slope to the left, and on looking down to the left its upper end would slope to the right. We might conclude from this that when the eyes occupy oblique positions they experience torsion equal to that of the after- image. But since those parts of the wall upon which the image falls in these positions are not perpendicular to the line of sight, the slope is exaggerated and the proof is not complete. To vary the experiment, commence again, but with the head rotated considerably to the left and kept immovably so. Now, after gazing at the ribbon, run the eyes up the wall immediately above it, when the image will appear to become more and more twisted to the right the higher it is raised. This proves infallibly that torsion does take place on looking up and to the right, though the amount is less than the previous experiment would have led us to suppose. Similar experiments could be made for the other oblique positions of the head, which show that torsion to the right occurs on looking upwards and to the right, or downwards and to the left. On the contrary, torsion to the left occurs on looking upwards and to the left, or downwards and to the right. Even this experiment, however, though it correctly indicates the presence of torsion and the true sense in which it occurs, does not enable us to measure it exactly, because the after-image is pro- jected on a plane which is not perpendicular to the visual line in the secondary position. To rectify this, let a gray drawing board be suspended near the ceiling, directly above the scarlet ribbon, and be provided (according to a method of Le Conte's) with a large knitting needle projected perpendicularly from its center. Now make the drawing board lean forward, like a picture, until this needle is seen "end on" by the experimenter. On looking at the needle, the after-image is projected on to the board in a manner which represents the exact torsion of the eye. It can, if we like, be measured by a long wire so attached by its middle to the foot of the knitting needle as to rotate against the board to any required angle. If this wire be inclined by an assistant till it exactly coincides with The Ocular Motions 39 the after-image, its deflection from the vertical exactly measures the torsion of the eye. Now let him place the ribbon horizontally on the wall, and holding the head erect as at first, with face square to the wall, gaze at it steadily, and then move the eyes horizontally to the right. The image will now, in most cases, appear slightly tilted, with its right-hand end dipping. On turning the eyes to the left, the after- image will appear tilted in the opposite direction, the left-hand end extremely dipping. If the face be turned somewhat downwards, the tilting of the image, on looking to the right or the left, will be greatly increased ; while, on the other hand, if the face be raised towards the ceiling, the sense of the tilt will be reversed : on looking to the right the left-hand end dipping and on looking to the left the right-hand end. This fully confirms the results of the previous experiment and shows that whether we study the vertical meridian of the retina or the horizontal meridian, torsion takes place in the directions indicated. If, however, the head be thrown only slightly backwards, a position is gained from which, when the eyes look to the right and left, the image no longer tilts, but remains strictly horizontal. The eyes while looking at the center of the ribbon are now said to be in their "primary position," which is defined physiologically as that from which motions of the eyes in directly vertical or directly horizontal directions are unaccompanied by torsion. On glancing, however, in oblique directions therefrom, torsion occurs.* After providing the head of the experimenter with some means of fixing it with the exact amount of backward tilt, which brings the eyes into their primary position, the ribbon may be fixed obliquely on the wall, at an angle of, say, 45, or any other. "What- ever the angle may be, on turning the eyes in the direction indi- cated by the length of the ribbon, the after-image will be found to remain in a straight line with the ribbon in all parts of its course. This proves that parallel movements of the eyes from their primary position, in no matter what direction, take place as if about axes at right angles to each line of fixation while in the primary position. The experimenter may, if he please, reach any point on the wall by a circuitous or even spiral route, but he will always find, at the end, that the after-image occupies exactly the same position as * Strictly speaking, the ribbon should be at an infinite distance ior the definition to be true, since only then would the visual lines bu parallel. 40 Tests and Studies of the Ocular Muscles if the eyes had been moved to that position directly from the primary position. By experiments of this kind, Bonders arrived at his dis- covery of the law that "to every position of the fixation line with reference to the head belongs a definite and unchangeable value of torsion." On this law of Donders, everything else rests. The laws of Listing and Helmholtz, to be described shortly, are necessary corollories. Listing's law relates only to those parallel movements of the eyes which have the primary position for their point of departure, and states that the position of the eye in any secondary position is what it would gain by a rotation from the primary position about a fixed axis perpendicular to the primary and secondary positions of the fixation line. All axes of this kind must lie in one plane, viz. , that which is perpendicular to the fixation line in its primary position, and which has been called, in consequence, Listing's plane. Suffice to say that any linear after-image, which possesses, during the primary position of the eye, a given obliquity, when projected on a gray wall facing the observer, preserves the same obliquity inviolate whenever the eye glances in the direction indicated by the length of the false image, or in a direction per- pendicular to its length.* Though all observers are agreed as to the truth of Listing's law, all are not, however, agreed as to the conclusions to be drawn from it. Agreement of Helmholtz and Donders. A good deal of dis- cussion has been made recently about the discrepancy which has been stated in America to exist between the laws of false torsion formulated by Helmholtz and those laid down by Donders. There can be no question that their statements, as they read, look diametrically opposed to each other. And yet a careful study of Helmholtz will show that he has chosen a different definition and index of torsion, so that his state- ments do not really contradict those of Donders, but perfectly agree, as indeed we could only have expected. *It is true that when the eye glances in oblique directions other than these, the after- image does appear to have its degree of obliquity altered, but this is fully explained in every case by the fact that it is projected on a flat surface which does not (in that part of it) face the observer, and the apparent discrepancies, when properly analyzed, only confirm the law. The Ocular Motions Let us look at them in the following parallel columns : HELMHOLTZ. 1 When the plane of fixa- tion is directed upwards, lateral displacements to the right make the eye turn to the left ; BONDERS. " On the diagonal fixation upwards and to the right, the vertical meridians of both eyes suffer a parallel inclina- tion to the right." and displacements to the left make the eye turn to the right." "On diagonal fixation upwards and to the left, the vertical meridians of both eyes suffer a parallel incli- nation to the left." " When the plane of fix- ation is lowered, lateral dis- placements to the right are accompanied by torsion to the right "On diagonal fixation downwards and to the right, the vertical meridians of both eyes suffer a parallel inclina- tion to the left." and vice versa.'" " On diagonal fixation downwards and to the left, the vertical meridians of both eyes suffer a parallel inclina- tion to the right." HelmholtZ's Plane Of Reference. The "plane of reference" adopted by Helmholtz is the "visual plane," by which he means the plane common to the two visual axes and to the line which joins the centers of motion of the two eyes. When the visual axes are elevated or depressed, the visual plane is elevated or depressed with them. In the primary position of the eyes, the visual plane passes through the horizontal meridian of the retina, which Helmholtz calls the "retinal horizon." In all the cardinal motions of the eyes, which, it will be remembered, are motions from the primary position directly upwards, downwards, to right and to left, the retinal horizon lies rigorously in the visual plane ; but in oblique motions it becomes more and more inclined to the visual plane, in the sense stated by Helmholtz in the first of the above parallel columns. Bonders' Plane of Reference. I do not know what plane of reference Donders selected, but (since the one which I have selected gives the same results) probably the same as that which 42 Tests and Studies of the Ocular Muscles I have adopted in what follows, namely, a movable, ever-vertical plane, passing through the line of fixation and moving with it. No Torsion with Reference to the Median Plane. Were we to estimate torsion by reference to the median plane of the head, or any plane parallel to it, we would have to conclude there is no torsion at all, for, thus tested, the vertical meridian of the cornea would be torted in one direction and the horizontal meridian to an equal amount in the opposite direction. Indeed, it stands to reason that motion about any axis in Listing's plane cannot have a component about a line perpen- dicular to that plane. The nature of false torsion depends entirely upon the point of view from which we observe it. Index Of Torsion. Since the eye is an optical instrument, I think the index of torsion should be an optical one, and, to my mind, the best plan is to imagine the point of fixation, or, in other words, the object looked at, to be an intelligent being, able to tell us what amount of torsion exists from his point of view. The torsion would thus be measured by the angle between the originally vertical meridian of the retina (i. e., the meridian which was ver- tical in the primary position of the eye) and the vertical plane pass- ing through the line of fixation. When calculated in this way, the rules of false torsion agree exactly with those of Bonders, and therefore with all the text-books which have followed him. Let us give the name of Dextrotorsion to that which takes place when the upper end of the vertical diameter of the eyeball is tilted to the patient's right, and Laevotorsion to similar tilting to the left. When we look upwards and to the right or downwards and to the left, there is dextrotorsion. Conversely, when we look upwards and to the left or downwards and to the right, there is laevotorsion. In fact, the paths of the after-images trace out a figure shaped like a sheaf of wheat. Torsion Calculator. I have constructed a simple little model, which, though difficult to describe on paper, makes it very easy to demonstrate the true nature of secondary torsion and even to indi- cate automatically its amount in degrees for any oblique motion whatever of an eye. In its home-made form it consists of a circular piece of card- board (shown in No. i of Fig. 14), with a vertical diameter V V The Ocular Motions 43 Fig The author's torsion calculator. No. 1. The eye in its primary position. No. 2. The eye looking up and to the right, showing equal and opposite inclinations of the horizontal and vertical meridians with reference to the fore-and-aft axis of the hend. No 3. The eye looking as in No. 2, but showing equal similar inclinations of both meridians to the patient's right with reference to the fore-and-aft axis of the eye. No. 4. Mode of reading same. 44 Tests and Studies of the Ocular Muscles drawn upon it and fastened to a knitting needle R R which lies against some oblique diameter, about which it can rotate as axis. The cardboard is transfixed through its center by another knitting needle /perpendicular to its plane and not, therefore, visible in No. I except as a round spot (being seen foreshortened) at /. From the extremity of this needle (compare / in No. 2) is suspended a small weight W by a fine thread. The cardboard is to represent the equatorial plane. The thread-bearing needle represents the "line of fixation," and is, therefore, perpendicular to the plane, under all circumstances. Mode Of Use. First adjust the whole as in the primary position of the eye (No. i, Fig. 14). The cardboard will be in a vertical transverse plane and the line of fixation will look straight forward. Let the anterior extremity / of the thread-bearing needle repre- sent the "point of fixation," and the thread itself be the ever- vertical line passing through the fixation point. Now, let the observer hold his eye so as to be in a line with the thread-bearing needle ; its extremity will then hide its length from view, and the appearance will be that presented in No. I. The thread will appear to coincide with the vertical meridian V V of the cardboard, showing that there is no torsion. To proceed to the next step : since any oblique motion of the eye from the primary position must take place about an axis in Listing's plane, let R R be that axis, and rotate the card, as in No. 2, just, as the equatorial plane of the eye would be rotated actually. Let us, for instance, make the eye look upwards and to the (patient's) right, as in the figure. The thread-bearing knitting needle, which represents the line of fixation, now, therefore, points upwards and to the right. While it continues to do so, notice that, if your own eye is still in the same position as before, the appear- ance presented is that of No. 2. From this point of view, the vertical meridian V V no longer appears parallel to the vertical thread, but slopes towards the patient's left, as if to indicate laevotorsion of the eye. The horizontal meridian, on the other hand (h //), appears tilted from the horizontal in exactly the opposite direction, as if to indicate dextrotorsion of the eye. Moreover, the apparent laevotorsion indicated by the tilt of the vertical meridian is exactly equal to the apparent dextrotorsion indicated by the tilt of the horizontal meridian. The Ocular Motions 45 From this point of view, therefore, namely, from directly in front of the patient's face, there is no torsion whatever. This proves what we have already said, that the eye is deprived of one degree of freedom, and that rotation about any axis in Listing's plane cannot have a component about a line perpendicular to that plane. Are we to conclude that there is no false torsion, then ? By no means ; but we must look at the eye from the true point of view to see it, namely, along the length of its own visual axis, as already specified. Let us do so. Leaving the model as in No. 2, let the observer move his own eye till he looks along the line of the knitting needle /, and now it is evident that both meridians tell the same story, for they are perpendicular to each other and both indicate that the eye is dextrotorted. This is illustrated in No. 3. The amount of dextrotorsion can be read off from the graduated arc on the cardboard disk by seeing what degree appears crossed by the weighted thread. To facilitate doing this, the eye may be held lower down in the same vertical plane, taking care, as in No. 4, to keep the thread in apparent coincidence with the needle it hangs from. A pretty way of demonstrating to others is to hold a strong light in such a position as to make the shadow of the thread pass through the center of the card, as in No. 4. This linear shadow will then record the amount of torsion on the scale. The light should be held on a slightly lower level than the center of the cardboard disk. For greater accuracy, a rather better-made apparatus is de- sirable. In my own model the card is replaced by a graduated circle, of some white material, like ivory ; and is pivoted at its center to the oblique axis, so that this can be brought to coincide with any required diameter, and the axis itself is, by a graduated half- circle, capable of adjustment to any required degree of obliquity. Moreover, the degree of rotation imparted to the cardboard disk is registered by a small graduated circle (S S in No. i) fixed on the oblique axis perpendicular to it. If accurately made, such a torsion calculator at once tells us the amount of secondary torsion which belongs to any motion of the eye from the primary position about no matter what axis or to what extent. 46 Tests and Studies of the Ocular Muscles A very little experimenting witu this apparatus will easily show the reader that if Listing's law be true the following facts are also true for binocular parallel movements of the eyes : (a) Fixation upwards and to the right is accompanied by parallel dextrotorsion. (^) Fixation downwards and to the right, by parallel laevotorsion. (f) Fixation upwards and to the left, by parallel laevotorsion. (a?) Fixation downwards and to the left, by parallel dextrotorsion. Clinical Import. Though of great physiological interest, the clinical importance of secondary torsion and of Listing's law, which it expresses, is very small. It, or rather the unnatural absence of it, accounts for the obliquity of the false image in paralyses of the internal and external recti, during diagonal fixation ; and when a strong paralysis of one of these muscles is complicated by a feeble paresis of another muscle, the proper paretic torsion due to the slight paresis might conceivably be overborne by the false torsion in the opposite sense. Geometry of False Torsion. The torsion calculator makes it almost unnecessary to lead the reader into an analytical study of false torsion, and I will simply show how to obtain my formula. To start with, we will assume Listing's law proved and suppose the eye to be in the primary position before the motion commences. Required : for any given rotation about any given diameter in Listing's plane, to find the amount of "false torsion." It will serve our convenience best to select, not the vertical meridian of the retina, or the vertical meridian of the cornea, by which to gage the amount of torsion, but that diameter of Listing's plane in which it is inter- sected by the plane which passes through the vertical meridian of the cornea and the retina and which, therefore, is itself strictly vertical in the primary position of the eye. This diameter corresponds to the line V V on. the cardboard model of Listing's plane in No. i of Fig. 14. We wish to ask, for any position of the eye : What is the angle included between this line and a vertical plane passing through the center of motion of the eyeball, and the point of fixation ? It is evident that if the eyeball were free to rotate unhinderedly about any oblique axis, this vertical diameter of Listing' s plane (as we may call it) would describe two right cones with their vertices .meeting at the center of motion of the eye, and they would be what are called "opposite" cones, since they have one axis in common, namely, the axis of rotation of the eye. Let Fig. 15 represent one of these cones, C being the center of motion of the eyeball, O C the axis of rotation and V C the vertical diameter of Listing's plane during the primary position of the eye, while C n is the same line after a given amount of rotation ; V C is, in fact, the generating line of the cone. The Ocular Motions 47 The triangle V ' S C lies in Listing's plane, and the angle formed between it and the plane n O C measures the rotation of the globe about the axis O C. Let us denote this angle of rotation ( O V), whose arc is n F, by the letter R ; and let / denote the angle O C V by which the axis of rota- tion O C is inclined to the vertical. It is this angle, indeed, by which we define the axis of rotation, for there are an infinite number of diameters in List- ing's plane about which the eyeball might rotate, but only one for each specified angle from the vertical, though we need to take account of whether the inclination is positive (to the patient's right) or negative (to the patient's left). From V, drop the perpendicular Vm upon O n. Then m Cn gives us the angle of false torsion required ; for n C is the position of the generating line at the close of the rotation, and shows the new position of the vertical diameter of Listing's plane, while the plane m C V is the vertical plane passing through the center of motion and the point of fixa- tion, the angle between these two being the angle of torsion. It is evident that the plane m C V is a vertical plane, since it passes through the vertical line V C. It is equally easy to prove that the plane m C V, if prolonged, would pass through the fixation point, for it is perpendicular to the plane n O C, to which the line of fixation is also of necessity perpendicular, and they both pass through C; therefore, the line of fixation must lie in the plane, and conduct it, so to speak, to the fixa- tion point. Taking V C as unity Since O V = Sin. I Om Fig Design proposed by author for solving false torsion. C. Center of Motion of the eye. VS C. Plane passing through C, perpendicular to the Visual axis. n O C. Same Plane after Rotation of eye Up and to the Right about an ob- lique axis O C. Vm C, Vertical plane through V C, perpendicular to n O C. While n C was the Original Vertical Diameter of the eye in the Primary Position (since before rotation it coin- cided with VC},m C is the New Verti- cal Diameter the angle between them (n Cm) being the angle of Torsion. and Moreover, But, Or, O = O m Sin. I Cos. O C = Cos. I O m _ Sin. I Cos. ~o~c~ Om R. R Cos. I = Tan. (I x] = Tan. I Cos. R. O C Tan. ( I x ).= Tan. I Cos. x. x = I Tan." 1 ( Tan - ! Cos - R 4 8 Tests and Studies of the Ocular Muscles Putting this into language : The false torsion is equal to the angle from the vertical, or from the horizontal, of the axis about which the eye rotates, less the angle whose tangent is the multiple of the tangent of the inclination of the axis of motion with the cosine of the angle traversed by the line of fixation. The following short table will give an idea of the amount of false torsion which takes place on looking in any diagonal direction midway between any two of the cardinal directions. Since the greatest false torsion of which the eye is capable occurs at the extremities of these diagonals, we may see at once that it does not ever much exceed 10. ROTATION ABOUT AN AXIS 45 FROM THE HORIZONTAL. Degrees 5 10 15 20 25 30 35 40 45 Torsion e#' 26' i i47' 2 4 9 ' 46 / 54t/ 733 / 944' Azimuth and Altitude. The ocular motions can, for exact work, be analyzed with reference to three principal axes, a vertical axis, a horizontal axis and an antero-posterior axis. When the eye looks directly upwards or downwards it rotates round a horizontal (or transverse) axis. When it looks directly to the right or left, it rotates round a vertical axis. These will be recognized as the cardinal movements of the eye. In astronomical language, we might call the upward and the downward motion, "motion in altitude," and the motion to right or left, " motion in azimuth," these being the terms that would be used were the eyes two telescopes. Motion in azimuth may be illustrated by that of a weathercock : it is motion about a vertical axis, Motion in altitude may be illustrated by a piece of cannon, or by a toilet looking glass : it is motion about a horizontal axis. It will be seen that the cardinal motions of the eyes are those of either pure azimuth or pure altitude. When the visual axis, however, is directed obliquely to an object, altitude and azimuth are combined. What is so wonderful is that they are combined in the same proportion at every instant during the motion, so that the visual axis instead of first moving sideways, and then up and down, moves at once by the shortest route into its new position. An astronomer would direct his telescope by KI K . KJ first moving it in azimuth and then in altitude, but To show how the eye reaches this is far too clumsy a plan for the eye, since it anv new position hv the . , , shortest possible route. means two motions instead of one, and a longer The Ocular Motions 49 route instead of the shortest. The visual axis, therefore, sweeps along what- ever inclined plane is common to its initial position and its new position, and loses no time (Fig. 16). It is evident that in motion of this kind the globe must rotate about an axis perpendicular to this inclined plane, an axis, therefore, which is neither horizontal or vertical, but somewhere inter- mediate. All the same, it can be described in terms of its component azimuth and altitude as if it had reached its new position like a telescope. The horizontal component of the motion is the azimuth, and its vertical component the altitude. When motion is to the right from the initial position, the azimuth is by astronomers called positive when to the left, negative. Similarly, motion upwards gives positive altitude, and motion down- wards negative. In analyzing any motion, it is a good plan to adhere to the rule of allowing azimuth the first place, or preference, over altitude, so that, for instance, a motion of ( 20 -\- 10) means that there is negative azimuth of 20 with positive altitude of 10, or, in other words, the eye looks 20 to the left and 10 upwards. For ordinary clinical work, however, it is well to substitute for motion in azimuth, motion "to right and left" (dextroduction and lo'voduction), which leaves it an open question whether it is about an axis strictly vertical, or with an inclination forwards or backwards. For motion in altitude, elevation and depression are suggested as terms which do not bind us too closely. Fig. 17 Varying altitude (to illustrate torsion- less motion ac- cording to Helm- holtz). is Constant altitude (to illustrate torsion- less motion ac- cording to Bon- ders). Helmholtz's Plan of analyzing the ocular motions was to consider the fixation plane (in which both the fixation lines lie) as first elevated or depressed, above (brow-wards) or below (chin-wards) its "initial position," by an angle called the "elevation angle" of fixation. Then, in this plane, the angle between its mesial line and the fixation line was called the side- turning angle. By this plan, however, the altitude of the fixation line steadily lessens as the lateral deviation increases, and it was partly its adoption which led to the apparent discrepancy between Helmholtz's laws of false torsion and those in the text-books. It may be illustrated in a simple way by a weathercock with a bent stem, as in Fig. 17, where motion in azimuth and in altitude are compounded. Fig. 19 illustrates pure motion in azimuth, and Fig. 18 motion in azimuth with a constant altitude, as in Donders' plan. 50 Tests and Studies of the Ocular Muscles Since many of our tests are conducted with the patient facing a flat wall, it way be well to point out in what respects the two plans differ with reference to such a plane surface. By Helmholtz's plan, horizontal lines on the wall represent lines of elevation of the visual plane, and if each is marked in tangents of degrees to right and left of a central zero, these represent the amount of lateral deflection. If, however, the lateral deflection take place first, during the primary position of the fixation plane, then elevation and depression of this plane makes the fixation line describe a hyperbolic curve on the wall, with its concavity outwards. By the other plan, lines of equal altitude on the wall are hyperbolic curves with their concavity upward when the eves are elevated, and down- Fig. 20 Horizontal section of an eye abducted from A to A', to show the author's conception of the difference be- tween the laws of false torsion formulated by Helmholtz and Bon- ders. Fig. 21 Side view of an eye, seen in (ortho- graphic) projection against a ver- tical plane, superducted from A to A', the three circles being pro- jected as straight lines, to illus- trate author's conception of what would be the path of no torsion according to Bonders, and what would be the path of no torsion according to Helmholtz, while the actual torsion is as if the cornea pursued the intermediate path towards P. ward when depressed ; but when the eyes are first deflected to the right or left, elevation or depression makes the fixation line describe vertical lines on the wall. In Figs. 20 and 21 I have represented graphically the different points of view taken by Helmholtz and Donders. Fig. 20 is a horizontal section of an eye, viewed from above, and abducted from A to A' ' . The diameter which I have named "agreed axis" is the one about which rotation would produce exactly the false torsion which all observers are agreed upon. Of the two authorities in question, each adopts the diameter indicated by his name as the axis about which torsionless rotation would take place. Since these diameters are inclined at equal angles to the "agreed axes," though on opposite sides, Helmholtz's tables hold good for Donders' plan if only the signs be changed. The Ocular Motions 51 Fig. 21 shows a side view of an eyeball, not in section but solid, though viewed as projected on to a vertical plane ; to demonstrate that for any given elevation of the cornea from A to A', the path of zero torsion adopted by such authority is that circle indicated by his name, the path which would give the actually observed torsion bisecting the angle between them. In fact, we may say that, for any secondary position of the eye whatever, the false torsion is the same as if the cornea had reached that position by vertical motion, and then through an arc of a circle passing through the center of the cornea (A'} and the primary position in space (P) of the posterior pole of the eye. It will not, of course be supposed that the cornea actually takes this path, but its torsion is the same as Af it had taken it. CHAPTER III Individual Ocular Muscles The Laws of Motion are not Explained by the Anatomy of the Muscles. Our studies of the ocular motions up to this point have been quite independent of the ocular muscles, and our deduc- tions from them would not have suffered had we possessed no knowledge of their anatomy. No sooner do we investigate the musculature, than we find the remark of Helmholtz to be true, that "The manner in which the eye is fixed presents no obstacles to any rotations whatever of a moderate amplitude : the existing muscles would suffice equally well to rotate the eye about any given axis whatever. ' ' But, he adds, " In the ordinary circumstances of normal vision the eye is far from executing all the movements of which the mechanical possibility is recognized."* These remarks are con- firmed to a considerable extent by the phenomena of paralysis of isolated ocular muscles. The Laws Explained by Innervation. The limitation, there- fore, of the parallel ocular motions to rotations about diameters perpendicular to the visual axis, is a limitation which finds no proper explanation from anatomy, but is due almost entirely to cerebral co-ordination. If we except the internal and external recti, any one of the other muscles, acting in an isolated way, would rotate the eye about an axis lying far out of the perpendicular ; but, as a matter of fact, they never do act in an isolated way, but in innervational conjunction with some other muscle in such a manner that the resultant axis is perpendicular. Brief Description Of the Recti. Each eyeball is controlled by four recti and two obliques. The recti spring from an oval tendinous tube at the apex of the orbit. Since, however, there is not quite sufficient room for their origins round the optic foramen, this tube spans the sphenoidal fissure to be attached to the well-known spine on its lower margin. From the orbital surface of this common tendon, so as least to affect the optic nerve by their contraction, *" Optique Physiologique," p. 598. 52 Individual Ocular Muscles 53 spring the muscular fibres of the four muscles, those of the superior rectus being almost continuous with those of the levator palpebrae at first, though separating later, while the internal rectus is con- tiguous to the origin of the superior oblique. From the upper span across the sphenoidal fissure, as well as from the spine itself, springs the external rectus, while the lower span gives rise to tht inferior rectus. The internal rectus proceeds almost straight forwards and lies rather close to the slightly-convex inner wall of the orbit. The external rectus proceeds forwards and outwards to the outer side of the globe. The superior and inferior recti proceed forwards and somewhat outwards to the upper and lower parts of the globe respectively. Spiral Of Insertions. The insertions of the internal, inferior, external and superior recti lie, in round numbers, five, six, seven and eight millimeters respectively from the corneal margin (Tillaux). The internal rectus, therefore, has the greatest mechanical advantage and the inferior next. Description Of Insertions. The insertions of the internal and external recti form two perpendicular lines, so that their tendons appear to have rectangular extremities. The tendon of the internal rectus is strong and well defined : that of the external is thinner, its margins passing almost insensibly into the lateral expansions connected with Tenon's capsule. The superior and inferior recti have oblique insertions, the outer extremities of which are rounded and lie farther back than the inner extremities. In operations on their tendons this should be remembered, so as to approach them on their inner, more accessible, side. Otherwise some difficulty may be experienced in hooking them up. Relative Strength. Of all the muscles, the internal rectus is the strongest, or at least the most bulky, weighing, according to Volkmann, .747 of a gramme ; the external rectus comes next, weighing .715 of a gramme ; the inferior rectus weighs .671, and the superior rectus (the weakest of all) .514 of a gramme. Description Of Obliques. The superior oblique arises from the medial side of the upper part of the origin of the internal rectus and runs forward ; but its tendon is reflected in a fibro-cartilaginous pulley or " trochlea " near the upper inner corner of the orbital outlet, whence it passes backwards and outwards over the orbital 54 Tests and Studies of the Ocular Muscles surface of the superior rectus tendon to be attached to the globe by a flattened expansion mostly in the upper and outer quadrant of its posterior hemisphere. The line of insertion is subject to con- siderable variation, its direction, according to Fuchs, being more longitudinal in myopes. In emmetropes it forms nearly equal angles with the antero-posterior and transverse meridians of the globe. Though the anatomical origin of the superior oblique lies on the apex of the orbit, the pulley may be regarded as its virtual origin. The inferior oblique arises from a little depression in the bone near the lower and inner corner of the orbital outlet, and passes backwards and outwards, beneath the orbital surface of the inferior rectus, continuing to curve round the globe between it and the external rectus, till, without a tendon, its muscular fibers gain insertion about a quarter of an inch from the tendon of the superior oblique, either in the same quadrant or between the upper outer and the lower outer quadrants of the posterior hemisphere. The line of insertion is not parallel to that of the superior oblique. Why a Pulley at all? Many must have wondered why one of the oblique muscles should have a pulley and the other not. There must, of course, be a reason, and the following con- sideration is advanced as a possible one : The speed with which a muscle's point of insertion moves is proportional to its length. By this it is not meant that a long muscle takes necessarily a different time to attain its maximum con- traction, from a short one ; but that in the same period of time it will move its insertion through more space than a short one and, therefore, with a greater speed of motion. The obliques, therefore, must have a certain length if they are to keep pace with the recti ; * and this length could not be afforded were they both to have origins similar to that of the inferior oblique, unless, indeed, they passed each other in curling round the globe, which would spoil their action, for it is advantageous that their insertions should be opposite each other, rather than side by side. To secure sufficient length for its muscular belly, the inferior oblique has to curl round more than its own share of the globe, and this would leave the superior oblique short. In fact, to gain more space, the inferior oblique dispenses with a tendon of insertion altogether, its muscular fibers extending quite to its attachment. * Really, the insertions of the obliques move more slowly than those of the recti during most movements of the eves. Individual Ocular Muscles 55 Why the Superior Oblique and not the Inferior? Why, then, it may be asked, does the inferior oblique not have the pulley instead of the superior, seeing it is the inferior which is supplied by the third nerve and thus might be expected to rise in company with the third-nerve recti, rather than the superior oblique, which is supplied by the fourth nerve ? The reason may just possibly be this : that prolonged looking downwards is more important for daily work than looking upwards, a view which is confirmed by the fact that the continuous down- ward excursions of the eye are more amply provided for than the upward excursions. The center of motion of the eyeball approximates more closely to the geometrical center of the eye on looking downwards than on looking in any other direction, showing that the mechanical resis- tance is least during this motion and, moreover, the eye can make a more extended excursion downwards than upwards. The superior oblique, therefore, which is a subductor of the globe, might be expected to have the most advantageous arrangement accorded to it. A free muscular belly, even though complicated with a pulley, is perhaps if not stronger, at least more delicately efficient than a muscular belly which traverses tissues in contact with the globe and is embraced by a check-ligament (Fig. n). If, on the other hand, both muscles had pulleys, the pulley-complication would be doubled unnecessarily. * Everything for Speed. In some other parts of the body the muscles are constructed for strength rather than speed, but with the eyeball everything is adapted for speed. The greater the number of muscular fibres ranged side by side the stronger is a muscle, and the greater the number arranged end to end the quicker it is. The bi-penniform arrangement, e. g. , of the rectus femoris, is a beautiful example of adaptation for strength at the expense of speed, the muscular length being considerably less, and the muscular breadth being considerably more, than the actual length and breadth of the muscle as a whole. The muscu- lar length is found by measuring along the lines of fibres from one tendon to the other. There is no such marked arrangement as this in the muscles of the orbit, where speed is more desirable *M;uithner has shown that in paralysis of the inferior oblique the vertical separation of the doulile images is greater than in paralysis of the superior oblique. But then the superior rectus is very much weaker than the inferior rectus, and Mauthner's observation may enly show that the difference between the obliques is less than that between the recti. 56 Tests and Studies of the Ocular Muscles than strength and where, therefore, a certain length of belly is indispensable. Touching Point. 'The point at which a muscle, as we trace it forwards, first touches the globe, or its momentary insertion, is continually changing its location with every movement of the eye. The "arc of contact," therefore, along which the muscle remains applied to the globe, and which extends from the touching point to the anatomical insertion, is correspondingly ever varying in length. Its variations, I believe, however, are tempered by the disposition of Tenon's fascia, so that in the extreme rotations of the eye, the arc of contact is not abolished so quickly as calculations based on the muscles only would lead us to expect. Thus, in the case of the internal rectus, it is easy to calculate that the arc of contact, while the eye looks straight forwards, is about 36 (from the center of motion), yet many eyes can be adducted 50 or perhaps even 60 at a push, and it is not likely that the arc of contact ceases at 36 or the eye would be tugged at and dis- torted. The "collarettes" or " intra-capsular ligaments," as they are called, must tend to bind the tendon longer to the globe so as not to let them so soon part company. Terminology. When the cornea is drawn toward the temple the eye is said to be abducted ; when towards the nose, adducted ; when raised, we may call it elevated ; when lowered, depressed. When the eye is twisted about its own axis so as to make the cornea revolve like a wheel, we may call it torted ; intorted when the upper segment of the cornea revolves towards the nose, and extorted when it revolves towards the temple. I have found it convenient also to speak of dextroduction, lacvoduction, dextrotor- sion and laevotorsion. Prime Muscular Functions. Each eye possesses one muscle pre-eminent for abduction the external rectus ; another for adduc- tion the internal rectus ; for elevation the superior rectus ; for depression the inferior rectus ; for intorsion the superior oblique ; for extorsion the inferior oblique. Subsidiary Functions. But besides these "prime" actions, each muscle has "secondary"* actions. This is least so with the internal and external recti, which are pure adductors and abductors respectively, except when the eyes are elevated or depressed. *At Prof. Savage's suggestion, I have altered th<; word "subsidiary" to "secondary," as more euphonious. Individual Ocaiat Ahiscles 57 They have subsidiary vertical and torsional effects, but I believe to a far less extent than has been supposed, owing to the restraints imposed by the collarettes and Tenon's fascia, which make the tendons share to some extent any change of direction imparted to the eye.* Medial Origins Of Muscles. With regard to the secondary effects of the superior and inferior recti and the obliques, we may assist the memory by recollecting that all the ocular muscles, with- out exception, spring from origins nearer the median plane than their insertions. Hence, the superior and inferior recti, being inserted into the anterior hemisphere of the globe, pull it nearer the median plane, i. e. , abduct the cornea ; whereas, the obliques, being inserted into the posterior hemisphere of the globe, pull their insertions towards the median plane, z. e. , abduct the cornea. Moreover, in consequence of the same medial disposition of the muscular origins, those muscles which proceed to the upper hemisphere of the globe by pulling their insertions inwards zwtort the cornea ; and those which proceed to the lower hemisphere of the globe, since they also draw their insertions inwards, ^.rtort the cornea. We may say then that the superior muscles cause intorsion, and the inferior muscles extorsion ; the obliques abduction, and the recti (superior and inferior) adduction. Thus the superior rectus, besides elevating the cornea, intorts the eye, because its insertion is "superior," and adducts it because it is inserted into the anterior hemisphere. The inferior rectus, besides depressing the cornea, causes extorsion (being "inferior") and adduction (being inserted into the anterior hemisphere of the globe). The superior oblique causes, besides its proper intorsion (from being "superior"), depression of the cornea from the upper character of its insertion, and abduction (being inserted into the posterior hemisphere of the globe). The inferior oblique causes, besides its proper extorsion (from being "inferior"), elevation of the cornea because its insertion is inferior and its origin anterior, and causes abduction from the posterior character of its insertion. *Thongh the reader need concern himself but little with these unimportant considera- tions, on looking up. the e a slight superductor and intortor. and on looking down a slight subductor and extortor. On looking up, the internal recttit musl he a slight superductor and extortor ; and on looking down, a slight subductor and intortor. 58 Tests and Studies of the Ocular Muscles Inverse Proportions of Prime and " Secondary " Actions. We now come to a point of considerable clinical importance. All the secondary effects of the various muscles which we have just recounted are at the expense of their prime actions. The energy expended in producing them represents so much loss in the prime action of the muscle. In those positions of the globe, therefore, where we find secondary effects of a muscle at their minimum, the prime effect is at its maximum, and vice versa. Lateral Superductors and Subductors. The superior rectus is a "lateral" sursumductor, and the inferior rectus a "lateral" dursumductor, because their elevating and depressing effect is greatest when the eye is sufficiently abducted towards the temple for their vertical power to be uncomplicated and their secondary effects to become practically nil. The more the eye is, on the other hand, adducted, the greater become their secondary adducting and torsional effects, and the less efficient they are for the vertical movements of the eye. When we come to the diagnosis of ocular paralysis we shall find the advantage of knowing that the maximum torsional effect of a muscle occurs when the eye looks to the opposite side from that in which its maximum vertical effect occurs. Medial Superductors and Subductors. The obliques have their greatest torsional effect when the eye looks outwards, because then they form the greatest angle with the optic axis, and since their greatest effect on the vertical motions of the eye is found on looking towards the nose, the superior oblique is a ' ' medial ' ' depressor, and the inferior oblique a ' ' medial ' ' elevator. While the reasons for these facts are no doubt self-evident, their fuller consideration requires a study of the ' ' muscular planes ' ' and ' ' muscular axes. ' ' Lines Of Force. Owing to the existence of an " arc of con- tact," the muscular forces acting on the globe must be tangential forces. There is, therefore, one tangent line to the globe for each muscle which indicates its direction of force. In the primary position of the eye the lines of force probably extend from the "touching points" of the several muscles to their orbital origins, or the trochlea in the case of the superior oblique. Individual Ocular Muscles 59 When the eye is moved away from the primary position, the lines of force are, I believe, more or less diverted by Tenon's fascia acting as a pulley, or at least exerting an elastic side-traction where the ' ' collarettes ' ' exist. Muscular Planes. When we have a force acting tangentially on a rotating body, confined as by a center of motion, and the line of that force is given, the plane of force is evident at once. It is the plane which passes through the line of force and the center of Fig. 22 The Muscular Planes and Axes. The muscular planes, and o o' the axes of the obliques. R The muscular planes, and r r' the axes of the recti. motion, and in the case of the eye, is called a "muscular plane," there being one muscular plane for each muscle. The muscular planes, therefore, all agree in passing through the center of motion, but differ in that each extends thence through the line of force of its own muscle. Three Pairs. The muscular planes of the internal and exter- nal recti are practically horizontal and identical. Those of the superior and inferior recti are generally also supposed to be identical, but vertical, forming an angle with the median plane, which is estimated by Landolt at 27 in accordance with the well-known anatomical fact that these recti, instead of 60 Tests and Studies of the Ocular Muscles running directly forwards, run forwards and somewhat outwards to their insertion. In a horizontal section of the eye, as shown in Fig. 22, their common muscular planes are represented in section by the lines R. The muscular planes of the obliques are similarly supposed to be identical and, therefore, vertical, but running, of course, back- wards and inwards instead of forwards and outwards, so as to form an angle with the median plane estimated by Landolt at 51. In Fig. 22 the common muscular planes of the obliques are represented in section by the lines O. Properties of Muscular Planes. Each muscular plane possesses this property, that if a single muscle were to contract, the mid point oi its insertion into the sclerotic would not move out of the plane, and though all the points contained in the plane would move, the plane itself remains fixed in space. Every muscular plane, since it passes through the center of motion, must approximately bisect the eye and cut its surface in a great circle. All points on the surface of the eyeball not lying in this circle, will, when the muscle contracts, also move in smaller parallel circles, and the greater the distance of each surface point from the muscular plane, the smaller the circle in which it moves. When the center of the cornea, therefore, lies actually in the muscular plane of a muscle, as it does, for example, in that of the superior rectus, when the eye is abducted 27, the motion of the cornea under the influence of that muscle is greatest, and it becomes less and less the more the cornea is moved away from the muscular plane by adduction of the eye. We have seen that the parallel circles of motion on the surface of the globe become smaller and smaller as they lie farther and farther from the muscular plane. This goes on till at last they are reduced to a point on each side which remains fixed in space while the globe rotates. These points, being those on the globe's surface, which are farthest removed from the muscular plane, are the poles of the "muscular circle," and the diameter of the globe which unites them and which, therefore, remains fixed in space with them, is the "axis of rotation" and is strictly perpendicular to the muscular plane. Axis Of Rotation. To find the axis of rotation for any muscle, we must first find the muscular plane ; then a line perpendicular to it, and passing through the center of motion is the axis of rotation. In other words, the muscular plane cuts the surface of the globe in a circle, the axis of which is the axis of rotation.* Fig. 22 shows the axes of rotation as usually figured. * Though it is usual in opnthalmolo^y to speak of the axis of rotation as a diameter, yet it is defined physically l>y the ratlins perpendicular to the muscular plane, on that side of it which represents the sense in which the contract'ion of the muscle makes the eyeball rotate. Individual Ocular Muscles 61 Since the superior and inferior recti are supposed to have one muscular plane in common, they have a common axis of rotation (r r' ) about which, however, they rotate the eye in opposite senses. The obliques are also supposed to have a common horizontal axis, shown by o o' in the same figure. Are the Axes, Supposed to be Identical, Really So ? Let us now ask : Do the superior and inferior recti really rotate the globe about one axis in common? I am not sure that they do, for were it so, isolated paralysis of either muscle would (cceteris paribus), during the primary attitude of the sound eye, adduct the other, so as to cause homonymous diplopia, and this is contrary to most recorded clinical experience.* Similarly, it is doubtful if the two obliques rotate the globe about an axis common to them both, for if this were true, paralysis of either would tend to cause crossed diplopia during the primary attitude of the sound eye, which is also contrary to usual clinical experience. It is true that sometimes crossed diplopia does occur in paralysis of the obliques, but this has hitherto been explained, on Mauthner's hypothesis, by the liberation of previously-existing latent divergence (exophoria) of the two eyes. The reader may feel inclined to object: "Surely, if contraction of a muscle causes adduction (as we know to be the case with the superior and inferior rectus), its paralysis will result in abduction !" This is true when the sound eye is raised or lowered towards the area of maximum diplopia, for the superior rectus is an adductor when it super- ducts the eye, and the inferior rectus when it subducts the eye ; but what we are now considering is what they do during the primary position of the sound eye. Double Contraction. It it were possible for both the superior and the inferior rectus to contract simultaneously, the effect on the rotation of the eye in its primary position would be nil if they really have but one axis in common. They would simply tend to draw the eye as a whole backwards and slightly inwards towards the apex ot the orbit, just as if an elastic string were tied by one end to the optic foramen, and by the other end (if possible) to the center of motion ot the eye. The eye, therefore, would be neither adducted" nor abducted. Single Contraction. If, however, either muscle contracted alone, the vertical motion of the cornea would be accompanied by a proportionately increasing adduction. Single Paralysis. So far the reader will readily agree ; but the next proposition is quite as simple : that if either muscle were paralyzed, the paralytic displacement of the cornea should (during the primary position of the sound eye) be accompanied with a precisely corresponding proportion of adduction also. For if the ax-is be common to both, paralysis of, say, the superior rectus, must cause the same effects as contraction of the inferior. *Since this proposition requires a good deal of thinking out before becoming evident to every reader, the smaller print may, with advantage, be skipped on first reading. 62 Tests and Studies of the Ocular Muscles When the eyeball is in equilibrium, each tension is balanced by the resultant of all the other tensions, and if the increase of any one tension rotates the eyeball about a given axis the resultant of all the other tensions would, in the absence of that one, rotate the eyeball about the very same axis, only in the opposite sense. But this is just, in kind, if not in degree, what the other rectus does when it contracts, for it also rotates the eyeball about this same axis in the opposite sense.* Arc Of Contact. This name has already been spoken of as given to the line along which a muscle and its tendon embrace the surface of the globe. The actual insertions of the tendons are in advance of the points where the muscles first reach the surface of the globe tangentially. The touching points are those we must take account of in studying the dynamics of the ocular muscles, but we really know much less about them than is usually taken for granted, since they are modified by Tenon's capsule in a way which it is impossible to determine precisely. This shows it to be all the more judicious not to study the ocular muscles synthetically, i. e. , by argument from their anatomy, but analytically, by close observation of the actual results of their physiological action and pathological failures. Bonders placed great emphasis on this principle. Ophthalmotropes. For teaching purposes the synthetical study is, however, needful and, provided we confine ourselves only to broad principles, we shall not go far astray. A number of ophthalmotrope<> have been invented from time to time, and of these I prefer Landolt's and Anderson Stuart's as the best. Their purpose is to represent, in the form of a model, the characteristic functions of the several muscles in isolated action, as in Figs. 23 and 24. Landolt's Ball. Another ingenious device by Landolt is his "india-rubber ball" (Fig. 25), which any reader can easily mark for himself. His own description is as follows : ' ' Take a simple india-rub- ber ball, depict upon it the cornea, the vertical meridian and the horizontal meridian. On the latter, mark, at 39 from the anterior *This amounts to saying that, during the primary position of the sound eye, the paralysis of the superior rectus produces the same effect as slight spasm of the inferior, if the usual single-axis hypothesis be true. Clearly, therefore, either it is not true, or else previous clinical observations in the primary area of the motor field have been misleading. I will not attempt to say which is the case. Individual Ocular Muscles 63 pole (center of the cornea), the anterior extremity of the axis of the obliques ((9), and at 63 on the opposite side (/?), the axis of the superior and inferior recti. ' ' Fig. 23 Landolt's Ophthaliuotrope. The eye is seen under the action of the superior oblique, O O being the axis of the oblique. Now suppose, for example, we wish to demonstrate the action of the superior oblique, we reason thus : this muscle makes all the points of the cornea describe parts of parallel circles about its axis. 64 Tests and Studies of the Ocular Muscles Taking a pair of compasses, therefore, we open them so that one of the points shall correspond with the anterior extremity of the axis of the oblique muscle, the other to the center of the cornea. Fig. 34 Anderson Stuart's Model of the Ocular Muscles (the inferior obliques should not have a pulley) Keeping the first point fixed, we trace with the other the circle, of which a part is traversed by the apex of the cornea under the influence of the contraction of the superior oblique. "If we wish to know where this apex of the cornea is found after a rotation, for example, of 40, we have only to trace a straight line, starting from the anterior extremity of the axis and forming an angle of 40 with the horizontal (below it for the superior oblique, above it for the inferior oblique). "The point, O 1 or R', where this line meets the circle indi- cates the position of the corneal apex which corresponds to the required rotation. We see thus, at once, in what direction and to what extent it deviates from the horizontal as well as from the vertical. "As for the slope which the vertical meridian of the cornea will have acquired at the same time, it is of necessity perpendicular to the line which we have just traced and passes through the point which it has discovered for us to be the center of the cornea. That is evident. This very line is, in short, no other than a part of the Individual Ocular Muscles 65 horizontal meridian, sloped by the muscular contraction ; it is perpendicular to the vertical meridian. " It is thus that in our figure the two black stripes indicate the inclination impressed upon the vertical meridian of the right eye by the superior oblique O 1 and by the inferior rectus R' . ' By dropping a perpendicular from the points O' and R' upon the horizontal meridian, we get the amount of depression ( O' h and R' /i) produced by the muscles in question. 1 ' The perpendicular dropped from these two points O" and R upon the vertical meridian correspond to the amount of abduction ( O 1 V) caused by the oblique, and to the amount of adduction caused by the rectus. ' ' With Landolt's Ball the Sound Eye is Supposed in its Primary Position. It should be remarked that Landolt's ball as thus made only represents the truth when the internal and external recti are Fig. 25 Laortnlt's Ball quiescent, for the more the eye is adducted by the internal rectus, the farther is the anterior extremity of the oblique axis removed from the cornea, making the arc for the oblique become that of a larger circle and the arc for the rectus that of a smaller circle. 66 Tests and Studies of the Ocular Muscles Conversely, the more the eye is abducted by the external rectus, the smaller becomes the circle for the oblique, till perhaps it becomes nil, showing that then the oblique is purely torsional in its action : and the larger becomes the circle for the rectus, till at last it becomes a straight line, showing that then the rectus is a super- ductor or subductor. Furthermore, since the circles on Landolt's ball map out the paths followed by the apex of the cornea under the action of indi- vidual muscles, and since it is based on the approximation that the obliques have one and the superior and the inferior recti also have one horizontal axis in common, we may well use it to illustrate that Fig 26 To show, if the axes are tilted, the nature of the tilting, /. R. and S. R. being the axes of the Superior and Inferior Recti, and S. O. and /. 0. those of the Superior and Inferior Obliques. if this be true, paralysis of any one of these muscles would bring about the opposite horizontal condition from that which is generally believed. For if contraction, say, of the superior rectus, move the apex of the cornea in a circle as marked on the ball, its paralysis will move it in the same circle but in the opposite direction, i. e., just as slight contraction of the inferior rectus would move it, causing, therefore, adduction in each case. Tilted Axes. If clinical observation shows abduction to be the undoubted result of uncomplicated paralysis of the superior or inferior rectus during the primary position of the sound eye, then the axes of rotation for these muscles must be regarded as inclined Individual Ocular Muscles 67 to the horizontal in opposite directions ; the axis for the superior rectus having its inner end lower, and that for the inferior rectus having its inner end higher, than the horizontal meridian. Similarly, if uncomplicated paralysis of either oblique cause adduction, under similar conditions, the axis of rotation for the superior oblique must have its outer end higher, and that for the inferior oblique lower, than the horizontal meridian. I have represented this in Fig. 26, which shows an india- rubber ball traversed by knitting-needles to represent the axes. As a matter of fact, it is extremely difficult to ensure that any paralysis is uncomplicated by previously-existing latent squint, so that clinical records are very little to be trusted on this point, unless they are made with special reference to it. The theory of tilted axes is, I find, far from new, Meissner having taught them, and later Continental writers having owned them theoretically as true, though deeming the convenient approxi- mation of horizontal axes sufficiently accurate for practical purposes, and for clinical deduction perhaps. CHAPTER IV Associated Muscles in a Single Eye Isolated Contraction of Some Muscles Unknown.- -Our study of the ocular motions in Chapter II will have shown us that isolated action is forbidden to at least four of the muscles of each eye, since neither the superior or inferior recti, nor the obliques, can act alone without violating Listing's law, by which, it will be remembered, all rotations from the primary position are forbidden to the healthy eye except those about axes in a vertical plane passing through the center of motion of the eye, perpendicular to the visual line in its primary position. But neither the axis for the obliques nor that for the superior and inferior recti lie in this plane ; therefore, no one of these muscles can contract without some associated muscle acting with it in that perfect proportion required to keep the resultant axis in this inevitable plane. The rotations which individual muscles would effect severally, have to be compounded with great nicety into one rotation. On looking, for instance, directly upwards, the eyeball must rotate about the horizontal diameter of the plane. The superior rectus cannot effect this, because its own axis is inclined by 27 from it. It is, however, so reinforced by a smaller con- traction of the inferior oblique that the resultant axis lies in the plane. Elevation of the cornea is effected by both of these muscles, but it is a "prime" action of the superior rectus and only a "secondary" action of the inferior oblique. On the other hand, intorsion is a "secondary" action of the superior rectus, and extorsion is the "prime" action of the inferior oblique. Since physiological elevation of the eye is always quite free from torsion, the two muscles must contract in such proportion that the intorsion by the one shall exactly counterpoise the extor- sion by the other. For this to be the case, the oblique must contract to a much less extent than the rectus and, in reality, only about three-tenths of the elevation of the eye is due to the inferior oblique, the remain- ing seven-tenths being due to the superior rectus. 68 Associated Muscles in a Single Eye 69 In an exactly similar manner depression of the cornea is effected by combined action of the inferior rectus and the superior oblique. In all this, we are confining ourselves to motions which start from the primary position. If the eye be adducted or abducted to start with, the case is, of course, different. Then, as Helmholtz says, the resultant axis no longer lies in the transverse plane of the head, already described, but in a plane which bisects the angle between it, and the plane fixed in the eyeball which originally coincided with the former plane when the eye was in the primary position, but which moves with the eyeball, so as to be ever perpendicular to the line of fixation. This is shown in Fig. 27, modified from Helmholtz,* where O B represents the fixation line in the primary position of the eye. The equatorial plane A A which moves with the eyeball and which passes through the center of motion perpendicu- larly to the fixation line, takes the posi- tion CC, when the fixation line is deviated from O B to O P. The eye is now in a secondary posi- tion, and whatever motion it may make from this position into any other, must be effected by rotating about some diameter of the plane ////which bisects the angle between the planes A A and C C. There are, of course, an infinite number of diameters in this plane (////) about which rotations are possible, so that it may be called "the plane of the axes of rotation" for that secondary position of the eye. Composition Of Rotations. There is a beautiful and well-known method of representing, in linear measure, the amount of rotation imparted to a rotating body, by simply measuring off along the axis, from the center, a distance proportionate to the rotation ( ' ' rotation vector " ). There are, of course, two senses in which a body can rotate about any one axis, and it is, therefore, needful to specify in which sense the rotation occurs. This is easily done, for since there are two directions in which we can measure along the axis from the center, we can choose one direction to represent rotation in one sense, and the other direction to represent rotation in the opposite sense. * Physiologik. Optik.," p. 624. To show how in an eye abducted from B to F, the axis of super- duction is not .4 .4 nor C C, but in a mean position (H H ). 70 Tests and Studies of the Ocular Muscles By convention, we imagine ourselves to stand at the center and look along the axis in that direction which makes the motion appear to us like the hands of a watch or the motion of a right-handed screw. By a single measured line, therefore, we can record no fewer than three quantities : (1) The axis of rotation, by the direction of the line ; (2) The amount of the rotation, by the length of the line ; and (3) The sense of the rotation, by the direction from the center in which the line is drawn. We may choose any units we please. Suppose, for instance, we decide to represent degrees by millimeters, then 10 millimeters measured along a direct line means 10 of rotation about that line as axis, and in the same sense as that of a screw being* screwed along the direction of measurement. We have only to seize the line at its origin, and screw, to understand the sense in which the rotation occurs. We may compound rotations, therefore, or resolve them as we please, on the same principle as the parallelogram of forces. Dynamics Of the Eye. In theory the dynamics of the eye are exceedingly simple, since the resistances are elastic (and conform, no doubt, to Hooke's law, " Ut tensio sic vis"), the forces are tangential, and the lines of the forces may with little error be reckoned as equally distant from the center of motion, so that the moments of the forces are proportional to the forces themselves. The ' ' moment " of a force about a point is the importance of that force as regards balancing or producing rotation about that point. The greater the distance of the line of force from the point, the greater is the moment of the force. Forces only Estimated by Results. The resistances to rota- tions of the eyeball are no doubt greater about some axes than about others, and since we cannot calculate this element, we are driven to study the forces as if measured only by the rotations they produce. Instead of compounding forces we are obliged to com- pound rotations, for the forces are unknown quantities to us, while the rotations can be investigated to a high degree of accuracy by the behavior of double images and after-images. Fig. 28 illustrates the composition of rotations in a rotating body whose center is at o. The arrowheads on the lines o a, o b represent the directions in which the lines are measured, and there- Associated Muscles in a Single Eye 71 fore the sense of the rotation which takes place about each as axis and which is the same as that of an ordinary right-handed screw screwed in the direction of the arrow. Thus the line o a represents a rotation proportionate to the length o a, and about o a as axis, in the sense of a screw driven from o to a. The line o b represents a smaller rotation, since it is a shorter line, about o b as axis and in the same sense as a screw driven from o to b. When two forces, capable when acting singly of producing these respective rotations, are impressed upon a body simulta- Fig. 28 Composition of Kotations neously, the rotation which results is represented by the diagonal o c of the parallelogram o a c b completed by drawing b c and a c parallel respectively to o a and o b. The resulting rotation, therefore, is about the axis o c, propor- tional to the length o c, and in the same sense as the rotation of a screw driven from o to c. The reason for this actual composition of the rotation is as follows : If the body were only subjected to one of the rotations o a or o b, any point in it would move over a distance proportional, firstly, to the amount of rotation, and, secondly, to its distance from the axis of rotation ; just as the rim of a wheel travels farther than the hub during a given rotation, in proportion to its distance from the axle. When the rotations o a and o b take place simulta- neously, points which lie between their axes would rise in conse- quence of one rotation and sink in consequence of the other, and there is a line of points (o c~) so situated that the rising and sinking exactly neutralize each other. The distance of each point in this line from the two axes is inversely proportional to the amount of Tests and Studies of the Ocular Muscles Fig. 29 Horizontal Section of a Right Eye rotation about the axes, so that the faster rotation of the body as a whole about one axis is compensated for in the case of the point under consideration by its greater distance from the other axis. These points, therefore, all remain stationary and form the new axis of rotation. All points which lie to the a side of it are depressed, be- cause of their distance from o b being too great to be compensated for by the greater rotation o a ; while points to the b side of o c are elevated for the contrary reason. Now let us apply these principles to the eyeball. Let Fig. 29 represent a horizontal section of the eye, where A is the anterior pole of the eyeball and P the posterior pole, so that A P is the optic axis. The line D E is the transverse axis ; / 6" is the axis of rotation for the superior and inferior recti, and /' S' is the axis of rotation for the obliques. A measured quantity {Or) along the line O S from O as origin, indicates a measured rotation of the globe in the sense of a screw proceeding from O to .5". This rotation elevates the cornea and is such as would be effected by the superior rectus acting, were it possible, alone. Similarly, any measured quantity ( O s) from O towards /' specifies a proportionate rotation by the inferior oblique, which also elevates the cornea, since the sense of rotation is that of a screw passing from O to /'. These rotations (Or and O s~), when they occur simulta- neously, are compounded into the single rotation O E, which takes place about an axis in Listing's plane. Now, as in Fig. 30, let us drop a perpendicular from A, the center of the cornea, upon the axis of the superior and inferior recti (/ S). What have we? The vertical plane passing through this line is the plane of motion for the center of the cornea during isolated action of either the superior or inferior rectus. The anterior pole of the eye under these conditions describes a circle in this plane, which we might call the corneal orbit for these muscles, since it is the path in which the center of the cornea travels under their guidance. (See Fig. 30). A plane, therefore, passing through A perpendicularly to the axis / 6" is the plane of the corneal orbit during rotations about that axis ; and we Associated l\fusclcs in a Single Eye 73 see at once that in whichever sense such rotation takes place, it must necessarily adduct the cornea, as well as elevate or depress it. In precisely the same way a perpendicular may be dropped from the center of the cornea (A) upon the axis of the obliques (/' S e ) : the vertical plane passing through this line is the plane of the corneal orbit during rotations about that axis, at once indicating that abduction of the cornea is a result of such rotations whether they are produced by isolated contraction or isolated paralysis of either oblique, if the usually accepted view is correct that the obliques have a common horizontal axis. Resolution of Rotations. Next, let us see how to resolve rotations due to indi- vidual muscles. Take the inferior rectus as an ex- ample, and in Fig. 29 let the distance O I represent the maximum rotation it can effect. Drop perpendiculars from / upon the transverse axis D E and the optic axis A P; these perpendiculars cut off dis- tances from O along these axes which represent the component depression and torsion respectively. Since thus O m represents the depression of the cornea and O n its torsion, we see at once that depression is the prime action of the muscle. The torsion occurs in the same sense as in a screw passing from O to n, so that it is extorsion. The lengths of the lines O n and O m are easily found ; for the proportion which they each bear to O I is simply that of the cosine of the angle included between each and O I, or, what comes to the same thing, of the sine, and the cosine of / O D. Suppose, for instance, we take the obliquity of the axis of the superior and inferior recti to be 27 from the transverse axis, the component O m will be .89, i. e., less than nine-tenths; and the component O n will be .45, i. e., about nine- twentieths of the whole rotation O I. The torsion, therefore, is only about a half .of the elevation. Co-ordination. Let us now see how much rotation the superior oblique must effect in order to be a perfect associate of the inferior rectus (Fig. 29). Clearly, if subduction is to be unaccompanied by torsion, the extorsion O n must be counterbalanced by an equal Hg. 30 Horizontal section of a right eye. The longer dotted liuefrom A indicates the vertical plane, to which the motions of the anterior pole of the eye are confined under the guidance of the Sup. and Inf. Recti. The shorter dotted line indicates the same for the Obliques. 74 Tests and Studies of the Ocular Muscles intorsion O ri . After marking off O rJ ', therefore, equal to O n but in the opposite direction from O, erect a perpendicular at n' to cut off along O S' (the axis of the obliques) a distance Op, which shows the exact proportion of intervention required from the superior oblique muscle, its rotation being resolved into a torsional component {O n') which balances the torsional compound of the rectus (On), and a subducting component equal to n' p, which supplements the subducting effect of the rectus. Indeed, the lengths O m and n' p exactly represent the relative proportion of pure subduction due respectively to the inferior rectus and superior oblique. The latter is scarcely more than two-fifths of the former.* Effect of Horizontal Displacement. When the eye to start with is ab- or adducted, the proportions are different. We imagine the muscular axes (SI and S / I') to remain fixed in space (though they do not do so absolutely), and the visual A P and D E to move with the eye. In abduction, the transverse axis of the eyeball approaches the axis of the superior and inferior recti. With 27 of abduction, therefore, the torsional component of the superior and inferior recti ceases, while it would reach its maximum were it possible for the eye to rotate in 63. Conversely, their vertical effect is theoretically greatest with abduction of 27, becoming nil with hypothetical adduction of 63. The torsional effect of the obliques is greatest theoretically! with abduc- tion of about 39 and nil with adduction of about 51, since in the former case the axis of rotation (S' I'} coincides with the optic axis (A P) and in the latter is perpendicular to it. Exactly the opposite is true of their elevating power, which is nil with abduction of 39 and greatest with adduction of 51. Though these calculations are at best only approximately true, we can by their aid determine with more or less approach to truth the provinces of the motor field over which different muscles hold chief sway, or sway of a special kind. A chart of the motor field on this principle was attempted by Duane. The only reliable way of constructing an exact chart of these provinces is by very careful examination and measurement of the motor field in cases of isolated paralysis, since there is some reason to believe that synthetical calculations are only true in a certain measure owing to the influence of Tenon's capsule, that measure being greatest near the primary position and less with increasing departure from it. * n' p = m Tan. 27, Tan. .39. fThe reason I use the word so freely in this section is because I suspect the muscular axes do not remain quite so stationary as is supposed. Associated Muscles in a Single Eye 75 The right-hand side of Fig. 29 (where we come to deal with the superior rectus and inferior oblique) shows that to resolve any given superduction of the eye, such as O E, we need only com- plete the parallelogram, of which that line is a diagonal, by drawing E r and R s parallel respectively to the axis of the recti and the axis of the obliques. Then the dimensions O r and O s show the component rotations effected by the rectus and its associated oblique. They are proportional to the sines of 51 and 27 and, therefore, about 17 to 10. It is true that isolated paralysis of the superior oblique is common. The double images therefrom indicate sometimes abduc- tion of the cornea, but far more frequently adduction. Moreover, the occasional abduction is most likely explainable, on Mauthner's hypothesis, by the liberation of a previously-existing latent squint, or tendency of the eyes to diverge (exophoria) when not engaged in single vision, from slack action of the converging innervation.* If this explanation of Mauthner's be true and adduction be the characteristic effect of this paralysis during the primary position of the sound eye, then the axis for the muscle must be tipped up above the horizontal plane at its outer end and dip below it at its inner end, as shown in Fig. 31. Even then, adduction would only occur during a moderate paralytic displacement of the eye, and would give place to abduction if it exceeded a certain amount, which it would be quite easy to assign were the exact tilt of the axis known. In fact, as soon as the depression of the eye were to become twice as great as the tilt of the axis, adduction would begin to give place to abduc- tion, provided the center of motion of the eye be fixed. Paralytic Exophthalmos. It should not be forgotten that since the four recti tend to draw the eyeball back into the orbit (and balance thus the tensions in the expansion from Tenon's capsule to the orbit with its check ligaments, and the oblique muscles, all of which tend to draw the eye forwards, assisted by the elastic resistance of the retro-orbital fat) it is more than likely that pronounced paralysis of a rectus, when physiological tone is lost, allows the center of motion to advance, and thus the eyeball to be translated forward as well as rotated. This, however, would only intro- duce a source of error into any quantitative calculation, for it would not alter the principles : the paralytic rotation of the globe would be the same in kind as if no translation occurred, but less in amount. It would, indeed, occur about an axis, exactly the same in direction as if there were no translation, but which instead of passing through the center * Perfect orthophoria (by whioh I mean orthophoria maintained if one eye he excluded for a week) is not found in one of a thousand : it is this which makes the horizontal element in paralysis so uncertain. 7 5 Tests and Studies of the Ocular Muscles of motion would lie to the opposite side of it from the paralyzed muscle, so as no longer to be a diameter of the globe. The rotation about this new eccentric axis, however, would be resolvable into an advance of the center of motion and a rotation about it, the latter being the same in kind, but less in degree than if there were no translation. The greater the translation the less the rotation. The translation in itself is of no clinical account, since it does not affect the diplopia directly, but only indirectly by lessening the amount of rotation. Model With Tilted Axes. On an india-rubber ball, like Professor Landolt's, I have represented, as in Fig. 31, the paths pursued by the center of the cornea during contraction or paralysis of isolated muscles whose axes of rotation are tilted to the horizon. Since there are four TEMPORAL lnf.Rect. /NA5ALA V SIDE / Sup RecC. Fig. 31 An india-rubber ball, marked so as to show the Paths of the Cornea during Contraction and Paralysis of the Muscles, if their Axes are tilted (the tilt being purposely exaggerated). muscles concerned, none of which have coincident axes, there must be four corresponding paths (or orbits) for the center of the cornea. Since the axes are not horizontal, the muscular planes, to which they are invariably perpendiculars, cannot be vertical planes, neither can the planes of the corneal orbits, for they are parallel to the muscular planes. Construction. We may, therefore, after deciding how much to tilt the axes in a model (e. g., an india-rubber ball), still follow Professor Lan- dolt's plan of placing one leg of a pair of compasses on the extremity of each axis in turn, and the other leg on the center of the cornea to describe a circle with the latter. These circles are the four corneal orbits for the respective muscles. Differences. In a horizontal-axis model there are only two corneal orbits, each common to a pair of muscles, and it will be seen from Fig. 23 that neither orbit transgresses the vertical meridian : though both touch it at Associated Muscles in a Single Rye 77 the corneal center, they keep strictly to their own sides, so that, in the pri- mary position, adduction is the only result demonstrable by the model of either contraction or paralysis of the recti : and abduction for the obliques. In Fig. 31, however, each orbit crosses the vertical meridian. It may be well to explain that the model does not represent the actual globe of the eye, but an infinitely thin sphere immediately surrounding it and fixed in space, so that the center of the cornea, in its motions, describes these paths upon it. The vertical meridian of this sphere coincides during the primary posi- tion of the eye, with the vertical meridian of the cornea, but is fixed while that moves. The cornea, when its center is found to the usual nasal side of this fixed vertical meridian, is adducted, when to its temporal side abducted. From the fact that each corneal orbit lies in part to one side and in part to the other, it is evident that both adduction and abduction occur with motions about each axis Each rectus (superior and inferior) on contracting, adducts the eye, and each oblique abducts it. The semi-orbit A s is that for contraction of the superior rectus, A i for the inferior, and both are entirely to the nasal side of the fixed vertical meridian, showing these muscles to be adductors. Moreover, since these orbits form an angle at the vertical meridian at the anterior pole instead of touching it by a continuous curve, as in Fig. 23, adduction is more marked and commences at once in such a way as not merely to be an incident of the motion of the cornea, but to be due in part to true rotation of the globe about its vertical axis, which we have shown cannot occur rf the axes of rotation for the muscular contractions are not tilted to the horizontal, since horizontal rotations cannot have vertical components. The more tilted the axes are, the greater are the vertical components of their rotations. The same may be said, mutatis mutandis, of the orbits A s', A i' for the obliques, which show the eye to be more vigorously abducted by con- tractions of these muscles than in Fig. 23. Paralytic Semi-Orbits. When we come to consider paralytic rotations, the two figures are in contrast. What we may call the "paralytic" part of each orbit lies to the oppo- site side of the meridian from the "contractile" part, for a certain distance, and shows that paralysis of a muscle causes, at first, the opposite horizontal diplopia from its contraction. Thus the arc A s" is the paralytic arc for the superior rectus, being continuous with its (already considered) contractile arc A s. It crosses the meridian at A, showing that the slightest paralysis causes abduction at once, which increases to its maximum at b when the depression of the eye is equal to the tilt of the axis from the horizon, and then as the paralytic rotation becomes greater the abduction lessens, till the orbit again crosses the meridian at c, thereafter to give place to adduction. This cross- ing of the meridian occurs when the depression of the eye is twice as great as the tilt of the axis, for c is twice as distant from the horizontal meridian of the fixed sphere as the extremity of the axis. 78 Tests and Studies of the Ocular Muscles I have carefully said "as the paralytic rotation becomes greater" instead of saying "as the paralysis increases," because the paralytic rota- tion does not necessarily keep pace with the increase of paralysis, as we have already seen, the latter perhaps expending its effect to some extent on translation of the globe forwards. When this is the case, what relation exists between translation and rota- tion? It will not do to compound them according to ordinary physical composition, for the translation is not something added to the rotation. The simplest way would probably be to look upon the pathological yielding of the muscle under remaining tension as represented by a definite linear quantity of lengthening. Without translation, this linear quantity, curved round the surface of the globe into a circular arc, measures the angle of paralytic rotation, being the arc which subtends that angle at the center of motion. When translation occurs, this arc of rotation is shortened by a linear quantity equal to the amount of translation. Thus, if the lengthening of the muscle be 3 mm., the angle of rotation, without translation, would be an angle subtended by an arc of 3 mm., but with i mm. of translation it would be an angle subtended by an arc of 2 mm. CHAPTER V Conjugation of the Two Eyes To the best of our knowledge, every innervation of the ocular muscles is conjugate. It is impossible for a nervous impulse to descend to the ocular muscles without being equally divided between the two eyes. In consequence of this the two eyes work together, to borrow Hering's expression, as one organ. It will be seen, therefore, that the "centrifugal" impulses to the eyeballs answer completely to the "centripetal" arrangements of vision. Homonymous halves of the two retinae convey impres- sions to the occipital lobe of the same side. An object, for instance, which lies in front and to the left throws its images on the right half of each retina. If the images fall on corresponding points they are blended into one in the visual center into the right visual lobe. In that case, if attention be directed to the object, both eyes move equally and simultaneously as one organ, so as to receive the images of the object on the two maculae and inspect it by direct vision. More often than not, however, the object is so situated at first that its images do not fall on strictly corres- ponding points. Then, simultaneously with the conjugate lateral movement, an adjustment of the convergence takes place, in an equally conjugate manner, the two movements being compounded into one. True Associates. Every muscle, therefore, has a yoke-fellow in the other eye. The superior rectus of one eye is associated with the inferior oblique of the other ; and the inferior rectus of one with the superior oblique of the other. Graefe based this view on the secondary as well as the prime action of the muscles. For example, the elevating power of the right superior rectus is greatest on looking also to the right : and that of the left inferior oblique is likewise greatest on looking also to the right. On looking to the left, the elevating power of both decreases, and the torsion of both increases. Moreover, their torsion is in the same sense. These muscles, therefore, work together more harmoniously than either could do with any other work-fellow. 79 8o Tests and Studies of the Ocular Muscles As a matter of fact, however, in the voluntary motions of the eyes this pair never works alone, without the other pair of associates, the left superior rectus and the right inferior oblique, as we learn from the study of secondary torsion (Chapter II). Graefe suggested that the best operative treatment for paralysis of a muscle would be tenotomy of its associate in the other eye. Thus, for a faulty right superior oblique, he would think of tenotomy of the left inferior rectus, from the consideration that if both associates are weakened a stronger impulse is all that is needed to remedy the defect in both. There are weak points in this practice, excellent as the reasoning is, one being that the propor- tion of elevation effected by the oblique is much less than that effected by the rectus, and the other that tenotomy does not really weaken a muscle much, but chiefly acts by altering the position and shortening the length of its arc of contact. It does weaken it a little, however, indirectly, owing to the lengthening of the check ligament, and to that extent, it keeps company with the paralysis. Practically, the operation is rarely advisable, unless the diplopia is considerable while the sound eye is in the primary position. Spasm Of Single Muscles. It is rare to meet with an unim- peachable example of those cases, described by others, in which there is true idiopathic spasm of an isolated muscle. It is possible that ocular muscles may be subject to "cramp," like that in the calf of the leg, but I have not met with it. Were it to occur in any but the internal rectus, it could easily be diagnosed. Its characters would be : (a) Sudden, extreme and temporary devia- tion of one eye. (3) Normal motions showing no paretic muscle in the other eye. (f) Absence of marked heterophoria in the intervals between the attacks, as tested, by occlusion or the glass rod. This is the most important point in the diagnosis, for sudden deviation of an eye may be due to liberation of a previously-existing high degree of latent squint and, unless paralytic, is always due to this. Spasm of the internal rectus is often simulated by spasm of conver- gence, and since the latter is always immensely more probable, it should be given the benefit of any doubt, (d} No other paretic muscle in the affected eye discoverable after the attack, the nervous energy intended for which may have overflowed into another, for "secondary deviation " can be a monocular as well as a binocular affection, (e) During pure spasm of a single muscle there would Conjugation of the Two Eyes 81 be a temporary loss of concomitancy, causing the squint to be greater on looking in some directions than in others. In chorea slight irregular contractions of the ocular muscles are said to sometimes take place, as evidenced by brief diplopia. In meningitis and other irritative affections of the base of the brain, irritation of the nerve trunks may cause spasm of individual muscles, though far more frequently paresis, or both. It is comparable to the rigidity that occurs in the limbs (Gowers). Crampy or epileptiform spasms of single muscles have been recorded by Hock, Gowers and Duane, in some cases occurring when the eye was moved into the field of the muscle, in others without any exciting movement of the eye. Hysteria is said to never affect single muscles. I have dwelt at this length upon muscular spasm, in spite of its extreme rarity, because it looks like an exception to the rule of "conjugation," though not really so, since the pathological does not disprove the physiological. Conjugate Innervations. The number of conjugate innerva- tions is, at present, unknown. Five have long been recognized : of which one elevates both corneae, another depresses them, a third turns both to the right, and a fourth both to the left. The fifth is the converging innervation. Of the conjugate innervations of the eyes these five only are voluntary. They are those for the four parallel movements of (1) Binocular elevation ; (2) Binocular depression ; (3) Binocular dextroduction ; (4) Binocular laevoduction ; and that for the totally distinct act of (5) Convergence. Even parallel motions are, however, I find extremely difficult to effect if the eyelids be kept closed, though they can be brought about by the greatest ease in perfect darkness if the eyelids be opened. The act of convergence is not so easy to effect in dark- ness as parallel movements, being rather more dependent than they on visual reflex government in ordinary life. It is most easily effected by thinking of a near object, and it probably has its cortical seat only in the occipital lobes (calcarine fissure). Besides these five innervations, which are more or less under voluntary control, there are two which trim the torsion of the two eyes to 82 Tests and Studies of the Ocular Muscles the right and left simultaneously and which may, therefore, be described as (6) Binocular dextrotorsion, and (7) Binocular Isevotorsion These innervations are absolutely involuntary. We know of their existence from physiological experiments, clinical observations and the phenomena of rotational nystagmus. Others must have noticed what I have sometimes observed, viz. , that after a careful correction of astigmatism the patient may come back needing a slight alteration of both cylinders by an equal amount to either the right or left, showing that a slight prepon- derance of one of the innervations for conjugate dextroduction or laevoduction has occurred during the interval. Moreover, in paresis of an oblique muscle, if it happens to belong to the best eye, it is not very rare to find the tilted image transferred to the unparalyzed eye, from corrective activity of one of these innervations. I have recently seen a striking illustration of the same kind of transference in a doctor, whose left eye had been blind for ten years with ripe cataract, the vision of the right eye being rather poor. After extraction of the cataract, vision was wholly transferred to the left eye and vertical objects appeared slanting to the left, prov- ing that the left eye had become extorted during its blind period. This extorsion, however, soon rectified itself, but in doing so intorted the right eye, as shown by the fact that on occluding the left, objects viewed by the right appeared slanting to the left. The correction, therefore, had been effected by a conjugate inner- vation, viz., that of binocular dextroduction. This correction did not take place once for all, but ceased as soon as ordinary objects no longer engaged the attention, so as to call for it, as shown by experiments with the glass rod. It is not scientific to speak of these corrections as effected by the obliques. This is well illustrated by rotational nystagmus, in which the two eyes, while experiencing simultaneous wheel-move- ment, strictly maintain their visual axes at the same horizontal level, which could not be if the obliques only were the active agents : for if they were, we should find that during double-wheel movement, say, to the right (binocular dextrotorsion) the superior oblique of the left eye would depress the left cornea, and the inferior oblique of the right eye would elevate the right cornea. Conjugation of the Two Kycs 83 The obliques could not of themselves get rid of their subordinate movements. Again, Javal showed, by the observation of astigmatic correc- tion, that when we slope the head towards either shoulder the principal meridians of the two retinse no longer remain strictly parallel with the median plane of the head, but lag behind it a little.; their inclination from the true vertical becoming slightly less than that of the head, though they are still parallel with each other, an observation verified by Helmholtz, with after-images, in a very beautiful way. We have, therefore, abundant evidence of the existence of these two innervations. It is extremely probable that there are innervations for regulat- ing the parallelism of the vertical meridians of the retina with each other, namely, one for (8) Binocular intorsion, and one for (9) Binocular extorsion. There is reason to believe that if only one of these exist, it is probably that for binocular intorsion, and that binocular extorsion is effected by its relaxation or inhibition ; for (though I speak from general impressions only, and not from statistics) my experience hitherto has appeared to show that want of general tone manifests itself rather by a tendency to binocular extorsion than by intorsion. Since the same loss of tone occasions relaxation of the converging innervation, it may be that binocular intorsion plays the same part with respect to the ze'/?e observed between the two, since we can, by an effort of the will, fix one point while (lii-eeting our attention to a neighboring one. We count, indeed, on the possession of this faculty in our patients when we test their field for colors, or their indirect vision by the perimeter. t " Diseases of the Nervous System," vol. ii, p. 195. ioo Tests and Studies of the Ocular Muscles creation, and it is probable that in most animals the whole retina possesses properties intermediate between the center and periphery of our own, so that their sense of the form of objects is inferior to that which direct vision, and superior to that which indirect vision affords to ourselves. Central Fixation. The point of fixation is surrounded by an area of acute vision said to be about three-fourths inch in diameter at the distance of a foot (Le Conte). For small objects, therefore, it may suffice to fix one point, since all the other points will lie in this area ; but for larger objects it is essential to glance from point to point. "The anatomical fovea has a breadth of 0.2 mm. to 0.4 mm. (Henfe), or, viewed from the posterior nodal point (which is 16 mm. from the retina) an angular breadth of 45' to i 30'. On looking at the sky the fovea would, therefore, cover a portion having two or three times the diameter of the moon, which corres- ponds to half a degree. The point of fixation has a much smaller breadth, for we can easily tell whether we are fixing the right-hand or left-hand margin of the moon. In general, as soon as we can distinguish that two points are discrete we can tell which we are fixing. It wasyatWwho emphasized this fact" (Tscherning).* By "central fixation" we mean fixation exerted to bring the images of objects on the point of acutest vision, and this is almost the only kind of fixation which exists in the ordinary use of healthy eyes ; but since it exists in the interests of direct vision, it becomes lost as soon as the power of direct vision is destroyed : as, for instance, by disease of the macula, or a central scotoma. The eye then tends to wander, since the central blind area of the retina is surrounded by a zone in which no point of acuter vision than the rest exists, or, if it does exist, the eye has yet to learn to use it exclusively for fixation. If central vision is impaired at birth, or shortly afterwards, true fixation is not often acquired, and nystagmus frequently results. Ophthalmoscopic corneal images, as Priestley Smith has pointed out, afford a simple means of observing whether an eye has central fixation or not. "Fixation-Line." Having defined the "point of fixation " as that point outside the eye which at any moment engages the eye, it is easy to conceive the "axis of fixation" or "fixation-line" as an imaginary straight line extending from this point to the center of motion of the eyeball. * " Physiologic Optics." p. 36. Fixation, Projection and Binocular Vision 101 Field Of Fixation. The field of fixation is the expression of the mobility of the eye in all directions. It is, for this reason, sometimes called the ' ' motor field. ' ' If we think of the eye as placed in the center of an imaginary sphere, the part of the sphere which bounds the extreme sweeps of the fixation line is the field of fixation. Fig- 37 gives Landolt's measures for the greatest possible excursions of an eye in all directions, while Fig. 38 gives Schuur- man's. Both of these observers distinguished between the lateral mobility of hypermetropic, myopic and emmetropic eyes. In both 34 M. 38 t. 42 H. 38 C Nose Fig 37 Landolt's Figures for the Field of Fixation. -41 M. -45 E. -47 H. (Nose) 57 Fig. 38 Schuurman's Figures for the Field of Fixation in Myopia, Enimetropia and Hypermetropia. figures the upward mobility of the eye is considerably less than the downward. Stevens gives 33 for the maximum elevation and 50 the maximum depression of normal eyes.* Binocular Fixation. It is frequently taken for granted that binocular fixation and stereoscopic vision are necessarily the same thing, but the latter requires a higher order of cerebration than the former On throwing the light from the ophthalmoscope into the eyes of a patient after operating for squint, it is not uncommon to find that while both eyes apparently fix the mirror quite truly, yet the subject of the experiment suppresses the image of one eye. It is, of course, very difficult to prove that binocular fixation under these *To departures from this proportion Stevens has given the names of "ancephoria" or "kataphoria," according as elevation exceeds or falls short of its proportion to depression. IO2 Tests and Studies of the Ocular Muscles circumstances is real instead of apparent, but it is well to bear in mind the possibility of its existence. Projection. Objects whose pictures are formed upon the retina are not themselves supposed to be within the eyeball, but are mentally relegated to some external position in space. This cerebral process is called "projection." The more perfectly it is performed the more truly the projected pictures of objects coincide with the objects themselves. Though projection is a congenital faculty, since there never was a time when we imagined objects to be located within our eyes, it is perfected during the exercises of childhood when the real position of objects is constantly being discovered by other senses. Given the direction of an object from an eye, and its distance in that direction, its position is known. It is convenient, however, to treat the ' ' perception of distance " as a separate study, and treat projection as if related only to "direction." It is important to recognize that projection is not a faculty of the retina, but is a mental act. Field Of Projection. Related by its own constant angle to the "line of direction" in which a picture focused on a single fovea is projected, there is a definite and unchangeable line of direc- tion belonging to every percipient element in the retina. Though the attention of the mind is generally concentrated upon whatever picture occupies for the moment the fovea, the whole retina is covered by a continuous sheet of pictures of other objects, both near and distant, some in and some out of focus. It is the projected images of these which constitute the field of projection. Since pictures on the retina are inverted and since the direc- tion of projection coincides practically with the axes of the incident pencils of light which enter the pupil from outside objects and which, therefore, cross each other in the crystalline lens, it follows that the field of projection is re-inverted, so that its right half cor- responds to the left half of the retina and its upper half to the lower half of the retina. For this reason objects, in spite of their retinal images being inverted, appear erect and as they are. Malprojection Of a Field. In natural acts of vision the direction of projection of an image on the fovea coincides with the visual axis of the eye ; but when, under some unusual or pathological conditions, the direction of foveal projection is displaced away from Fixation, Projection and Binocular Vision 103 the visual axis, every part of the field of projection is equally dis- placed in the same direction, so as faithfully to retain its relation to the foveal projection. In other words, projection in a field is always true,* though projection of & field may be " false." Coincidence of the Two Foveal Projections. Pictures formed upon the two foveae are projected under all conditions very faith- fully to the same spot in space. Thus, in recent paralytic squint, two candles held in line with the two visual axes invariably appear as one. Again, if a piece of paper be pricked through with a pin at two points separated by the interocular distance, and be held up close to the eyes so that distant objects can be seen through them, the two holes themselves are seen as one, in the median line. Another well-known experiment is to create a tiny foveal after- image by looking at a bright point of light with one eye ; then, whatever object be fixed by the other eye and however much the spectralized eye may be artificially displaced even by forceps, or made to squint, the after-image still clings tenaciously to whatever point is fixed by the other eye. Corresponding Points. What is true of the fovea is also true, in a less-pronounced way, of all other parts of the retina. Every percipient element in one retina has a corresponding element in the other (situated similarly with respect to its fovea, i. e., at an equal distance in the same direction from it, so that its projection in space is identical). Double Set of Corresponding: Points. In very old-standing squint (strabismus incongruus) it sometimes happens that certain, at least, of the percipient points in one retina have two correspond- ing points in the other, of which one was originally true, and is, therefore, again true when the eyes are put straight by operation, and the other created during the condition of squint. Rotation of the Field of Projection. It has seemed to me possible that without any actual translation of the field as just described, the corresponding points may become altered in some cases by rotation as a whole about the point of fixation. In a case, for example, of complete traumatic paralysis of the superior oblique I found, years after, that the field of projection was still perfectly untorted on looking straight forward. Since the eye gave much evidence otherwise of so-called secondary contracture (consecutive *This statement, ot course, supposes absence of anatomical changes in the lens, etc. IO4 Tests and Studies of the Ocular Muscles deviation), it can scarcely be conceived that in this case there has been no actual paralytic torsion. Physiological Diplopia. In every ordinary act of vision a vast number of objects do not throw their images on corresponding points of the two retinae. For every position of the point of fixa- tion there is what is called a " horopteric surface," all objects in which are seen single, while all other objects would be seen double were they closely analyzed. If the two forefingers be held before the face in the median plane, one in advance of the other, and the farthest one be fixed, the near one is seen double ; and by momentarily closing the right eye it is easy to assure one's self that the left of the two images belongs to the right eye and the right image to the left eye. This proximal diplopia (as we may call it) is, therefore, crossed. (See Fig. 40.) If, on the other hand, the near finger be fixed, of the two images of the distant one which now appear, the right one disap- pears on closing the right eye, and the left on closing the left, showing that the distal diplopia (as we may call it) is komonymous, By such experiments, we learn that all objects nearer to us than the point of fixation (and the horopteric surface connected with it) have crossed images, while all objects beyond have homonymous ones. Our higher intellectual powers are insufficient to inform us whether any double images which we see are crossed or homony- mous, proximal or distal ; but some inferior center seems to have no such difficulty, for as soon as an effort is made to unite the images, it always commences in the right direction without any preliminary trial to discover whether it is an effort of convergence or one of divergence that is called for. Suppression Of Images. In the physiological diplopia of people who are right-handed, the image which belongs to the right eye is apt to appear more substantial-looking than the other (Tscherning), and this is probably especially the case with those who are accustomed to frequently use the right eye separately, as, e. g., in aiming. Con- sistently with this indication, while attention is diverted from the diplopia and concentrated upon the point of fixation, the less sub- stantial image of objects out of the horopter is in most persons so entirely ignored by the mind as to be what is called " suppressed." Even when it is not so, the diplopia attracts no attention, because from absence of critical analysis it is undistinguished by the mind Fixation, Projection and Binocular Vision 105 from that other kind of indistinctness which is due to the object being out of focus. Origin Of Projection. In projecting the retinal field into space the mind must have some " point of origin " for the radius vector, or "line of direction" in which the projection is made. Hering places this origin midway between the two eyes, as if they were united into one cyclopic eye. His idea is supported by the double pin-hole test previously mentioned, and is no doubt true of those whose eyes are of equal value in binocular vision. Some, however, and possibly the majority even of those who have equal visual acuity in the two eyes, seem to use one eye rather as the aide-de- camp of the other, than as an equal partner in projection. One is then called the "directing eye" (Javal), since the origin of projec- tion appears to be displaced to coincide with this eye. Tscherning finds this condition in his own case and that of several others, and himself evidently judges of the position of objects much more truly with his right eye than with the left, probably because it has been most often used separately. Test for True Projection. The following modification of a very old test by Hering enables the co-ordination of hand and eye to be well tested : Take a large piece of cardboard, marked in the middle of each surface with a short vertical line, these lines being exactly counterposed, which can easily be ensured by pricking the cardboard through at their extremities. Holding the card vertically six inches before the patient's eyes, let him endeavor, by passing his hand behind the cardboard, to place his finger exactly behind the vertical line which he sees. He should make the attempt as carefully and judgingly as possible, with first one eye shut and then the other, using also in each case first the right hand and then the left. The surgeon standing behind the cardboard can see perfectly from the line on the back the nature of any failure in projection, while the patient never, himself learns in what direction his aim has been missed. Herein lies the advantage of modification, for if once the patient knows his error, he makes mental allowance for it in his next attempt. Illustrative Errors Of Projection. In near vision, as we may see elsew r here, an excluded eye generally deviates outwards, and hence arises an error in monocular projection. Fig. 39 illus- trates, as an example, the case of an aurist examining the drum of an ear. The left eye, having nothing to do, diverges, and the io6 Tests and Studies of the Ocular Muscles Fig. 39 Mis-projection by an aurist who closes, or does not use, his left eye. apparent position of the drum, as represented in dotted outline, lies in consequence midway between the visual lines. Another error of projection may be demonstrated by quickly thrusting the finger at a pencil held about a foot away from the eyes at the extreme lateral limit of the motor field while the eyes are strongly turned toward it. The finger will generally miss its mark to the outer side of the pencil. The reason of this error is that the ordinary cal- culations of the mind are formed from the more habitual smaller obliques of vision, and the excessive effort required to produce unusual obliquity of the visual axes creates the impression of a proportionately displaced object. At the limits of the motor field, strong increments of effort produce smaller increments of result, owning to mechanical difficulties in the motions of the eyes. Malprojection, kindred to the last, is seen to a more marked extent when a muscle is paralyzed. Since the mind is counting on every muscle to do its duty, the least failure in contractile response to stimulus results in malprojection proportionate to the failure, and in the direction which is suggested by the greater effort put forth. Fusion. Since corresponding points have their pictures pro- jected to the same point in space, the mind cannot but regard them as one, since it cannot conceive two objects occupying the same place at the same time. But there is to be considered more than the mere existence of single vision : there is a natural love of single vision, expressed by a strong sub-conscious desire to bring together and thus fuse any double images of the same object while even one of them engages the attention of the mind. Withdrawal of attention to another object almost, if not quite, abolishes this desire ; also anything which makes one image differ from another, either in color, size or shape. It is the absence of this "abhorrence of double images" or "love of single vision," as it has been called, in very long-stand- ing cases of strabismus which is the chief difficulty encountered in training the eyes to work again together. When entirely absent, Fixation, Projection and Binocular Vision 107 it is said that there may be even a desire to separate the images in order to see one of them more clearly, a condition described by Graefe as antipathy to single vision. But this antipathy is, I fancy, merely a mental choice, not a sub-conscious contrast to the "love." It is quite reasonable to expect it, since one field embarrasses the other less in proportion to its displacement therefrom. The Power Of Overcoming: Prisms. If a prism be held before one eye, its effect will be to displace the image belonging to that eye to another part of the retina, so that, for a moment, vision will be double. The image seen by the naked eye is the "true" image, since it is mentally referred to its true position in space : the image seen through the prism appears displaced from the true position of the object in the direction of the apex of the prism by an angular departure equal to the angle by which the prism deviates light, and which for brevity we call the "deviating angle of the prism." If the prism be weak enough, the co-ordinating centers endeavor to overcome the diplopia by directing the embarrassed eye towards the apex of the prism, so as to again receive its image on the fovea. This is done by the conspiracy of at least two conjugate innervations, and when it is effected, vision is again single.* Apparent Prismatic Displacement. Now, however, the object does not appear to occupy the position either of the former true one or of the former false one, but lies exactly midway between the two. A person with both eyes open and a prism before one eye, will, as I have shown elsewhere, misjudge the position of objects, even though he see them single ; but his malprojection will only equal half the deviating angle of the prism. If he cover the naked eye with his hand, under these circumstances, the image may appear to move slowly till the malprojection is doubled. All this proves that the innervations at play are conjugate, according to the principles already mentioned. \Yhen distant objects are viewed, a prism higher than No. 8 (with 4 deviation therefore ; for the deviation of light by a prism is by an angle about half its apical angle), with apex out, cannot generally be overcome. Since the prism is held before one eye, we have to mentally divide its effect between the two eyes. It follows, therefore, that a *For diagrams to illustrate this, vide " Ophthalmological Prisms" (J. Wright & Co.), pp. 62, 63. io8 Tests and Studies of the Ocular Muscles divergence of 2 from parallelism, of each eye, is the greatest diver- gence which even the love of single vision can usually induce the co-ordinating centers to effect. It is far otherwise, however, with prisms whose apices are placed inwards, and which can be increased to much greater strengths without inducing diplopia. Since, in the vertical motions of the eyes, there is nothing known correspond- ing to the converging innervation, it is remarkable that prisms with their apices upwards or downwards can be overcome at all. As a matter of fact, to be overcome, they must be very weak. A prism of from 2 to 4 (i d. to 2 d.) before one eye, is the strongest vertical prism that can generally be overcome, without practice. Breadth Of Fusion Power. When diplopia is created artificially, as by prisms, the smaller it is, i, e., the less the double images are separated, the greater is the desire, and the easier is the task, to effect fusion. There are limits to the separation of the images, beyond which the diplopia becomes insuperable. These limits define the ' ' breadth of fusion ' ' (as it is generally called for brevity). Three Conditions. When the breadth of diplopia is ( i ) greater than the breadth of fusion power, no effort can unite the images. When they are (2) almost equal, the images may be united by a great effort for a short time. When the breadth of diplopia is con- siderably (3) less than the breadth of fusion power, the images are easily united. These three conditions are found respectively in "permanent squint, " in "periodic squint" and in " latent squint" (heterophoria). The difference between them is merely a question of degree. There is a great difference between the breadth of fusion power in different individuals, and it varies also for different dis- tances of the object-point, and according as the diplopia is homony- mous or crossed ; according, too, as the health and the will- power vary. The power to fuse horizontally separate images is much greater than the power to fuse those which are separated vertically ; and "crossed" diplopia is more easily overcome than "homonymous," since converging effort is easy and diverging effort difficult. The best way to estimate the breadth of fusion is to find the strongest prisms which the eyes can overcome : the strongest prism base in added to the strongest prism base out, gives the horizontal Fixation, Projection and Binocular Vision 109 breadth, while the strongest prism base down added to the strongest prism base up, gives the vertical breadth. A convenient convention to adopt is that prisms base in measure the negative breadth of fusion, and that prisms base out measure the positive breadth : the two together, of course, constitute the total amplitude. In making tests of this kind with prisms it is necessary to remember that anything which makes one image differ from the other lessens the desire to unite them ; hence, in using strong prisms which alter the image by chromatic and prismatic aberration, it is best to divide them equally between the two eyes, so that the two images shall be equally perturbed. Caution. In cases of defective converging power at reading distance some have tenotomized the external rectus with the result of producing homonymous diplopia in distant vision. Such mistakes would have been avoided by taking the trouble to test how much negative breadth of fusion the patients possessed (or, in other words, how strong a prism, base in, they could overcome) in dis- tant vision. A defective negative breadth is an evident contra- indication against division of the external rectus tendon, for homonymous diplopia in distant vision is the most difficult of all the horizontal forms of diplopia to overcome. Binocular Fixation. We should, perhaps, draw a distinction between (a) Binocular fixation, (^) Binocular vision and (c) Stereo- scopic vision or perception of relief. The first of these is beneath the region of consciousness, the two eyes jointly fixing the same object from habit, even when the mind suppresses or at least pays no regard to the vision of one eye. I have seen cases in which it seemed that binocular fixation was preserved, both visual axes being directed correctly, so far as objective tests could discover, even though diplopia could not be elicited by prisms. However that may be (for it is confessedly difficult to understand), there is no doubt that the second should be distinguished from the third, for binocular vision, in which certain objects seen by one eye are mentally recognized simultaneously with vision by the other eye, is inferior to the pow r er of erecting bodies into ' ' relief ' ' with such an instrument as the stereoscope. Some have this last power much more intensely than others. Monocular Perception of Distance. It is well to know in how many ways a single eye can gain an idea of the third dimension, so as not to be deceived when testing for true binocular vision. They are : no j esis and Studies of the Ocular Muscles (*z) Aerial perspective. More distant objects are veiled by a greater depth of atmosphere, and the greater the depth of atmos- phere the bluer also this veil is. In mountainous districts, when the atmosphere is unusually clear, distances are judged to be less than they really are ; the reverse being the case in a fog. (6) Shadows and overlappings. With the source of light behind us, an object which throws its shadow on another object is, of course, nearer. So also is an object which hides part of another object. (r) Visual angle of known objects. The size of many objects is so well known, that their distance can be estimated by their apparent magnitude, as, for instance, in the case of men, horses, etc. (*/) Mathematical perspective. The gradual decrease in the size of similar objects and the gradual approximation of parallel lines, is too well known to need further description here. The number of intervening objects also influences our judgment ; hence, distances at sea appear less than on land, it is stated. (w;z one object to another we learn their relative distances. Fixation, Projection and Binocular Vision in Fig. 40 To show that Proximal Diplopi crossed, and Distal Diplopia homonvmous. a is Stereoscopic Vision or Perception of Relief. It has been said by Dove and others that objects appear solid when seen by so instantaneous an illumination as that of an electric spark. If this be so, the appearance of solidity must be due to the physiological diplopia of those parts which are not seen single. For the analysis, however, of "relief" and the quan- titative perception of depth, it is necessary that the eyes should unite in succession different parts of the object by consecutive increase and decrease of convergence (Briicke). Hence, many find that with a stereo- scope the appearance of relief does not appear until after a few such motions have been made. Fig. 40 shows how this principle works in the case of a lead pencil, held pointing forwards in the median plane a little lower than the eyes. When the far end of the pencil is fixed, the near end is seen double. By converging a little more so as to fix the middle of the pencil, both ends exhibit diplopia of half the magnitude which the near end at first ex- hibited, and by converging still more to look at the near end the far end exhibits wide diplopia. Stereoscope. Fig. 41 gives the plan of a Brewster's stereo- scope, A and B being the pic- tures. These are taken from slightly different points of view with a photographic camera, so that the distance between identical objects in the foreground of the two pictures is less than between identical objects in the back- ground. To fuse the former, therefore, more convergence is called for than to fuse the latter. Foreground objects and background objects cannot both be fused simultaneously : if they could the Fig. 41 Plan of an ordinary Stereoscope (the pic- tures, howpTer, being separated mote than usual for diagrammatic purposes). 112 Tests and Studies of the Ocular Muscles sensation of relief would disappear. While looking at the fore- ground of the scene depicted, there is physiological diplopia of the background, and vice versa, just as with the lead pencil of Fig. 40. The decentering outwards of the lenses enables the eyes to converge somewhat, as, for instance, to C ; but the united picture is generally only projected to D, since the knowledge of the convergence, which acting alone would project it to C, is in part overborne by the conscious knowledge of the size of the stereoscope which tends to bring the projection towards the plane of A B, unless the picture itself is one of a scene we are accustomed to think of as distant. In that case the projection depends a good deal on the powers of imagination which make the stereoscope forgotten. The pictures (A, B) generally lie slightly within the focal length of the lenses, so that accommodation is not wholly relaxed, and there is nearly always a certain amount of associated convergence. The distance between the pictures is generally made greater than the inter-ocular distance, to allow room for larger pic- tures to be used. A handy form of cheap stereoscope is shown in Fig. 42. Of the expensive ones, prob- ably the best for clinical purposes is Javal's " St6- r6oscope a cinq mouve- ments. " So various are the experiments which can be made with a stereoscope that the interested reader is referred to some book which, like Javal's, is wholly devoted to the subject. One of the best devices is that by Green, in which the letter L is placed before one eye and a letter F before the other. The patient who uses both eyes simultaneously sees them combined into an E. This is a test for binocular vision, but not for stereoscopic vision. No stereo- scopic test for the notion of relief is quite so clinically satisfactory as Hering's drop test, for in most others we have to rely upon the patient's statements, without being able to verify them in the same unmistakable wav. Fig. 43 Fixation, Projection and Binocular Vision 113 Mr. Berry's Sterescope. This is a very ingenious and satisfac- tory arrangement. Before each eye, in a stereoscope is placed a fixed circle, with a small movable circle within it, as shown in Fig. 43. By a simple mechanism the two small circles can be made to mutually approach one another, as shown by the small continuous circles, or mutually recede from one another to occupy the position shown by the small dotted circles. When their separation from each other is at its least, they resemble the images in the foreground of a landscape, so that the device is seen Mr Bcrry 8 ; sterescope . in relief, like a truncated cone or a bucket upside down. But when their separation increases, so as to be greater than the separation of the large circles, they resemble images in the background of a landscape and produce the appearance of a hollow cone or empty bucket. During the motion from one position to the other the stereoscopic effect is one of move- ment in the third dimension, the small circle appearing to sink from a plane above the great one to one which lies beneath it. This appa- rent movement, says Mr. Berry, is so evident, especially if the experi- ment be made in semi-darkness, that young children can at once say whether they see it or not, and seeing it, of course, implies the exercise of stereoscopic vision. Lecture Controlled. Javal's long-known plan of holding a pencil vertically midway between the patient's eyes and a page of print to see whether he can read continuously without suddenly bobbing his head to avoid the pencil, is a test not of stereoscopic vision nor even exactly of single binocular vision, but of the power of rapid alternate binocular vision. (#) If one eye be amblyopic he cannot, of course, read that part of the print which lies behind the pencil, as viewed from the good eye, without bobbing his head. (b} If both eyes have sufficient visual acuity and yet are not working together, there must either be a head-bobbing or else a pause from disconcertment when the deviated eye has to suddenly take up fixation, followed immediately by a second pause before the sound eye can resume it. An excellent arrangement by George Bull enables the patient to place against his forehead a light framework which supports both the print and a vertical rod in Tests and Studies of the Ocular Jlfuscles front of it. Previous to this he employed a bent strip of brass to be held against the book by a wooden spring-forceps. A somewhat similar arrangement, but to be held by the thumb only, has been used by Priestley Smith. What I use myself is a Holmes stereo- scope with two crosspieces, one for holding the print, the other for holding a series of upright strips of metal or whalebone, the strips being made of a dull black. Javal, too, has constructed a ' ' multiple controller" consisting of five bars side by side. Bering's Drop Test. In this test the patient sees an object for so brief an interval that there is scarcely time for a full movement of convergence to occur. It tests, therefore, rather what has been called the "notion" of relief, than the "measurement" of it. It requires a flattened cylinder or shallow rectan- gular wooden box about ten inches long by three or four broad, and open at both ends. From the farther end two wires project for- wards and outwards, connected at their ex- tremities by a horizontal thread which is pro- vided with a small bead at its mid-point for the patient to look at through the cylinder. Fig. 44 shows a very satisfactory home-made arrangement consisting of two cylinders of cardboard fixed together, the only disadvan- tage of which is that the two circular extre- mities are apt to solicit their own fusion and thus interfere with the free movements of con- vergence. Whatever form is used, it is impor- tant to exclude all vision of the operator's hands. Small objects, such as beans or marbles, of different sizes, are dropped from one hand into the other, some beyond the thread and others within it, taking care that on the whole those which fall beyond the thread are a little larger than those which fall within it. If stereoscopic vision exist, he will almost always give a correct answer to the question on which side of the string the ball falls ; but if not, nearly half the answers will be wrong. Fig. 44 Home-made form of Ber- ing's Drop Test. CHAPTER VII Strabismus Definition. Strabismus may be briefly defined* as " inconcert of the fixation lines," or as "a defection of one fixation line from the other. It exists whenever the two visual axes are not directed simultaneously to the point of fixation. Only one fixation line deviates as a rule, and the angle of its defection measures the squint. Chief Division. The chief division of true squints is into paralytic and non-paralytic. This division is almost identical with that into incomitant and comitant squints, since in nearly all paralytic squints the conjugate movements of the eyes are incomitant, z. e. , are unequal in certain directions of vision, as evidenced by increas- ing separation of the double images ; while, on the other hand, in nearly all non-paralytic squints their equality is so preserved that the squint remains of the same magnitude in whatever direction the eyes look, provided accommodation remains unchanged. We shall see, too, further on, that in paralytic squints the "secondary ' deviation, i. e. , that of the better eye when it is placed behind a screen so as to oblige the squinting eye to take up fixation, is greater than the primary, while in non-paralytic squints they are equal. (Paralytic squints are treated in the. next chapter.) Horizontal or Vertical. When an eye squints in or out, the squint is horizontal and is called "strabismus convergens," or "divergens," as the case may be. When an eye squints up or down, the case is one of vertical squint and may be " s. sursumver- gens " or " s. deorsumvergens,"f according as the squinting eye is higher or lower than its fellow. Horizontal and vertical elements very frequently co-exist, and it is rare to find a pronounced old convergent squint that has not a slight vertical element as well. Alternating: or Unilateral. In the first, alternating squint, the patient fixes with either eye at pleasure, the other squinting while *lt will be seen that I have not felt able to adopt one author's suggestion to make defect ol the fusion faculty a necessary part of the definition of squint. To do so would make the definition far too narrow and leave unprovided for several varieties of squint due to quite other causes. tl prefer the more manageable terms, "s. ascendens" and "s. descendens," but have retained those in the text in deference to usage. 115 Ii6 Tests and Studies of the Ocular Muscles he does so, for the reason that the two eyes are of such equal value that he has no preference. Worth finds that fifteen per cent, of constant squints belong to the alternating variety and divides them into "accidentally alternating squints" and "essentially alternating squints." The first class only differs from monolateral squints in the accident of the eyes being of equal refraction. The second class has a congenital total inability to acquire fusion. Since there is no ' ' anopsia ' ' in alternating squints, there is, of course, no ' ' amblyopia ex anopsia. ' ' Alternating squints of the ' 'essential ' ' class are, of course, only capable of cosmetic correction. A large number of squints are transitions between the com- pletely alternating and the completely monolateral varieties, one eye squinting very much more than the other, but not exclusively. Needless to say, even the occasional use of the generally squinting eye greatly retards the development of its amblyopia, though there is little doubt that the longer such a squint is neglected the more it tends to become completely monolateral. In contrast to squints of this kind, in which either eye takes up fixation indifferently, most squints are "unilateral," the patient having a distinct preference for one as the "working" eye. The way to distinguish to which of these classes a squint belongs, is to screen the working eye ; this makes the other take up fixation. If, on unscreening, the transference continues unchanged, the squint is alternating ; if, however, the squint reverts to its original eye, it is unilateral. In unilateral squints the squinting eye is nearly always determined by some diminution of visual acuity, either retinal or from higher ametropia, astigmatism or corneal nebulae, conditions which always predispose to the development of squint. Traumatic cataract and macular hemorrhage are mentioned by Percival. Strabismus Convergens Concomitans. The great majority of convergent squints are of this kind, being purely due to excessive activity of the converging innervation. Nearly all cases of concomitant convergent squint disappear under chloroform, showing that the internal recti are not contrac- tured or structurally altered, but only unduly innervated.* In most cases this activity was at first occasioned simply by association with excessive accommodative effort called forth either by hypermetropia * I have seen one case, but only one, in which the eyes (previously divergent) converged under chloroform. Strabismus 117 or possibly, in a few cases, by paresis of the ciliary muscle, as sug- gested by Javal. Convergent concomitant squint is sometimes con- genital, but far more frequently commences about the age of three years, when children first begin to regard small objects attentively. Possibly at this age accommodation begins to require a greater effort than before, from changes in the consistency of the lenses or a diminution of its early rotundity. Or, it may be, that sometimes at the age when a squint begins, the insulation between accommo- dation and convergence is still more incomplete than usual, so that strong accommodation is impossible without equally strong associated convergence from overflow of nervous force. The fre- quent association of squint with some other defects of the nervous system has been pointed out in France. It has always been a difficult question why some hypermetropes squint and many others of similar refraction do not. Later develop- ment than usual of the insulation just spoken of is an extremely probable cause, while congenital deficiency or late development of the love of single vision or feeble intensity of the fusion faculty is another possible cause, of which we must take an equal account. The success obtained by Worth in training the fusion faculty in very young squinters shows how rarely the faculty is completely absent. It proves, therefore, that other causes must co-exist in the great majority of cases, since it would be extremely unlikely that after using the fusion faculty for three years it would be surrendered at the usual age when squint comes on, unless its surrender were compensated for by some other gain. It is evident that any squint due solely to defect of the fusion faculty would date from infancy. When once formed, a squint persists from innervational habit. The influence of habit is seen in the fact that accommodative squints are generally not lessened by as many meter angles as there are diopters of refracting power in the correcting lenses (Berry). This is proved by measuring the squint first with, and then without, correction. Since the innervation is common to the two eyes, it affects them both equally, and only the desire for fixation keeps them both from squinting. When one eye looks straight forwards, in order to fix an object, doing so doubles the squint in the other eye, so that one eye bears the blame for the squint in both. A squint is often increased temporarily by nervous excitement a fact which also points to its innervational character. This 1 1 8 Tests and Studies of the Ocular Jlfuscles nervous element must be distinguished from the accommodative element. In some cases emotion seems to excite certain oculo- motor centers more than others,, so that a squint is temporarily increased under its influence. This increase is not necessarily an increase of convergence only, but if there be already a vettical element in the squint, that too may increase under the influence of emotion. The surgeon's measurements, therefore, may lead him to form an exaggerated opinion of the squint, since it becomes greater during consultation. Happily these cases, in their marked forms, are rather on the rare side. They should be approached with great caution, if the question of operation has to be considered. It is well known too how frequently such reflex irritation as helminthiasis accounts entirely for a temporary squint, and I have proved that even slight irritation of the primes vite from indigestion may stimu- late the converging center enough to cause a temporary latent squint. Another cause of temporary squint is hysteria ; cases of this kind are, however, more frequently classified under the name "spasm of convergence." Accommodative Squint. This name is given to a squint which disappears during a vacant stare, appears when attention is fixed and increases markedly as an object of fixation is made to approach the eye. In its incipiency every accommodative squint was at first only occasional, occurring during close vision of near objects, and therefore (Javal) likely to be unnoticed, owing to the inclined posi- tion of the head. The child, finding that by allowing the squint to occur, he can see distinctly with less effort, forms the habit of squinting more and more. Accommodation is effected more easily when supported by a full, or more than full, share of associated convergence. Once formed, the habit of thus assisting the accom- modation cannot be broken, a new relation is formed between the two efforts, and the squint becomes less and less confined to near vision. At this stage, even though in distant vision squint should never actually occur, the tendency to it is evidenced by the way in which an eye deviates inwards as soon as it is screened by Javal's disk of ground glass, or in any other way "dissociated" from its fellow (Chapter XII). There is, therefore, at this stage, '''latent" or " super able" squint in distant vision, combined with "insuperable" squint in near vision. At a later stage the squint becomes, even in distant vision, insuperable, and thus what is called a " permanent clement'' 1 is by degrees developed in addition Strabism us 119 to the " variable element," which is added to it whenever accommo- dation is active. Treatment of Accommodative Squint. Tne treatment of accommodative squint lies evidently in the correction of refrac- tion. Less accommodation is then called for and, therefore, Itss associated convergence. The cure takes place completely and at once if the squint be in its early periodic stage ; but since at the time when we first see a squint the hypermetropia has generally become much less than when the squint began, owing to the development of the eye, the diminution of accommodation by the spectacles is much less than the excess of accommodation which originally brought about the habit. The correction of refraction does not always cure even an accommodative squint at once, but lessens it by degrees. Occlusion of the fixing eye for a considerable time, to improve the working powers and the visual acuity of the habitually-squinting one, is a good adjunct. Another treatment for accommodative squint is the instillation of pilocarpine drops, which make the ciliary muscle respond more readily to impulses, thus lessening the effort of accommodation and with it the " associated convergence," but it seems to me to be only palliative. Convergent Squint Without Hypermetropia. A fair proportion of convergent squints are found to exist without hypermetropia or hypermetropic astigmatism. Some of these may possibly, though not very probably, be due, as Buffon believed, to imperfect visual acuity of one eye leading to voluntary squinting in order to get rid of the disturbing effect of a blurred image. A far likelier history in many cases is that hypermetropia existed at an early age, which has since disappeared. In others the want of balance seems inherent in the musculature, while yet others may have had at an earlier date some paresis either of the ciliary muscles or, and this is far more common, of one or both of the external recti at birth. In another important class the patient squints because there is congenital deficiency of the fusion faculty, just as there is deficiency of another kind in color blindness, and the cure of the one is as hopeless as of the other. Any inequality in the visual acuity of the two eyes lessens the value of binocular vision, so that the more ametropic eye is readily relinquished if by so doing less accommodative effort is required. The squinting eye is generally more astigmatic than the other, but not always, for sometimes an astigmatic eye is the fixing one, while its much more hypermetropic 1 20 Tests and Studies of the Ocular Muscles fellow squints. It is then simply a question of choice between superior visual acuity or minimum effort, for of those which have astigmatism in one eye and higher hypermetropia in the other, some prefer distinct vision with a great effort and use the hyperme- tropic eye, while others prefer less distinct vision with less effort and use the astigmatic eye. Corneal Nebulae, though they do not cause squint, predispose to it by lessening the value of binocular vision, and thus favor the surrender of the eye if, by that means, accommodation is facilitated or the image freed from haze. Congenital Amblyopia, from imperfect development somewhere, probably plays an important part in many, if not most cases of squint and is to be distinguished from that amblyopia which, being simply due to disuse and to habitual mental suppression of the pictures in one eye, is called amblyopia ex anopsia. In nearly every case of the ordinary convergent squint, no matter how amblyopic the squinting eye may be, its fundus appears perfectly normal and the macula tantalizingly perfect. The element of the amblyopia which is due to disuse can, I think, to some extent be distinguished from the congenital element by a considerable difference in the visual acuity of the outer and the inner halves of the retina, so that if both of the surgeon's hands be held up simultaneously, one on one side and the other on the other side, while the patient looks straight forwards, the movements of the outer hand appear much more vivid to the patient than those of the inner, for the probable reason that the inner half of the retina, since it looks outwards, has been less disused than the outer half. The same " ex anopsia" element is, of course, still more clearly demonstrated by the rapid, though generally only partial, recovery of visual acuity which attends continuous occlusion of the better eye. Even a few days makes a difference, and Javal has pointed out that if the occlusion be long continued, improvement takes place some- times by sudden accessions, since the eye at first is not only wanting in acuity but is awkward in seeing, like a raw recruit, and this takes prolonged practice to remedy, and is sometimes overcome suddenly, as in learning to swim. Javal lays great stress on imposing- monocu- lar vision in the treatment of squint, without any intermittence, so that if on special occasions it is desired to permit the use of the better eye, the louchette should be transferred for the time being to the squinting eye. Strabismus 121 Development of the Fusion Faculty. The normal development of the fusion sense has been made the subject of special study by Claud Worth. He finds, as others have done, that from the earliest hours after birth the pupillary light-reflex is present. Indeed, the interesting fact was demonstrated long ago that both pupils respond to light incident on one eye only, and that the reflex closure of the lids upon the sudden stimulus of light is obtainable also, though the conscious perception of objects, so far as this can be tested by closure of the lids when an object suddenly approaches the eye, is absent during the first few weeks.* Volun- tary convergence, as in watching the approach of an object towards the face, appears about the third month. Worth has shown that the preponderance of the macular region exists at birth, since light suddenly thrown into an eye by an ophthalmoscope makes the eye immediately fix the mirror, but only for a moment. The duration of this monocular fixation increases during the first few weeks and becomes binocular at about the fifth or sixth, though still somewhat uncertainly so. During the last half of the first year of life, Worth has convinced himself by prism experiments that true binocular vision has been obtained, and that towards the end of that period the eyes will make a considerable effort in the interest of binocular vision. From the results of fusion training in the case of squinters, he concludes that the fusion faculty is fully developed before the end of the sixth year. Defect versus Neglect of the Fusion Faculty. Congenital deficiency of the "desire for single vision" is comparatively rare. It doubtless accounts for the class of alternating squints, with slight refractive error if any, in which Javal pronounced all efforts to draw out the faculty absolutely hopeless. Worth describes these as ' ' essentially ' ' alternating squints, and contrasts them with the " accidentally " alternating, which only differ from monolateral squints in having approximately the same refraction in each eye. Of all constant squints, he finds fifteen per cent, are alternating. There is reason to believe that what is largely attributed to congenital deficiency of the fusion faculty is very frequently due rather to neglected training of that faculty in the early years of life ; not because of any fault in its mechanism, but because the inferiority of one eye to the other (which may be transient, as are * Preyer, 1884, quoted by Priestley Smith. 122 Tests and Studies of the Ocular Muscles early nebulae, retinal hemorrhages, etc. , or permanent, as in astigmatism or anisometropia), so much reduces the value of binocular vision that it is surrendered more readily in favor of any greater advantage, as, for example, that obtained by squint- ing in a hypermetrope. whose accommodation without it is affected with difficulty. In the absence of any such advantage, binocular vision is generally retained, even when one eye is highly astigmatic. The love- of stereoscopic vision is, however, undoubtedly more intense in some individuals than in others. The easy success obtained by Worth in training the fusion faculty in so large a proportion of young squinters appears to show it had suffered from neglect more than from anything else, and it may be noticed that squinters are, as a class, apt to be naturally unobservant. Suppression Of the False Image. The longer a squint lasts, as we have seen, the more difficult it becomes to elicit diplopia, because the mental habit of suppressing i. e., of disregarding one image, becomes confirmed, and, in addition to this, the longer diplopia is absent the more difficult it becomes to re-awaken fusion reflexes. Depth of the Suppression. In cases of suppressed diplopia it devolves on the surgeon to ascertain the depth of the suppression ; in other words, whether diplopia, though absent in general, can be artificially elicited with ease or with difficulty. (#) If with ease, a colored glass held before the working eye will restore it when a flame is looked at. () Failing that, a prism, edge up or down, will be more likely to succeed, by throwing the image of the flame upon an unusual part of the retina. (r) Last of all, when other means fail, the rod test, made of red glass, often succeeds, if held before the working eye, and especially if its effect is heightened by a black velvet screen placed behind the source of light for "contrast. Sometimes a blue or green glass held before the squinting eye assists. Nature Of Suppression Of Vision. Nearly every human faculty can be quickened by concentration of attention upon it and dulled by withdrawal of attention. That this is true in the domain of fusion I have shown by a simple experiment with the visual camera, described elsewhere. In recently-squinting eyes, two different objects throw their pictures on the two maculae, but whichever Strabismus 123 object engages attention for the moment extinguishes the mental perception of the other. Indeed, the only object whose mental appeal is effectual, is the false image of the object under attention, produced by its picture, which falls on an eccentric part of the retina of the squinting eye. The same mental process which obliterates the macular picture of the squinting eye can in time extend itself to the false image as well, and always does so in young squinters with disastrous effect, for the vision of an eye thus repudiated becomes rapidly impaired (amblyopia ex anopsia). As Priestley Smith has well put it, for the squinting eye, the advice not seldom given, "to wait and see," too often means waiting and not seeing. That part, however, of the retina of the squinting eye which answers to the extreme temporal portion of the field in a squint of low degree, is still of value in the monocular perception of objects which are hidden from the other eye by the root of the nose. The retention ot vision to a physiological amount in this extreme portion of the field, as compared with its defect in the opposite (nasal) portion of the field, constitutes a point of differ- ence between amblyopia ex anopsia and congenital monocular amblyopia. This latter is undoubtedly rare, and when it exists is probably due to a defect of some cerebral cells rather than to the retina itself, for congenital defects of the eyeballs are nearly always bi-lateral, as witness high hypermetropia, astigmatism, lamellar cataract, coloboma, iridis, etc. Imperfect Central Fixation. When central fixation is deficient from birth in both eves, it nearly always causes nystagmus, though not necessarily if only one eye be defective. Probably most cases of imperfect central fixation are acquired rather than congenital and, according to Javal, can even be recovered by exercise pro- longed for years by intelligent subjects, and absolutely free as regards their time, though, as he says truly, the advantage gained is out of all proportion to the necessary pains. Worth has never seen "lost fixation" in any case of squint first appearing after six years of age. The central region of the retina may suffer so much from neglect as no longer to be able to count fingers, and Worth even states that it may go so far as to have only bare perception of light within an area extending 25 to 30 from the center of the field. In congenital amblyopia without squint, on the other hand, he has never found the central vision 124 Tests and Studies of the Ocular Muscles lower than / ff ; probably for the simple reason that had it been lower the eye would have squinted, from binocular vision being of so little value. The earlier squint commences, the more rapid is the progress of the blindness, provided it be monolateral, and Worth states that at the age of six or eight months the power of central fixation is often lost within eight or ten weeks. When a squinting eye has lost its fixation power it either wanders indefinitely, when the fixing eye is covered, or tries to fix with some part of the retina around the fixation point ; or, as a third alternative, it may squint still farther inwards to use the temporal part of its field, which has still retained the exercise of its functions (eccentric fixation). The highest vision possessed by such eyes is to count fingers at 3 or 4 meters (Asher), and Alfred Graefe says that often greater visual acuteness is obtained when the test objects are held in a line with the macula of the deviated eye, in spite of the fact that from sheer want of habit the eye does not move so as to use its macula. Its Diagnosis. Defective central fixation is easily diagnosed by making the patient cover his good eye and try to fix the sight-hole of the ophthalmoscopic mirror with the amblyopic eye (Priestley Smith). The corneal reflexion, instead of occupying its steady and proper posi- tion, will appear to wander about. As akin to this defect Javal notes a certain number of cases in which there is a trembling of the image seen by the defective eye, even after the power of simultaneous vision by the two eyes has been restored, just as a weak hand trembles more than a strong one. I have noticed this too, and it is not infrequent. Newly- Acquired Field of Fixation (Perverse Projection, or Strabismus Incongruus, of Graefe). It occasionally happens that in squints of unusually fixed amount, which began in early life, the squinting eye has so far accommodated itself to its new conditions as to project objects in accordance with the working eye, so that a kind of second-rate binocular vision is retained. On putting such an eye straight by operation, crossed diplopia of high degree imme- diately appears, which fades away in time. It is not, of course, the eye itself which projects, but its cerebral center ; so that the name, "false macula," often given to this condition, is a fallacious one. It is doubly fallacious, since it is not a macula that is called into being, but a new field. It has been wrongly described as a small part of the retina which has retained its function in virtue of receiv- Strabism us 125 ing companion images to those received by the macula of the best eye. Were this view correct, there would be no post-operative diplopia, for diplopia means two images of the same object, and the supposed solitary functionating spot of the retina cannot, when displaced by operation, receive a second image from the same object as the good macula. The diplopia observable is that of images received upon the two true maculae, but the whole field of the squinting eye having been cerebrally displaced for many years, its macular impressions share the displacement as much as all other parts of its retina. In rare cases objects of similar appearance placed in line with the two foveae may be seen close together, as well as in the form of crossed images far apart. It is evident, therefore, that in consequence of the squint, the faulty eye has acquired a new projection without entirely forgetting the old. Javal's view is that there may have been fusion of the fields of the two eyes, with mental suppression in the case of each of the part which corresponds to the field employed by the other, since it is generally not until after operation in these cases that there is any complaint of spontaneous diplopia at all, and it is sometimes even difficult to elicit it before operation by red glass before one eye and a candle. Strabismus Convergens Myopicus. In myopia of not very high degree, and in which the value of the two eyes is too equal to make it seem desirable to cheir possessors to surrender either, a strong effort of convergence (relatively to accommodation) has to be made in near vision, since the converging innervation is so unsupported by any effort of accommodation, and the difficulty arises from having to strongly assert one, and restrain the other, of two cerebrally associated innervations. This relatively strong con- verging activity, exerted for long periods at a time by those who are engaged in reading or near work, cannot always at once be easily surrendered when distant objects are looked at, and thus esophoria in distant vision becomes developed in consequence, gradually increasing as its cause continues till homonymous diplopia threatens, then appears, persists and increases. In near vision the diplopia is less, and may even give place to slight exophoria. A certain proportion of these cases, if con- cave lenses and prisms do not relieve them, are grateful for operation, if care be taken not to create insufficiency of conver- gence in reading. 126 Tests and Studies of the Ocular Muscles Deficient Abduction of the Squinting Eye is found not only in cases of paralyses of the sixth nerve, but also in ordinary concom- itant convergent squint under certain circumstances, though as a rule the restriction in outward movement is considerably less than the amount of squint. When the restriction is very marked, it is natural to suppose^ the primary cause to have been an affection of the sixth nerve, and if there be any corresponding want of concomitancy, the supposi- tion is, without doubt correct : it may even be correct when con- comitancy exists over the whole motor field up to the area of restriction, for though the concomitancy shows that the nerve has recovered its power, its paralysis may have been the original cause. But in many cases the restriction is simply due to want of habit, and has no pathological meaning. It is when the squinting eye is highly amblyopic in all parts of its field of vision and when, there- fore, the amblyopia existed from infancy and preceded the squint, that this explanation is most probable, there being then no object gained in turning the eye outwards. Secondarily, perhaps, the rectus may be weak for want of use ; but this corrects itself, I believe, in time if the eye is brought into use. If the deficient abduction be due to defect of innervation, instead of tenotomizing the internal rectus of the squinting eye that of the sound one should be divided, so as to call the defective innervation into play. It is sometimes better, however, to advance the external rectus of the squinting eye. When restricted abduction is really due to an evident defect of the sixth nerve, advancement of the external rectus is the only justifiable operation, reinforced, if needed, by tenotomy of the internus of the same eye. A very useful adjunct to tenotomy I find to be stretching the soft cicatrix if a greater effect is desired. It can be done daily for several days after the operation. The way I proceed is as follows : After pressing a small plug of cotton wool dipped in cocaine solution and held by fixation forceps, against the conjunctiva close to the outer margin of the cornea, the conjunctiva is tightly gripped and the eye drawn slowly and steadily out while the patient fixes with his other eye an object on the other side of the room. The eye is then held in this position of divergence for about a minute, during which it yields a little more. The idea was suggested by the so-called "mechanical" treatment of squint by stretching the Strabismus 127 muscle without operation, of which, however, I have.no experience, as it does not sound a practical idea. Divergent Strabismus. The eyes when free from active in ner- vation tend to settle down into divergence. Healthy eyes diverge under chloroform and during sleep, and even the so-called "per- manent" element of a convergent strabismus may completely dis- appear under the chloroform. This seems to confirm Bonders' view, that while the development of convergent squint is an active, that of divergent squint is a passive process.* With a few, sometimes rather inexplicable exceptions, blind eyes in emmetropic individuals tend to diverge, especially in adults. The exceptions consist of those who had esophoria previously, either from weakness of the external recti, from anatomical anomalies of the ocular muscles, from ciliary paresis, or from habitual over-tonicity of the converging innervation. In Myopia. While there may be some truth in the statement that the elongated shape of myopic eyes opposes an obstacle to convergence, the want of support to convergence due to the absence of accommodative effort, is no doubt the chief cause of that myopic exophoria in near vision which often exists to so high a degree, even in eyes of equal refraction. The higher the myopia the greater is the effort of convergence in reading or fine work, and this effort being unsupported by its companion innervation may cause sufficient fatigue of the converg- ing center to allow at times one eye to deviate. If it does so at all, it does so considerably, so as to minimize the trouble occasioned by the diplopia. When once the habit has commenced, it gains in frequency and may lead to a permanent squint in both near and distant vision. The treatment in the early stages is evidently to correct the myopia in whole for young people or in part for older ones, so as to lessen the convergence and introduce at the same time an act of accommodation. With Anisometropia. When any considerable difference exists between the value of the two eyes, the effort of convergence may be greater than the usefulness of the worst eye, which the patient, therefore, at times allows to deviate outwards by discontinuing the converging effort, especially if he finds that by so doing accommo- dation can be more completely relaxed, or if the print, as seen by * Ponders' antithesis is : " Hypermetropia causes accommodative asthenopia, to be actively overcome by strabismus convergciis. Myopia leads to muscular astheuopia, passively yielding to strabismus divergeus. i j.s '/; .v/j- and Studies of the Ocular Muscles one eye, is more distinct than when seen by both. As Javal points out, binocular vision has less value for reading than for most other acts of vision, since in a page of print there is no " third dimension. For this reason habitual latent divergence (or "suppressed" squint) is all the more apt to give place to " manifest " squint on occasions which becomes more and more frequent, until it persists altogether, and involves distant vision as well. Age Relation. While, therefore, convergent squint is of infan- tile origin, the divergent variety commences towards adult life. This is due to the fact that children are so rarely myopic, and to the ado- lescent increase of myopia. The practice of steady reading, too, increases in the years of adolescence. And again, as mentioned elsewhere, the tonic activity and excitability of the converging center seem to lessen with age, which also favors divergence. Refractive After-Treatmerit for Squint. Over-correction of cotwergent squint is generally advantageously followed by either no correction or a considerable under-correction of any existing hypermetropia, and it is sometimes, I think, a good plan to make young emmetropes wear even weak concave lenses. On the other hand, operative under-correction of a convergent squint clearly indicates full correction of the hypermetropia in dis- tant vision and perhaps even an over-correction in near vision, in order to gradually supplement the effect of the operation. Operative over-correction of myopic convergent squint should be followed by constant use of the full refractive correction. Over-correction of divergent squint, if it remains, interposes a serious difficulty in the restoration of binocular vision, because a diverging effort is much more difficult to make in the interest ot fusion than a converging effort. Stronger plus lenses, or weaker minus ones, are therefore indicated. Under-correction of divergent squint indicates weaker plus or stronger minus lenses. The above rules, it need hardly be said, apply only to the "post-operative" treatment of squint, i. e., after everything has been done that is indicated in an operative way ; and they are only intended to give the ' ' fine adjustment ' ' at last. For every operator knows that after putting a squint straight to the perfect satisfaction oi the patient, the effect is generally either a trifle less or a trifle more than his own ideal. It is true that when binocular vision is restored the defect is entirely covered, and exists only as a Strabismus 129 " heterophoria" ; but there it is, all the same, and a slight modifi- cation of the optical correction, when the patient is (as usual) young enough not to mind it, can either increase or lessen the activity of the tonic convergence, so as to correct by degrees even that heterophoria. Thus, with residual "esophoria," a low hyper- metrope might dispense with glasses altogether for a time, and a high hypermetrope wear a somewhat weaker pair. There is no need to make the modification a great one, so long as it is in the right direction. Nor, of course, is optical adjustment intended to take the place of fine second operative adjustment in the event of the first operation being markedly insufficient. It is essentially " post- operative." In long-standing convergent squints, where the restoration of binocular vision is impossible, perfect straightness is to be regarded as undesirable, since it leaves no margin for the natural diminution of converging activity, which goes on year after year. In such a case, therefore, a slight reduction should be made from the full hypermetropic correction, unless the patient be willing for another slight operation a year or two later on. At least three degrees of residual convergence should be aimed at in cases where binocular vision is irrecoverable, and, thanks to the high angle gamma, which generally exists in hypermetropes, the eyes do not betray a residuum of even five degrees to ordinary observers. Author's Combined Bar-Reader and Squint Stereoscope. This instrument is intended to be placed in the hands of a patient. A saw-cut is made round the lenses of a Holmes stereoscope and two hinges put on, so that the lenses can fall down out of the way when the instrument is used for bar-reading. Two pieces of talc or mica are pivoted into saw-cuts and are scratched by gentle grada- tion more and more from their inner or lower edge to their outer edge. By gradually lowering one of these before the non-squinting eye, its vision is gradually lessened till the image that belongs to the squinting eye springs into view. Another plan is to begin with the talc shutter down, and while the squinting eye is examining some near or distant object, to gradually allow the good eye to be uncovered, while still trying to keep the squinting eye in use. In this way a monolateral squint can be readily trained to be alternating. For bar-reading, the oval containing the lenses should be turned down on its hinges. Another new feature of the instrument is that it is provided with an 130 Tests and Studies of the Ocular Muscles extensible median partition, which stretches from the middle of the card to between the two lenses.* Natural Cure and Natural Increase. Convergent squints become gradually less as years go by. In the case of young children this is partially due to the physiological growth of the eye causing diminution of their hypermetropia, its exciting cause. But, besides this, the converging center appears to lose its excita- bility with age, and even squints in which no hypermetropia at all is to be found, tend to get less in course of time. Divergent squints, however, generally get worse as years roll on. Treatment of Fixed Convergent Squint. We have seen that convergent squints tend in the course of years to undergo a natural cure. The correction of any hypermetropia, or hyper- metropic astigmatism, of course, expedites this natural cure and should always have a good trial, except in squints of very high degree, in which the patient might have to wait many years and thus lose the likelihood of regaining binocular vision. In such cases a good plan is to wear glasses for six months, and if the squint is found, by measure- ment before and after, not to have notably decreased, an operation should be performed, so as to lose no more time. In the mean time, steps should be taken to lessen amblyopia in the squinting eye, by occlusion of the good one, and to recover any lost faculties by training. Since, however, it is easier to lose faculties than to regain them, it is important, as emphasized by Javal and Priestley Smith, to commence the treatment of squint at as early an age as possible. Refraction should be corrected, or, if that be impracticable, atropine may be used continuously for a time in the best eye if the squint be unilateral or in both if it be alternating. The effect of atropine should be watched, so as to discontinue it if it does not markedly diminish the squint. Occlusion of the squinting eye (Buffon, Javal) is better still. Personally, I am not much in favor of atropine for continued use. Both Javal and Priestley Smith speak of operating as early as two years of age, if necessary. To do this we must first feel confident about the certainty of restoring binocular vision, otherwise the eye will turn out in later life. With congenital squints we cannot have this confidence, and it is wise to approach them with caution ; but when there is a definite history of the squint having been preceded by straight eyes, with an interval during which it was periodic, and especially if we can still elicit * The instrument can be obtained from E. Long, Tangley, Bournemouth, England. Strabismus 131 diplopia or excite fusion by stereoscopic devices, an operation can be done without fear. Recovery Of Lost Faculties. Javal has been the chief pioneer in this direction. The steps in cure, according to him, are : (1) Restoration of the power of simultaneous vision, as evidenced by diplopia, by overcoming the habit of suppressing the image of the faulty eye. The chief agent to this end is the permanent monocular occlusion. (2) Overcoming this diplopia \>y fusion. For this the stereo- scope is useful, and also exercises without the sterescope, with flames and prisms arranged to excite the desire to see single. For convergent squints of high degree, Javal uses Wheatstone's stereoscope. (3) The perception of relief by suitable motions of the eyes. For this the stereoscope may again be pressed into service. The length of time required to re-establish binocular vision, according to his experience, is nearly equal to that which has elapsed since the squint began. Let us look at these in detail. Occlusion. The object of this is to overcome the mental sup- pression of the false image. Javal' s idea is to have either one or the other eye always covered, so that more than one image at a time is never seen, in the hope that the brain will forget how to suppress the second image. It is best to cover the good eye, but, as a luxury, the cap may sometimes be transferred to the squinting one. Few oculists pursue this treatment so completely as Javal recommends ; occlusion for a few hours a day is a more common prescription but not nearly so effective. To equalize the vision of the two eyes I sometimes prescribe a deeply-tinted glass before the better one, with one or two opaque bands across it. Orthoptic Training. Javal' s cartons contain numerous devices and are of great service. They are intended for use with either an ordinary stereoscope or (if cut in two) with a modification of a Wheatstone stereoscope, arranged by Javal under the name of a "sterescope a charniere. " As a preliminary to stereoscopes, however, I prefer to use a very simple device lately introduced by Priestley Smith and to which he has given the name of " fusion tubes." They consist of two short tubes, held together by chains, for the squinter to look through. Each is provided, at the eye end, with a convex lens whose focal length is equal to the length of the 1^2 Tests and Studies of the Ocular Muscles tubes, and the other end is closed by an opaque disk perforated with two translucent holes. The hole in the center of each disk is white, while the neighboring hole is red in one tube and green in the other. On looking into these tubes, a squinter with binocular vision sees four holes, two of which are white, one red and the other green. By moving the tubes, the two white holes can be brought together and fused. Three holes only are then seen, red, white and green. The red and green holes act like Javal's "control marks," to insure that true fusion exists and not merely suppression of images by one eye. By now moving the tubes so as to slightly separate the white holes and thus making a strong cerebral effort to fuse them, the eyes can be trained to overcome a squint. The great quality required is perseverence, and when I have been able to meet with it, the fusion tubes have proved very successful. One nurse maid, for instance, with a periodic divergent strabismus of 15, restored her eyes to perfect orthophoria in three months. A boy with more than 5 of vertical squint restored his eyes to parallelism within the same time. Fusion tubes mounted in a more elaborate way, so as to measure the squint, constitute the heteroscope of Priestley Smith. By affixing translucent gum paper to the farther end of the fusion tube before the better eye, the holes seen by that eye can be darkened. This enables the other eye to see better. Landolt has introduced a stereoscope to facilitate this plan, in which similar tubes are used, but pictures are employed and the farther end can be darkened by an iris diaphragm. I do not know if Tourmaline plates have yet been suggested, but they would doubtless act very well. Quite recently C. Worth, of Lon- don, has brought out an arrangement of tubes which, in principle, is as if the "stere"scope & charniere" were mounted in tubes, with translucent pictures mostly like Perlia's, intended to be unequally illuminated by lamps placed opposite the two tubes. By lowering the lamp before the good eye, the picture .before the other eye becomes visible. The instrument promises to be useful for squints of higher degree than the fusion tubes can suit and has the advantage of permitting any number of designs. When an ordinary stereoscope is available, Perlia's excellent pictures may be used. Magnetic Stereoscope. This is a new apparatus which I have constructed and which appears likely to be very effective. The patient looks into an ordinary stereoscope fitted with small electro- Strabismus 133 magnets which move a black feather at the end of a straw, so as to cut off the vision of either eye at the will of the surgeon, who sits in a chair at any convenient distance and presses a button on a separate piece of wood to occlude the right eye or another button to occlude the left. The movement of the feather is almost instan- taneous. By interposing an interrupting hammer in the circuit or a metronome with a wire across its lever, the two ends of which dip alternately into pools of mercury, the work of the surgeon can be done mechanically. The apparatus can be used in several ways. One of Javal's cartons is placed in the stereoscope, such as an L before one eye and an F before the other, or a pictorial representa- tion of a stable before one eye and a horse before the other. Any one of the following plans can be adopted : (a) By intermittent occlusion of the fixing eye alone in a case of deep suppression of the false image, the latter comes into view. By degrees the intermission can be made so rapid that the true image is not lost at all, and thus both images are seen simultaneously. (^) The feather can be made to occlude each eye in turn, at first slowly and then more rapidly. This keeps both eyes "alive," as it were, and incites them to act more and more simultaneously. (r) With the good eye occluded, the briefest possible uncov- ering of it may be made, before and during which the patient is told to carefully watch the image before the suppressing eye so as not to lose sight of it. The interval can then be lengthened by degrees. The apparatus was suggested by some physiological experiments I made with the visual camera about seventeen years ago, but I have only recently applied it clinically. Extension of Partially-Preserved Faculties. The slightest retention of binocular vision in comitant squints affords a very encouraging factor in prognosis and squints can be approached for operation with far more confidence if it exists, since, once restored, it has a keeping power which prevents the return of the squint as well as the power of perfecting the straightness of the eye utterly beyond any operative ability. For this reason, operations for unilateral strabismus should be preceded by at least a month of permanent occlusion of the better eye, if there is any hope of restoring binocular vision. In incomitant squint, when binocular vision still remains in no matter how small a corner of the field, its extension by judicious 134 Tests and Studies of the Ocular Muscles operation is feasible, and even without operation it may be extended, as Javal suggests, by daily training in which the patient fixes some bright object with great attention while slowly moving his head so as to bring the vision of it to the furthest limits of his field of single vision. Evidences Of Squint. The most decisive evidences of a squint are diplopia and the appearance of a manifest deviation of one eye. The diplopia may, however, be missing, from blindness of one eye or from the habit of mentally suppressing the false image, and the deviation may be apparent rather than real. This makes it necessary to have tests at our disposal to make sure. Exclusion Test for Squint. As this useful old test is frequently spoilt by the student too indefinitely shifting his hand from one eye to another, it may be well to describe it minutely. Direct the patient's attention to some small and rather distant but perfectly distinct object, and after ensuring, by watching his eyes for a moment or two, that his gaze is steadily fixed, suddenly cut off the vision of one eye say the right by a swift lateral movement of the left hand, made from the wrist, with the fingers extended and the dorsum towards the patient's eye, but without touching any part of his face. If the left eye make no corrective movement but remain as immobile as ever, it is acquitted from squinting. But the excluded eye is not yet acquitted, for it may be the squinting one ; therefore, now, after waiting again a moment or two to ensure that the patient is steadily fixing, cover his left eye sud- denly with the right hand. If the right eye remain immobile, it also is innocent. No squint, therefore, exists. If, however, either eye should make a little inward ' ' correc- tive " movement when its neighbor is covered, it must have been previously squinting outwards, and if it make a little oiitward "corrective" movement, it. must have been previously squinting inwards. The test for manifest squint must be distinguished from the exclusion test for suppressed squint, described later, in which the procedure is entirely different. Majiifest squint .is that which exists when both eyes are naked. Latent or "suppressed" squint is that which only arises when one eye is excluded from vision, or is in some way dissociated from its neighbor. Strabismus 135 Subjective Screen Test. The following is translated from Alfred Graefe : "Suppose, for example, that, on account of paralysis of any muscle of the right eye, slight deviation of the visual axis is present for some determined position of the object. If we now cover the right eye during fixation, the image of that eye will disappear and that of the left retain its position, since the sound (left) eye, engaged in fixation, will continue undisturbed therein. ' ' Let us, however, under similar conditions, cover the left eye ; its image will correspondingly disappear, but simultaneously the still remaining image of the right eye will exhibit a change of posi- tion, since the right eye now for the first time directs itself for fixation and has to make a proportionate excursion to bring its hitherto eccentrically-placed retinal picture to the spot of central vision."* Secondary Deviation. When we compel a squinting eye to take up fixation by placing a screen, such as the hand or piece of ground glass, before the eye which naturally fixes, the squint is transferred from one eye to the other, and the deviation of the eye behind the screen is called the "secondary deviation," to distin- guish it from "primary deviation," which is the deviation of the squinting eye under ordinary conditions. In alternating squint^ we have seen that the transference of the squint from one eye to the other remains after withdrawal of the screen, one eye being as prone to squint as the other and the patient having no preference as to which he uses for fixation. With alternating squints, therefore, we cannot draw any distinction between primary and secondary deviations : in these cases it will generally be found that there is great approach to equality in the visual acuity of the two eyes. In unilateral squints, the secondary deviation gives place again to the primary as soon as the screen is withdrawn, the squint being again transferred to its original seat. It is easy to observe the relative amplitude of the two devia- tions, for the extent of the secondary deviation can be watched through a piece of ground glass (Javal) or even behind an opaque screen, if the latter be held obliquely so as to let the eye be visible * Alfred Graefe ; " Motilitatsstfirungen " (1858), p. 21. t There is a large intermediate group of cases which are nearly alternating, the patient being able to fix with either eye but preferring one above the other. In these cases the secondary deviation remains for some time after withdrawal of the screen. 1 36 Tests and Studies of the Ocular Muscles whik- yet it is cut off from fixation ; or, better still, the hand can be withdrawn instantaneously long enough to see the deviation without giving it time to disappear. When there is the slightest paralytic element in the squint the secondary deviation is infallibly greater than the primary, and the more so the more the direction of fixation becomes such as to require contraction of the affected muscle. An extremely delicate test, therefore, for paresis is to test the secondary deviation at the extreme periphery of the motor field in the direction of action of the paralyzed muscle. In comitant squint the primary and secondary deviations are equal and the amplitude of the squint remains unchanged over the whole motor field, except sometimes near its periphery from mechanical hindrances : this kind of squint is due to anomaly of a conjugate innervation.* Persistent Secondary Deviation. It sometimes happens that in a patient affected with paresis of an ocular muscle, the affected eye has so much the better vision of the two that he prefers to use it for habitual fixation. In this case he goes about with a "persistent secondary deviation " of the unparalyzed eye, which may deceive a careless investigator and make him blame the wrong eye. Such cases are somewhat rare, however, for the paralyzed eye can only be used at the cost of giddiness, malprojection and unsteadiness, to reduce which to the minimum the patient goes about with his head so inclined as to give the weak muscle as little work to do as possible. Reason Of Secondary Deviation. It remains to explain why, in paralytic squint, the amplitude of the secondary deviation is greater than that of the primary. It is simply due to the fact that no considerable f impulse can travel to any muscle without an equal impulse being sent to its associated muscle in the other eye. The normal eye faithfully responds to its received impulses and moves in exact proportion to their strength, but the paralyzed muscle is unable to respond in the same proportion, if at all. When the squinting eye, therefore, is compelled to take up fixation, half of the great effort required is vainly spent on the weakened muscle, while the other half produces a high degree of movement in the nor- rnal^eye a degree which measures the amount of effort put forth. Cases of non-paralytic squint I have met with in which the secondary deviation is dis- ly greater than the primary. In others, especially with H. or H. As., the reverse may be the case, from want of visual effort in the squinting eye ' "considerable," because in the interests "of fusion the eves can in a small degree icmselves in a way which does not seem to obey the laws of conjugate motion. Strabismus 137 Fallacy from Anisometropia. There is one fallacy to be guarded against in testing the secondary deviation : if one eye be more hyper- metropic than the other, the secondary deviation may be greater or less than the primary, owing to the greater amount of accommoda- tion required from one eye, producing a proportionately greater amount of associated convergence. Apparent Squint. The appearance of squint, as has been said, may be illusory. Myopic eyes frequently give the impression of a Fig. 45 Ophthalmoscopic corneal reflections in Emmetropic eyes ; (a) with both eyes looking at the center of the mirror (6) with both eyes looking to the right, showing asymmetry of the corneal images owing to the angle alpha. slight convergent squint, while, on the other hand, hypermetropic eyes, though not quite so deceptively, often appear divergent. We can settle any doubts in our mind as to whether the squint is a real one, by the "exclusion test," already described; or, better still, by placing the patient with his back turned three- quarters towards the window ( or with his head near a flame, if the room be dark), and reflecting the light on to first one eye and then the other from the mirror of the ophthalmoscope, held about nine inches from the face. The observer should look through the aperture of the ophthalmoscopic mirror as if about to examine the patient's fundus, and also direct the patient' s attention carefully to the same aperture. A tiny circular reflection from the mirror will now be visible in each eye, as in Fig. 45, about -^ inch in diameter, but smaller still the farther away the mirror is held. In emmetropic eyes the reflection will appear, as in the figure, slightly to the inner side of the center of each cornea, and if they are symmetrically disposed in the two eyes,, the existence of squint may be safely denied. I 3 8 Tests and Studies of the Ocular Muscles If the deceptive appearance has been due to myopia, the reflec- tions will lie nearer than usual to the center of each cornea ; but if to hypermetropia, they will both appear displaced farther inwards than usual. The fuller treatment of this subject in a subsequent chapter makes it unnecessary to pursue it much further in this one. Intrinsic Aberrations. In the eyeball itself are : (1) Angle alpha of Danders. The angle between the antero- posterior axis of the eyeball (which Bonders assumed wrongly to coincide with the axis of the cornea) and the visual line. Varia- tions of this angle cause deceptive appearances of squint. (2) Angle alpha of Landolt. The angle between the visual line and the major axis of the corneal ellipsoid. This angle con- tributes nothing to a deceptive appearance of squint. (3) Angle gamma. The angle between the antero-posterior axis of the eyeball and the fixation line. This angle differs but slightly from the last, since the fixation line so nearly coincides with the visual line and therefore has a bearing on apparent squint. The fixation line proceeds from the point of fixation to the center of motion of the eyeball. The visual line also proceeds from the point of fixation but to the anterior nodal point. Linear Strabismometry. This method of measuring squint has almost died out of use. It took account of the linear dis- placement of the pupil and was, at one time, the popular method, owing to Graefe's view that a displacement of the pupil, measured by so many lines or millimeters, could be rectified by setting back the tendon by an equal number of lines or millimeters. In practice, however, this has been found to be impossible, and Landolt pointed out that, owing to the different lengths of different eyes, an angular measurement was the only rational one. Nevertheless, the linear method did good service in its day, and flat pieces of ivory with a concavity to fit the lower lid are still to be met with, being relics, more or less faithful to the original, of Lawrence's strabismometer, once much used in England. The concavity is graduated in millimeters from a central zero and is intended to be used in this way : Place the patient facing the window and, with the good eye covered, direct his attention to some distant object. Place the zero of the scale just under the pupil of the now straight but usually squinting eye, and then uncover the better eye : at once the squinting eye asserts its habit and the figure which now lies under its pupil measures the squint. The relation between Strabismus 139 angular and linear measurements may be expressed by saying that each millimeter along the sclerotic means about 4^2 of squint. Hirschberg'S Method. A lighted candle is held one foot in front of the patient's face, the surgeon placing his own eye near to the candle and looking just over it at the eyes of the patient, who is made to look at the candle. The position of the corneal reflection on the squinting eye indicates roughly the amount of squint. Since the breadth of the cornea is about 12 mm., a squint which brings the reflection to the margin of the cornea is one whose linear measure- ment is half the diameter of the cornea, namely, 6 mm. Half this displacement means a squint of 3 mm., and so on. Hirschberg points out that a 6-mm. squint in which the reflection occupies the margin of the cornea, means one of about 45, while one in which the reflection occupies the margin of a medium pupil, is about 15. Owing to the angle gamma between the optic axis and fixation line, w^hich the Hirschberg method neglects, a reflection situated over the outer margin of an average pupil, means a greater and sometimes a much greater squint than a reflection situated over the inner margin. Nevertheless, Hirschberg' s method, as far as it goes, is a very useful one and often enables an excellent guess to be made, provided the precaution be taken to keep the surgeon's eye, the flame and the squinting eye in one straight line. If not quite so accurate as the use of ophthalmoscopic corneal images, which were introduced at a much later date, it is nearly so, and it possesses the advantage that ' ' lights ' ' are generally found more readily than ophthalmoscopes. For the more exact measurement of squint, however, one of the following methods is necessary : Perimeter Method. This mode of measurement assumed sway as soon as the linear method began to wane and, in the manner recommended by Javal, has been greatly used, its advantage being its accuracy, and its two disadvantages lying in the absorption of time by the preliminary arrangements and in the difficulty of measuring slight convergent squints by it, since for them the surgeon's head interferes with the fixation line of the sound eye. The patient should be seated so as to bring the squinting eye (S, Fig. 46) into the center of the perimeter, while straight in front, at a distance of five meters, is placed a candle for the fixing eye F to look at. It is only some perimeters which permit this.* *The useful addition to the perimeter, introduced by Landolt and now become general, namely, a piece of soft wood to be gripped by the teeth, is very useful for strabisuiometry. 140 Tests and Studies of the Ocular Muscles Another flame, or, better still, a small electric light, is then moved along the arc of the perimeter, with the surgeon's eye ever behind it, till its reflection appears to occupy the center of the cornea or rather that part of the cornea which our knowledge of the angle gamma leads us to select and which I have called else- where the "fixation-position" (Chapter XI). The squint is now measured by that figure on the perimetric arc which lies against the flame. For great accuracy the angle gamma can be measured separately by screening the good eye and making the squinting one fix the ivory disk of the perimeter while a flame (with the surgeon's eye kept strictly in line behind it) is moved along the arc till its reflection appears to occupy the exact center of the corneal circum- ference. The figure reached by the candle enables the angle gamma to be at once read off from the perimeter. This angle should be subtracted from the record of a convergent squint, and added to that of a divergent, premising, of course, that they have been measured by the center of the cornea. In the figure, d is the ivory disk of the perimeter, and were there no squint, the visual axis of the left eye would pass through this, as shown by the dotted line. The angle d S /, therefore, is the angle of the squint and is measured by the arc d I. By bringing the distant flame nearer to the perimeter the squint can be measured under different accommodative conditions ; or, finally, the ivory disk may itself be made the object of fixation. The figure makes evident, also, how in a squint of low degree the method is rendered impracticable by both the flame E and the surgeon's head behind it, interfering with the vision of the distant flame by the fixing eye F. Charpentier's Method. The difficulty just spoken of is evaded in the plan illustrated in Fig. 47, where advantage is taken of the Fig. 46 Javal's method. Fig. 47 Charpentier's method. Strabismus 141 law that angles of incidence and reflection are equal. The flame is placed over the fixation spot of the perimeter, and the surgeon's eye is made to travel along the arc till its reflection appears to lie in the center of the cornea of the squinting eye. The squint is then measured, its angle being half the angle of the arc. One little fallacy in this test seems to have escaped notice, and is illus- trated in Fig. 48. Owing to "spherical aberration," the image formed by reflection from a convex mirror alters its position with every change in the angle of incidence and its ever equal angle of reflection, so as to lie on the caustic curve shown in the figure. This caustic curve is one whose cusp F lies in a line drawn from the center of curvature O of the cornea parallel to the incident pencil i. By producing the reflected pencil r back- wards till it meets the caustic curve at F' the position of the image is found. Moreover, secondly, the sur- geon judges by its projection against the plane of the iris, not against the center of the cornea ; so the while the image lies at F f on the caustic curve, it is pro- jected on to the plane of the iris, where it clearly would appear eccentric. In the absence of a candle, a circle of very white paper, mounted in the peri- meter, gives a recognizable reflection from the cornea, and this, indeed, has been utilized in De Wecker and Masselon's "Arc Keratoscopique," which also has a little mirror in which the distant object is reflected which serves for the point of fixation. It seems superfluous, however, to have a special apparatus for strabismometry, if instruments already in possession for other purposes serve as well. To any who think otherwise, the "arc keratoscopique " will be found very handy. Priestley Smith's tape method will be found described in the chapter on " Ophthalmoscopic Corneal Images." The Tangent Strabismometer. The tangent scale (latest edition best) constructed by the author for use with his rod test is the only apparatus needed, and since it hangs on the wall, it occupies no space in the room and is ever ready. It is in principle Fig 48 Section of Cornea, to show that light reflected from the center of the cornea symmetrically does not produce an image in the optic axis, but rather to one side of the center of the pupil. 142 Tests and Studies of the Ocular Muscles a flattened-out perimeter, but has the advantage over the perimeter of being time-saving. An immense number of squints are operated on without being measured in degrees simply for want of time, and the tangent strabismometer is meant to meet this difficulty. It serves Fig. 49 First step in tangent strabismometry ; adjusting distance of patient by a meter-string. for both the objective and the subjective measurement of squint, and whenever diplopia can be elicited, both kinds of tests should be made ; for while, on the one hand, subjective measurement is far more delicate than any objective measurement could possibly be, on the other hand there is the chance of meeting with one of those occa- sional fallacies in the projection of the false image, which need check- ing by the rougher though more dependable objective observations. The large figures on the tangent scale are not intended for ordinary strabismometry, but rather for latent deviations or slight squints (under 10), since they represent degrees for a distance of 5 meters. The row of smaller figures is added for strabismometry and, being intended for use at one meter, since they mark degrees at that distance, a piece of string one meter long hangs from the candle ready to adjust the distance of the patient. Mode Of Use. Placing the patient facing the candle, at the meter distance, the surgeon introduces his own head between the two, but a little lower down, about a foot away from the patient and Strabismus 143 so that the root of his own nose is vertically under the rays of light which proceed to the patient's eyes. At once the tell-tale corneal reflections reveal which eye is the squinting one, and the amount of squint being guessed at by the degree of eccentricity of the reflec- tion, on Hirschberg's principle, the patient is told to look at the figure which numerates the guess.* If the guess of the amount of squint, as revealed by the corneal reflections, be true, the squinting eye has been brought straight for the candle and the reflection upon it occupies its proper position. If the guess be only partially correct, successive figures are mentioned, one by one, for the patient to look at, till the surgeon is satisfied as to the right one. For rapid work this suffices and takes scarcely more than half a minute. Since the height of the patient is immaterial, no time is lost in adjusting it, as in the use of the perimeter. Greater accuracy still may be secured by screening the work- ing eye and making the squinting one fix the flame for a moment to see what the fixation-position of the corneal reflection is and whether it is similar to that of the working eye. (i) Concomitancy can be measured by repeating the observa- tion with the patient's face turned to one side and the other (Berry). Fig. 50 Tangent Strabismometry. To show the path of the light from the candle. (2) The secondary deviation can be measured by screening the working eye and making the squinting eye (and the face) look at a figure which brings the working eye straight for the candle, as proved by momentary unscreening to look at the corneal reflection ; or by a little adeptness the working eye can be screened from the figure looked at by the other eye, yet not from the flame. *According to Ilirschherg, when the reflection occupies the margin of a moderate-sized pupil then' is about ln or 2e, since this keeps the fixing eve e cool and comfortable. The wire gauze should have a velvet edge Strabismus 147 the undoubtedly worthless plan, practiced by a few, of atropizing both eyes. He prescribes daily morning instillations into the fixing eye, adding the practical procedure of giving the mother a card on which are written the directions and the date of the next visit. "The best results," he adds, "are obtained in children whq are not more than four or five years of age. After six years of age, usually not much improvement in vision can be obtained." (4) Training the Fusion Sense. If under six years of age, this is carried out by Worth by the use of his " amblyoscope," an ingenious departure from Priestley Smith's fusion tubes, possessing the novelty of a mirror in each tube, which facilitates their conver- gence, while keeping the graphic designs so far apart as to make it easy to illuminate the one seen by the amblyopic eye more brilliantly than the other, by means of two lamps or electric lights placed over against them. "The favorable time for fusion training is between three and five years." (5) Operation. For cases in which the deviation is not over- come by other means ; advancement for moderate deviations, com- bined with tenotomy in the higher degrees. As regards tenotomy for convergent squint, the plan of choosing the squinting eye for operation is doubtless the best one, since it agrees with the wishes of the patient, who does not understand "conjugation." Yet I have an impression that a slightly-greater, effect would be gained by tenotomizing the other. If an advancement be done, it is better performed on the external rectus of the squinting eye, especially if there be any deficient abduction. In divergent strabismus the most valuable rule to remember is that if, on approaching the finger towards the straight eye, no con- verging effort is visible in the diverging eye, tenotomy is an entirely useless procedure ; its effect will be nil. Advancement is indicated. In absolute divergent strabismus my experience confirms that of Javal, that " there is no fear of producing an exaggerated operative effect." If, after a surgical interference, a little convergence is left for certain directions of fixation, this effect pretty rapidly disap- pears. "We cannot," he remarks, "count upon a durable cure unless optical means are employed immediately after the operation. A squint is not definitely suppressed unless the subject has acquired the habit of reading binocularly. When we have to do with an adult whose divergent strabismus has become permanent for an 148 Tests and Studies of the Ocular Muscles extremely long time, even the most successful operation must be followed by the stereoscopic exercises continued several hours a day for months. I shall quote some examples of success," he adds, " up to the age of forty-five years, but the patients say with truth that the remedy is worse than the disease. Even with females I would not advise undertaking such a cure after the age of twenty or twenty-five years. With young girls from fifteen to twenty years, on the contrary, one is generally seconded by a courage and a patience proof against everything, and success is absolutely assured. ' ' As a commentary on the above remarks about the gradual decrease of operative over-effect, the following illustrative account of one of my cases will be of interest : M. L., school girl; myopia corrected by .5 D. Left eye deviates outwards when tired. By objective strabismometry left eye diverges 20 (abbreviated thus : 20 L. Concomitant. Very low angle alpha. September 5th. The r. and 1. external recti were tenotomized under chloroform, and the r. internal rectus advanced. Examination by the glass rod and tangent scale gave the following results : DATE ON LOOKING TO RIGHT ON LOOKING STRAIGHTFORWARD ON LOOKING TO LEFT September I4th + 12 + 10 + 3 September isth + 10 + 7 - o October 5th . . + 10 + o - 3 October 26th . + 4 2 ,0 November 8th . + 4 2 - 4 February 5th . 4 2 2 2 Subjective Strabismometry. The subjective test is made by holding a disk of red glass rods before the squinting eye and read- ing off that figure on the same scale which appears crossed by the streak of light. A piece of blue or green glass before the working eye improves the effect by making the images more dissimilar in color and more equal in intensity. (See M. L.'s case, above cited.) Concomitancy can be measured by turning the face to one or other side, as before, and comparing the readings. Direction of Fixation. In both the above tests the patient's face can, if desired, be turned towards the figure he is fixing, so as 149 to gain the advantage of measurement under the usual conditions of vision with the fixing eye looking straight forward. Paralytic Equilibrium. So far, we have left out of calculation the modifying effect of Tenon's capsule and its adnexa. Let us now take that into account also. Since the eyeball is so nearly spherical and the center of motion so nearly at its geometrical center, we may, with little error, assume that they are quite so and that equal forces have equal moments. This enables us to say that when a single muscle contracts, the tension in its tendon is equal and opposite to the resultant of all the other tensions, of which there are two groups, namely, those in the remaining tendons and those in the orbital fasciae. When the same muscle, however, is paralyzed, the eyeball is under the influence of two now opposing groups of tensions, those of the fasciae (which tend to keep it in the primary position*) and those of the tendons of the still unparalyzed muscles (which tend to rotate it away from the primary position). The resultant of these two groups is equal and opposite to the tension which existed during health in the paralyzed muscle. As if guided by this resul- tant, therefore, the eyeball rotates in the opposite direction about the same axis. It must be remembered that, in paralyses, though the belly of the affected muscle has lost its contractility, it does not lose its elasticity at once and in some pareses does not wholly lose at once even all its physiological tone, so that the new position into which the eye settles is resisted not only by the tension in Tenon's capsule, but also by the remaining elastic tension in the paralyzed muscle. For this reason paralysis of a muscle only produces a very slight effect at first, while the healthy eye is in the primary position, /". e. , so long as voluntary innervations are quiescent. Secondary Contracture, or Consecutive Deviation. But as time goes on, the lamed eye deviates more and more, owing to the loss of vital resistance in the paralyzed muscle, to which w r e may perhaps add what physicists call "fatigue of elasticity" in it and in the resisting portions of Tenon's capsule. Thus arises what is gener- ally called " contracture of the antagonist." Mr. Berry believes that there is no real contracture. I am inclined to believe that in the course of years a slight contracture does occur in the opposing muscle or muscles, but as a consequence rather than as a cause of * Probably in a more divergent position than the primary, as Hanson Grut has shown. 150 Tests and Studies of the Ocular Muscles the increase in the paralytic deviation. When the lame muscle becomes stretched and its resistance enfeebled more and more, the others move the eyeball, without their, however, becoming stronger than they were before. My impression is that the consecutive deviation (as I prefer to call it, since this name commits to no theory) will be found great in proportion to (1) The absoluteness of the paralysis ; (2) The long-standing of the paralysis ; (3) In proportion as the paralytic deviation is supplemented by a pre-existing latent deviation ; (4) In proportion to the degree of atrophy of the paralyzed muscle from (a) Want of innervation, (<) Want of use ; (5) The more yielding Tenon's capsule is, and the more readily it experiences fatigue of elasticity ; (6) The more the habit of the patient is to turn the eyes away from the side of the paralyzed muscle ; (7) The more the patient uses the paralyzed eye ; (8) In the case of paralytic convergent strabismus, the greater the hypermetropia and the more sensitive the converging center ; and vice versa in paralytic divergent strabismus. CHAPTER VIII Ocular Paralyses In the absence of any visible squint, the most evident symp- toms of an ocular paralysis, beginning with the more objective, are : (1) Vicarious inclination, or unusual pose, of the head; (2) Imperfect movement of an eye ; (3) Magnified secondary deviation ; (4) Malprojection ; and, under certain conditions, (5) Giddiness, and (6) Uncertainty of gait ; (7) Diplopia. (8) In addition to these it sometimes happens that asthenopia, headache and a strained feeling of the eyes are caused by the con- tinual efforts required to preserve single vision in the presence of a slight muscular paresis, though care must be taken to exclude other more likely causes of these symptoms. If really of muscular origin, they cease when the attempt to maintain single vision is given up. A good practical test, therefore, is to keep the suspected eye covered for a sufficient time and note whether so doing causes the disappearance of the symptoms. Let us now discuss each symptom in detail. Symptom No. 1: Vicarious Inclination of the Head. The object of posing the head is to avoid the inconvenience of diplopia, so that these two symptoms are alternate. Whenever the eyes look in a direction which calls for activity in the paralyzed muscle, its inefficiency is manifested by diplopia. To avoid any call upon the muscle, therefore, the patient turns his head so that the eyes may look in the opposite direction to that of the most troublesome diplopia. It was called "vicarious" inclina- tion of the head by Graefe because the neck muscles do the work instead of the paralyzed eye muscle. Anyone well acquainted with the subject can generally guess the associated pair of muscles of which one is paralyzed, whenever a patient enters the room with a marked inclination of the head. It is quite easy to guess, if it be remembered that the patient's face looks in the direction of the paralytic diplopia. I 5 2 Tests and Studies of the Ocular Muscles There are six directions in which the face may look (if we assume that a single muscle only is affected), and each of these six directions is in relation with its own pair of muscles. Thus, if the face look to the left, one of the two kevoductors is at fault, either the right internal, or the left external, rectus. A face directed down and to the right impeaches the dextral* depres- sors ; and so on. But, after all, we should never trust implicitly to the inclination of the head without proceeding to other tests, for it may be mislead- ing. A fallacy is sometimes introduced by the fact that different components of the diplopia are not equally troublesome to different patients. Some find the torsion of the false image trouble them disproportionately, and others the vertical displacement ; and, since the inclination of the head is merely adopted by the patient to avoid embarrassment, it does not supply mathematical information. Some patients, indeed, have not yet discovered the best inclination, and need to have it pointed out to them. Differences in different patients arise chiefly from various latent conditions of equilibrium (heterophoria), which pre-existed. This subject may be closed by a chart of the positions of the face as follows : IF THE FACE LOOK THE AFFECTED MUSCLE IS EITHER WHICH ARE To right R. Ext. R. or L. Int. R. . Dextroductors To left --'. L. Ext. R. or R. Int. R. . Laevoductors To right and up . . R. Sup. R. or L. Inf. O. . Dextral t elevators To left and up ... To right and down . L. Sup. R. or R. Inf. O. . R. Inf. R. or L. Sup. O. . Lseval elevators Dextral depressors To left and down L. Inf. R. or R. Sup. O. . Ljeval depressors Symptom No. 2 : Imperfect Movement of an Eye. Though this may be due to some obstruction or incease of resistance as by a tumor or pterygium, such are, in practice, too evident to cause any mistake. *Th e word "dextral" must be carefully distinguished from " dextroducting." Bv a Jxtral elevator" we do not mean a muscle that elevates and dextroducts, but one that Mwtei most when the eye happens to be dextroducted by another muscle. The left superior jue, e. g., is a Isevoductor and yet a dextral depressor. It must not be forgotten tha't the superductors and subductors are not called dfxtral or because of turning the eyes to the right and the left, but because their vertical effect is hen the eyes are turned to the right or to the left by other muscles. Ocular Paralyses 153 Order of Examination. It is good order to test first the comparative mobility of the two eyes, with the conjugate mobility of both together ; followed by the examination of the converging power, and ending with the absolute mobility of each. (1) Comparative Mobility. Commencing, then, our examina- tion by testing the comparative mobility of the eyes, we make the patient, with both eyes, follow the point of a finger as it is moved upwards, to right and to left, and intermediate directions. During these manoeuvres we watch both eyes closely to see whether they move equally in every direction, or whether one eye tends tc linger or "lag" behind the other; and if so, in which direction the lagging is most apparent : this direction will invariably be found to agree with the direction of greatest diplopia.* (2) Conjugate Mobility. It may be, however, that both eyes are equally mobile and concomitant and yet are equally defective in their movements in one or more directions ; this is spoken of as a defect in their "conjugate mobility." For instance, on attempting to follow the finger in its upward path, the two eyes may manifest a perfectly symmetrical inability to rise to the usual elevation. A dis- tinctly less common condition is for them both to fail in their move- ments to the right or to the left. A little practice is required to learn the normal limits of movements, in order to decide whether a defect of this kind is sufficiently pronounced to be considered pathological, especially as a good deal depends on the amount of effort made by the patient. Nystagmus should be carefully watched for at the limits of the motor field ; also during the passage of the finger from one place to another any jerky or irregular movements of the eyes should receive attention. (3) Near Point of Convergence. The object of the fourth manoeuvre, namely, passing the finger nail towards the root of the nose, is to estimate the power of "convergence." Here, again, a little practice with normal eyes is all that is required to learn the average converging power, though the result will be found to depend a good deal on the effort made, and the concentration of the attention. Even when testing a patient who has divergent squint, the estimation of converging power should not be omitted, for though it is only possible for one eye at a time to fix the finger, *It is, at the same time, well to notice whether the lagging eye manifests any " torsion " in its ineffectual effort to follow the sound eye up or down, for, if it does, the integrity of the oblique muscle which causes the torsion can be taken as proved. 154 Ttsls and Studies of the Ocular Muscles an inward movement observed in the other eye as the finger approaches the root of the nose affords a valuable indication that the faculty of convergence has not been lost, though perhaps for long unused. It is well known that without such converging power, tenotomy of the external rectus will have practically no effect ; but \\ith a fair amount of it remaining, tenotomy may be undertaken with more or less prospect of success. Should greater exactness be required, either Landolt's well-known dynamometer can be used, or simply a vertical line on the back of a visiting card, approached to the patient's eyes till he can no longer by any effort keep it from parting into two. The shortest distance from his eyes, measured by a dioptric tape, at which he can still see single, gives the number of meter angles of positive convergence. (4) Absolute Mobility of Each Eye. We may next find the greatest possible excursion of which each eye is capable, while covering the other, by invoking the patient's highest voluntary effort to follow an object to the extreme limits of the motor field in all directions. The value of the test is impaired by the fact that voluntary effort is such a variable quantity, and the palpebral aperture by which we judge the extent of movement is liable to such variations of size and shape in different individuals. In comparing the excursions of the two eyes, however, these disadvantages are reduced to their minimum. Under normal conditions it is easy to make the outer margin of the cornea touch the outer canthus by strong abduction of the eye, while in full adduction the inner margin of the cornea should be slightly buried beneath the caruncle. Alfred Graefe's rule is that the inner margin of a moderately- dilated pupil should be brought to touch an imaginary vertical line ascending from the lower " punctum lachrymale. " While inciting these extreme movements, watch again care- fully for any appearance of nystagmus, and if it should seem Fig. 52 Landolt'g Dynamometer for esti- mating the near point of convergence Ocular Paralyses 155 desirable to repeat the test with more approach to accuracy, adopt LandolC 's method with the perimeter. Place the patient's head so that the eye under examination shall lie in the center of the arc of the perimeter ; fix the head, and pass a small piece of diamond type along the arc of the perimeter till the patient ceases by any rotation of the eye to be able to read it. If the eye be amblyopic, it will be necessary to conduct the test objectively, which can be done by passing a small lighted candle along the arc of the perimeter till its reflection occupies the "fixation position" on the cornea, while the patient strives his utmost to look to that side. Schweigger's hand perimeter (Fig. 53) would be the most convenient for this purpose were it provided with a strip of wood for the patient to grip with his teeth. By either of these methods the motor field can be plotted out for each eye. Its limits are a little greater when tested ob- jectively than when tested sub- jectively. An excellent sugges- tion by Casey Wood is to fix a strip of paper with a row of letters on it to the perimeter, and let the patient read along the row till he can read no longer. Symptom No. 3: Dispropor- tionate Secondary Deviation. The ''primary" deviation is that which is found in the par- alyzed eye during fixation of the good, or, as it has been called, the "working" eye. It occurs spontaneously whenever the eyes look in the direction of diplopia. The "secondary" deviation is an artificial phenomenon pro- duced by screening the good eye, so as to compel the paralyzed one to take up fixation as well as it can. The effort required to make the paralyzed muscle contract is out of all proportion to the result, and since half the effort must go to the other eye, its deviation becomes greatly exaggerated ; this is then called the secondary deviation. Fig. 53 Schweigger's Hand Perimeter 156 Tests and Studies of the Ocular Muscles When the primary deviation, in a slight paresis, is too small to be discerned, the secondary deviation may enable a diagnosis to be made, but to obtain its full effect the eyes must be made to look as far as they can in the direction which makes the greatest demand on the suspected muscle. Since it is not easy to see the behavior of the good eye behind a cover, Javal ingeniously introduced a ground-glass screen, of a circular shape, and which is now to be found in most trial cases as a companion to the colored disks. This is intended to be held as close as possible \.Q the good eye, while the paralyzed one is made to follow the surgeon's finger in the direction of greatest demand on the muscle. The patient's eye can be seen through the ground glass, though he cannot himself see through it, and the secondary deviation can be quietly observed. Practically, however, the obscured disk is rarely used, because a more accurate idea of the deviation is obtained by suddenly withdrawing the hand, or some quite opaque screen, and observing, first, the amount of deviation ; and, secondly, the extent of the visible corrective movement which the eye makes to reclaim its fixation. When the affected eye has the best vision or the most useful refraction, the patient will sometimes still use it as the working eye, and then the sound eye deviates. These cases are exceptions to the statement that the secondary deviation is artificially created, for the patient goes about exhibiting it. Symptom No. 4: Malprojection. This never occurs except when the affected eye is at work, either alone or in company with the other eye. If alone, the malprojection is just twice as great as when fixation is binocular. The principles on which this phe- nomenon are based have been gone into so fully in earlier pages that little need be added here. For Horizontal Ductors. The usual plan of testing is to make the patient cover the good eye with a hand, and then suddenly dart his right-hand forefinger at the surgeon's finger held upright, at an arm's length distance from the patient, in such a position as to make a demand upon the paralyzed muscle. The stab must not be a slow cautious one, neither must the patient aim with his finger before making it. He will miss the mark to the side of the implicated muscle : thus, if the muscle be the right external rectus, he will judge the surgeon's finger to be more to the Ocular Paralyses 157 right than it really is and will miss it to the right, really stabbing at a phantom, namely, the false image, the reason being that the mind estimates by the nervous effort expended on the muscle, as if the muscle were responding to it. I have nothing to add to the usual mode of performing the test, unless that after the patient has learned to correct for his mistake, which he often does after a few stabs, it is interesting to uncover the good eye and cover the bad to see if he now at first misses the mark to the other side. A few attempts with each eye alternately thus, makes the test a more reliable one. For Vertical Ductors. When the affected muscle is super- or subductor, the projection test is equally simple to make. The sur- geon should hold his finger horizontally above the horizontal plane, if the muscle be a superductor ; in which case the patient will aim too high, or beiow it if the muscle be an elevator, when the patient will aim too low. Symptoms Nos. B and 6: Giddiness and Uncertain Gait. The relation of these symptoms to each other and to the last is obvious. They occur only when demand is make upon the paralyzed muscle. Since, in the case of the ocular muscles, the muscular sense is central and not peripheral, it miscalculates when a muscle does not truly respond to its stimulus. It is when depressor muscles are affected that the inconvenience reaches its maximum, since they are needed both for walk and for work. This is seen frequently in the not uncommon paralyses of the superior oblique. Covering the affected eye stops it at once, and sometimes a prism, base down before the weakened eye, and another, base up, before the good eye, will earn the hearty thanks of the patient. Their strength can be selected after an examination by the glass-rod test, and the vertical scale (described in Chapter XII). Symptom NO. In practice we rely chiefly upon the nature of the diplopia for the diagnosis of the affected muscle. The first step is to make sure that the diplopia is not monocular, by covering each eye in turn to see whether one image disappears in each case. The image which disappears belongs, of course, to the affected eye. That this precaution is not a needless one may be shown by the fact that I have seen a case of monocular diplopia deceive one of the best of surgeons. The case was, however, peculiarly decep- tive in that the diplopia was noticed by the patient only on looking to one side. By the employment of ophthalmoscopic corneal images afterwards I found that there was no deviation of either eye, in any direction of vision, and monocular diplopia was thereupon searched for and found. Common but Incorrect Aphorism* The statement, so often made, that the affected muscle is the one which physiologically turns the eye in the direction of greatest diplopia, is not strictly correct. Take the superior rectus, for instance : its greatest diplopia when paralyzed is up and out ; whereas, its physiolog- ical action is to turn the eye up and in. Corrected. If we qualify the statement by saying that "the lame muscle is one which in health turns the eye in the cardinal direction of the diplopia," it becomes at once unfailingly true. The cardinal directions are up, down, right and left. Diplopia, *I generally finft it best to place the glass rod before the good eye, with or without a green glas efore the other, the source of light being brilliant, and backed by a velvet screen. Ocular Paralyses 159 greatest in the upper half of the field, is undoubtedly due to one or more of the elevators ; in the lower half to one of the depressors : in the right half to one of the dextroductors ; and in the left half to one of the laevoductors. * There can be no mistake here, if mechanical obstructions are excluded ; but this aphorism only helps us to find the group to which the affected muscle' belongs. Second Aphorism. Since every paralytic deviation makes the false image travel faithfully in the opposite direction to the eye by an equal angle, and since also the physiological displacement of the eye by the muscle before the paralysis was in precisely the opposite direction to its paralytic deviation, it follows that the false image is displaced exactly -as the healthy muscle originally displaced the eye. To speak figuratively, when the muscle fails to move the eye, it moves the false image instead in the same direction that it would have moved the eye. As it moves the image in disease, it moved the eye in health. This makes it very easy to detect the muscle. Is, for example, the false image (relatively to the true) elevated, adducted and intorted ? Then the muscle must be an elevator, adductor and intortor. Only one muscle in each eye is this, namely, the superior rectus ; so the case is solved. Complications. If there were no complications, this "second aphorism" would suffice for all our need. But only a part of the displacement of the false image may be due to the paralysis, the remainder being the result of latent squint (heterophoria) which may have pre-existed for years, though now set free by the paralysis. This introduces a fallacious element and requires that we should so make our tests as to avoid it. Again, more than one muscle may be affected, and we might, if unwary, be caught in a trap. It is better, therefore, to reserve the "second aphorism" to the end of our investigation and use it only for confirmation. Even then, to get the full benefit of it, account must be taken of the direction in which the sound eye is looking, for muscles have dif- ferent effects in different positions of the eyeball, and the position in which the muscle is most valuable is that in which its loss is most felt, and the paralytic diplopia, therefore, is greatest. The superior rectus, for instance, is a more efficient elevator when the eye is *The convenience of these terms will at once be perceived 160 Tests and Studies of the Ocular Muscles abducted to start with ; therefore, in abduction, its vertical diplopia from paralysis is greatest. In adduction it is a more efficient intonor ; therefore, in this position of the eye, its torsional diplopia from paralysis is most marked. And so on. Clinical Procedure. For clinical work we must employ the method which, while thoroughly simple, is freest from pitfalls. Instead, therefore, of merely considering the one displacement of the false image, we should investigate separately its vertical, horizontal and torsional components, giving to each its relative value, since they are not equally trustworthy for diagnosis. We have to weigh the evidence, and not merely count it. Narrowing Circles. Instead of rushing straight for our muscle, we reach it by stages, just as a botanist with a flower enquires successively into its natural order, its genus and its species. (a) Cardinal Groups. We begin by finding to which of the four cardinal groups the muscle belongs, whether that of the eleva- tors, the depressors, the dextroductors, or the Isevoductors, in which group a paralysis makes the diplopia increase respectively upwards, downwards, to right or to left. If two or more groups seem affected, begin with the worst, not forgetting that vertical diplopia is rela- tively more important than horizontal diplopia, since the latter, if it extended all across the field, may be due to some anomaly of the converging center. The most convenient test object is the ever-ready white handle of an ophthalmoscope, and it is quite enough in simple cases. If, however, the false image be faint, or the patient unobservant, a colored glass before the sound eye may be necessary, used in con- junction either with a lighted candle, or a strip of white paper mounted on black velvet,* to obtain a contrast effect. Place the patient with his back to the window, and charging him to hold his head erect and follow the test object with his eyes, move it upwards, downwards, to right and to left, over the surface of an imaginary hemisphere, of which his head is the center and with a radius of about a meter. While testing the horizontal motions of the eyes, hold the handle of the ophthalmoscope vertically, but in testing above and below, hold it horizontally, since in these positions the vertical component *Tho Weal test object would be a luminous glass rod about six inches long and mountea urainst black velvet. Ocular Paralyses 161 of the diplopia is the most important and it is more readily estimated by a horizontal than by a vertical test object. If the diplopia is found only on looking upwards, there is some defect among the group of sursumductors ; if on looking downwards, among the group of deorsumductors ; if to the right, among the group of dextroductors ; and if to the left, among the group of laevoductors. () Affected Eye. Having found the group, the next thing is to find the eye, which is easily done, while the test object is still held in the area of maximum diplopia, by rapidly screening one eye two or three times in succession with the hand, in order to find which image belongs to the screened eye. It is, of course, the image which disappears and reappears. The image which lies farthest in the direction of increasing diplopia belongs to the paralyzed eye. If the affected muscle be an internal or external rectus, our work is done when we have found the group and the eye, for each eye has only one dextroductor and one laevoductor. (Y) Delinqueut Vertical Ductor. But if the fault be among the group of elevators or depressors, one more step is needful, since each eye has a pair of each, of which one is ever a rectus and the other an oblique. The next thing to do is, while holding the test object (itself horizontal) in the diplopic half of the field to pass it first to the right hand and then to the left, to note in which position the ver- tical component of the diplopia appears greatest to the patient. If, on looking to the same side as the paralyzed eye, the difference in height is greater than on looking to the other side, the affected muscle is a rectus. If the difference in height is greatest on look- ing to the side of the sound eye, the affected muscle is an oblique. If we do not know which eye is wrong, we may still decide in the same way, whether the affected muscle is dextral or laeval in its action.* Dextral and Laeval. It is very easy to recall which muscles are dextral and their vertical effect, and which laeval, since the dextral are those whose tendons point to the right, and the laeval *T!ie reader who is accustomed to speak only of lateral and medial elevators, and adduc- tion and abduction, may possibly challenge the change to dextral and heval. dextroduction mid leevoduction. The reason tor it is that since the former terms refer to the median plane, students and beginners make fmjxi'nl mistakes when the tast object lies to the opposite side of the median plane from the affected eye, calling an adducted image an ahducted, and soon. This constant liability to error is entirely removed by the change of terms I have employed. It would not have been made otherwise. But this is not all : the change of terms enables us to adhere more closely to nature, for dcxtrodncting and Iwvoducting innervations exist, but we have no certain knowledge of an abducting iunervation. 162 Tests and Studies of the Ocular Muscles those which point to the left. Fig. 54 makes this very clear and is easily borne in mind after a few moments' contemplation of it, the tendons of the obliques being treated as if they pointed forwards and inwards, instead of back- wards and outwards. A moment's considera- tion will show how it must be that a muscle alters the height of the cornea most when the visual line comes to lie in its muscular plane. Since a dextral muscle is one whose muscular plane points to the right and a laeval muscle one whose muscular plane points to the left, it will have their directions Fig. 54 To show how those Muscles whose vertical effect is dextral have Iheir directions pointing be seen that the recti rightly describe themselves, for the right recti are dextral in their action, and the left recti, Iceval. The obliques are contrary. The dextrals, therefore, are the right recti and the left ob- liques ; the laevafs, the left recti and the right obliques. There is no need, how- ever, to commit anything to memory, since the ana- tomical disposition of the muscles can always be called to mind sufficiently to recollect whether its line of force points to the right or to the left. The attitude oi Fig. 55 may come to the help of any one unable to conjure up the muscles. Torsional Purchase and Vertical Purchase Reciprocal. While the vertical purchase of a muscle is greater in proportion as its muscular plane is approached by the visual line, its torsional effect, on the contrary, increases as its muscular plane is departed Fig. 55 Mnemonic attitude for the Muscular Planes (borrowed in part from Landolt). Ocular Paralyses 163 from by the visual line. Thus, the figure shows that when the eyes look to the right the dextrals have the greatest elevating or depress- ing effect, and the laevals have the greatest torsional effect; and vice versa on looking to the left. The corollary is that the greatest torsion of the false image is always to be found on the opposite side of the median plane from its greatest vertical displacement, z. c. , if the greatest vertical sepa- ration is up and to the right, the greatest torsional displacement will be up and to the left ; or if one is down and to the right, the other will be down and to the left. As soon as we have settled whether the muscle is dextral or laeval, our task is done, and the diagnosis made. Recapitulation. Tc summarize, we find* : f Elevators ? s ITT, , Depressors? (1) Which group < . Dextroductors ? ^ Laevoductors ? (2) Which eye? The eye which sees the most advanced image in the direction of diplopia. (3) Rectus or oblique? Rectus if the maximum vertical diplopia be on the side of the paralyzed eye. Oblique if on the side of the sound eye. Confirmation of the Diagnosis. It is well, if time allow, to study the three components of the diplopia in different parts of the field. While we do not trust much to the torsion, or to the minor degrees of horizontal diplopia, in discovering the muscles, they both, but especially the former, afford valuable confirmation ; and if the torsion conflicts with our discovery, the initial investigation should be repeated. The best plan for confirmation is to draw up a motor chart in the usual way, dividing the field into nine areas, as showr in Fig. 57, and carefully representing the false image over as many as it appears in. The non-diplopic areas constitute the "field of single vision," and this can, if desired, be also filled in by the aid of the glass rod, though hitherto the field of single vision has been generally left unanalyzed. * Personally, I prefer to find the eye last, but have adhered to the usual order in the text u being more easily explained. and Studies of the Ocular Muscles Measured Charts. If more accuracy be required, the three components of the diplopia (vertical, horizontal and torsional) can In- iinusiirfd in degrees tor each area by the glass rod and the tangent scales. Construct, then Scrutinize. It is well to fill in the entire chart before reasoning on it, so as to be unprejudiced in the observations. Then, see if the false image corresponds in each area to the physio- logical action of the suspected paralyzed muscle during vision directed towards that area For this we recur to the italicized rule previously enunciated. Another way of putting it, more handy than elegant, i s ._" What the muscle does ; the false image is." Example. For example, suppose the left inferior oblique to be implicated. We know that it becomes a purer and stronger sursum- ductor on looking to the right (Fig. 54). The false image, therefore, in the right upper part of the field will be more purely and greatly sursumducted above the true, than anywheie else (Fig. 68). We know, too, that it still has some laevo-torsional purchase, even on looking to the right, and that it still slightly laevoducts the left eye ; therefore, the false image will be laevotorted and slightly laevoducted. accordingly, in that same area. We know, further, that on looking to the left the elevating power ot the left obliques almost ceases, and their torsional and abducting effects reach their maximum ; therefore, in the left upper area of the field the false image will be but slightly higher than the true, though greatly laevotorted and moderately laevoducted. In the lower part of the motor field the muscle has but trifling sway, and, therefore, here diplopia disappears, the false image running into the true. Names Of the Areas. In drawing up the chart, let everything be denominated by the patient's right and left, and not by the surgeon's. This is an invariable rule for everything in ophthal- mology. Fig. 74, which will appear in its numerical order, shows the names which I think it best to permanently give to the areas, in view of a namesake principle to be described later on. They are names easily recalled, because simply descriptive of their posi- tion from the patient's point of view. Inscription. The mode of inscribing the false image or the diplopia is a matter of taste, and I have not yet settled on a final choice. My favorite way at present is to represent the true image by a dot in the center of each area, leaving the imagination to Ocular Paralyses 165 construct a vertical line through the dot. Then a thin vertical line from the dot represents the vertical element, a horizontal line from the end of that the horizontal element, and a larger dot at the end of that line represents the false image. The advantage of this plan is that we are not bound to inscribe the torsion if the patient's account of it is unsatisfactory, while if we do wish to inscribe it, a thin line through the second dot shows it at once (as in Fig. 56). Moreover, if we wish to make a quantitative record, we can use dotted lines and make each dot represent a degree, to re- present the horizontal and vertical element ; or an inch, or any unit we like to choose. The torsion can also be marked in degrees. Never forget to record on the chart Fig 66 to which eye the false image belongs. Even Incorrect Statements are Valuable, if True Comparatively. It need hardly be said that comparative statements about the diplopia in the different areas are more common with patients than absolutely true measurements, yet though the patient's idea of an inch may be far out, it does not matter if he is consistent otherwise, and maintains his peculiar inch throughout. To enable him to do so, care should be taken to hold the test object at the same distance from the eyes throughout the test. With the ophthalmoscope handle, three or four feet is a convenient distance ; with a candle, six feet. If more than one muscle be affected, the diplopia may increase in more than one direction, and each direction may then be studied independently. Thus, if a depressor and an elevator be both paralyzed, diplopia will increase both upwards and downwards, and become almost nil on looking straight forward. In dealing with multiple paralyses, a careful inscription should be made in every area of the chart, without bias or prejudice, and then the affected muscles should be puzzled out from it. To Read a Simple Cnart. At the risk of being tedious, I will give one example of a single paralysis. An inspection of Fig. 61 : (#) Shows diplopia upwards ; therefore, involving one ot the group of sursumductors. (6) The highest image (say) belongs to the right eye ; there- fore, the muscle is one of the elevators of the right eye. 1 66 Tests and Studies of the Ocular Muscles (r) Its maximum vertical diplopia is up and to the right ; therefore, it is a dextral superductor. But there is only one such muscle of the right eye the superior rectus. Found, therefore. Does the torsion agree ? Yes ; for though there is none ia the right superior area, there is marked laevotorsion in the left superior. Had it been a case of the right inferior oblique, the greatest elevation would have been to the left side, and the greatest torsion to the right side ; moreover, the torsion would have been dextrotorsion. The diagnosis is confirmed, therefore. TO Read a Multiple Chart. (a) Begin by noticing in how many and which cardinal directions the diplopia seems to increase, and if it does so in more than one direction, begin with that of greatest diplopia. Observe which group this greatest diplopia points to. () If the observed diplopia be horizontal, the muscle is found, for the image most removed from the center of the chart belongs to the affected eye. (*:) If the diplopia be vertical, the muscle affected is a rectus, if the area of greatest vertical diplopia is on the same side as the eye that sees the false image : it is an oblique if on the opposite side. Next find the direction of second greatest (independent) diplopia and study that in the same way. Then the third, and so on. Independent Diplopiae. Diplopise in opposite halves of the motor field in which the false image occupies opposite sides of the true image are independent. If the false image remain on the same side as the true all across the field, then it is not a case of two independent diplopiae, but there is a concomitant element, due either to an anomaly of the converging innervation or to what is generally called the "secondary contracture."* If the separation of the images is constant in amount, the diplopia is entirely concomitant ; but if it differs in degree in different areas while ever the same in kind, there is a paralytic element as well as a concomitant one. Concomitant elements are distinguished by pervading the whole field and, therefore, an investigation of every area in the field of single vision, as by the rod test, leads to a fair estimate of their amount. In all multiple paralyses the diplopia produced by one muscle ma y a 'ter that due to another, so that any untypical features of Theconconiitancy of 'secondary eontracture," or ''consecutive deviation," as it is better di SOD ** Vety lml>er ' ectl the < and slightly Malprojection. the field. ) ( downwards. [ In the lower ] f Outwards False Image Displaced. outer parts of \\ and slightly J the field. } ( upwards. Maximum Diplopia. On looking down and out. PARALYSIS OF RIGHT INTERNAL RECTUS This muscle turns the cornea to the left, therefore there result : 1 . Primary Deviation (of paralyzed eye) To the pat'ent' s right. 2. Face looks 1 Defect in Motion of Eye Secondary Deviation (of sound eye) Malprojection Maximum Diplopia False Image Displaced io tne patient's left. 172 Tests and Studies of the Ocular Muscles Diplopia. Horizontal, crossed, increasing on looking to the left, and also with approach of the test object. Greater, too, on looking up ; less on looking down. Fig. 59 PARALYSIS OF LEFT INTERNAL RECTUS This muscle turns the cornea to the right, therefore there result 1. Primary Deviation To the patient's left. 2. Everything else To the patient's right. Fig. 6O Ocular Paralyses 173 Diplopia. Horizontal, crossed, increasing on looking to the right and also with approach of the test object. Greater, too, on looking up ; less on looking down. QUALIFICATIONS IN PARALYSIS OF THE INTERNAL RECTI Two unimportant refinements (akin to those for the external recti) require notice. First Qualification. The horizontal separation of the images is apt to increase on looking upwards and lessen on looking downwards, due, as in the case of the external recti (q. v.), to the habit of the converging center. Second Qualification. On looking up and in, the superior rectus of the paralyzed eye ; and on looking down and in, the inferior rectus of the same eye, lose torsional purchase over the eye from the fact that their muscular plane forms a smaller angle with its visual axis than when adduction is efficient. Hence, on looking up and in, the false image should theoretically be lower than the true and intorted ; and on looking down and in, it should be higher than the true and extorted. Absolutely isolated paralysis of the internal recti are, however, so rare that this could scarcely have been dis- covered from actual experiment. It is chiefly from the analogy of the external recti that it is assumed to occur. It will be seen represented in the charts. PARALYSIS OF RIGHT SUPERIOR RECTUS Fig. 61 This muscle turns the cornea upwards, and somewhat inwards, with intorsion. Its power as a superductor is greatest during vision to the right. It is, therefore, a " dextral superductor." Jests and Studies of the Ocular Muscles Its power, on the other hand, as adductor and intorter is greatest during vision to the left. Therefore, there result from its paralysis : 1. Primary Deviation (/. e., of paralyzed eye) Downwards ; and, in the upper half of the field, outwards ; with extorsion. 2. Face looks Up and to the right. Defect in Motion of Eye Upwards; most marked when the eyes are also turned to the right. Secondary Deviation (i. e., of sound eye) Upwards, and slightly to the left, probably with a little laevotorsion. Malprojection Upwards, and slightly to the left. Maximum Diplopia On looking up and to the right. False Image Displaced Upwards and slightly to the left, with laevotorsion. Diplopia. Vertical diplopia, increasing on looking up, and especially up and to the right ; crossed and torsional diplopia increasing on looking up and to the left. The nature of the diplopia during the primary position of the sound eye should be carefully investigated, and the equilibrium should be examined (by the rod test, e. g.~) in the non-diplopic half of the field. This remark applies equally to all the succeeding paralyses. PARALYSIS OF LEFT SUPERIOR RECTUS Same as the last, with substitution of "left" for right, and LEFT SUPERIOR RECTUS Fig. Ocular Paralyses 175 vice versa. It turns the cornea up and in, with inward torsion, and is a laeval elevator. (Fig. 62.) PARALYSIS OF RIGHT INFERIOR RECTUS This muscle turns the cornea downwards and somewhat inwards, with extorsion. Its power as a subductor is greatest fig. 63 during vision down and to the right. It is, therefore, a "dextral subductor." Its power, on the other hand, as an adductor and extorter is greatest during vision down and to the left. There result from its paralysis 1. Primary Deviation (/. e., of paralyzed eye) Upwards and, in the upper half of the field, outwards ; with intorsion. 2. Face looks Down and to the right. Defect in Motion of Eye Downwards, most marked when the eyes are also turned to the right. Secondary Deviation (i. . Tfi and 77 in the original work were omitted by the author in this revised edition, which explains the break iu the sequence of the figures at this point. 200 Tests and Studies of the Ocular Muscles squinting eye. To be expert, a little practice is necessary, but the same is true of every method of examining the eyes. Many a perplexity would be at once dispelled if these corneal images came to the rescue. Fixation Position of the Corneal Reflection. When the vision of babies is imperfect, or the two eyes do not work well together, it is easy to find whether each eye possesses the power of central fixation by observing whether each corneal image occupies the ' ' fixation posi- tion," with steadiness. In order to describe the " fixation position," let us mention a third precaution to be observed namely, to allow for the imperfect collimation of the visual line* and its variations. We will, for simplicity, suppose that the eye has only two axes, as in Fig. 79, viz. , ( i ) the geometrical axis, and (2) the axis of vision, and explain these briefly : The optic axis ( G) is the geometrical axis on which, so to speak, the eye is built, passing from the center of the cornea in front to the posterior pole of the eye behind. With this axis, however, the line of vision ( V) does not coincide, for, curiously enough, we do not see straight out of our eyes, but obliquely out of them. This is due to the fact that the " fovea To show the obliquity of the ,. visual Axis (v f) with centralis ( / ) does not lie exactly at the reference to the Geometri- . i i N ,- cai Axis (/>). The Fovea posterior pole of the eye ( f>). but slightly (/) is to the outer side of . , , , the posterior pole ( p). to its outer side and below it. Consequently, the line of vision ( V) intersects the geometrical axis at the nodal points ( the optic axis. The visual line passes from the point looked at through the nodal e fovea. The line affixation, on the other hand, extends from the point looked at and h ,,?i motlon . of the eye. The discrepancy between the aberration of the visual line Doniil V ,,,,' ^amrna in a given eye is greater in proportion to the nearness of the object, g/f alpha, since be assumed the major axis of the corneal ellipsoid to coincide with uU-aft axis of the eyeball, is the angle between this axis and the visual line. Ophthalmoscopic Corneal Images 201 appears to the inner side of the center of each cornea (see Fig. 78). The average aberration of the visual line is, in emmetropia, 5. In hypermetropia the angle is greater, the average given by Donders being nearly 8, and in myopia it is less, sometimes even negative, the average given by Donders being less than 2. From the fact that Donders called this angle the angle alpha, some confusion has arisen in the use of that term. (See foot note, p. 200. ) Apparent Squint. In consequence of these differences, hyper- metropic eyes appear slightly divergent and myopic eyes slightly convergent, for we are so accustomed to the emmetropic aberration as to think any greater or less aberration peculiar, and hence arise the two well-known varieties of "apparent squint." The apparent position of the corneal image on the cornea, while the center of the mirror is fixed by the patient, may, as already mentioned, with advantage be called the "fixation position" of the image. We have seen that in emmetropia the fixation position is to the inner side of the corneal center ; in hypermetropia it is still farther to the inner side, because the angle gamma is greater ; in myopia it is less to the inner side or even, in some cases, slightly to the outer side of the corneal center, because the angle gamma is smaller or even negative. In emmetropia the most common condi- tion is, as represented in Figs. 78 and 80, for the pupil to be slightly to the inner side of the center of the cornea, and for the corneal image to be again slightly to the inner side of the center of the pupil. It is important, however, not to trust much to the position of the pupil lest it should mislead, and if the pupil be misplaced the position of the image in the cornea should be studied rather than its position in the pupil. In an eye free from nystagmus and which possesses the power of central fixation, the corneal image occupies the fixation position with great steadiness. If central fixation, however, be lost, the image is seen to wander aimlessly about the cornea, though really, of course, it is the cornea itself which wanders. Priestley Smith has made the interesting and valuable observa- tion that in tobacco amblyopia the power of central fixation is retained, while in some cases of acute retro-bulbar neuritis it is lost. An absolute scotoma involving the macula would, of course, destroy central fixation, which also might very likely be impaired by functional or organic changes at the macula, produced by looking at strong light or by over-use of the microscope, etc. 202 Tests and Studies of the Ocular Muscles Refraction Surmisable. With a little practice it is quite easy to surmise from the corneal image alone whether an eye is much hypermetropic or myopic, and I have pointed out elsewhere that a high angle gamma, as indicated by an unusually-displaced corneal image, should, in an apparently emmetropic eye, make us suspect the presence of latent hypermetropia and induce us to paraly?e the accommodation.* It is well, however, to remember that exceptions to the rule are not infrequent. The angle gamma in astigmatism does not appear to have been studied fully yet. In some cases of hypermetropic astigmatism in which the deficient curvature was horizontal, I noticed a greater angle alpha than in emmetropia ; and my impression is that, as a rule, a cornea which is too flat horizontally has a higher angle gamma than usual, whatever the vertical meridian may be. The beauty of ophthalmoscopic corneal images is that we are able, as it were, to actually see in a moment what point of the cornea is traversed by the line of vision (cf. Figs. 78 and 79), and by the distance at which this point lies from the center of the cornea to guess approximately the amount of the angle gamma. Any instance of an unusually high or low angle at once strikes us and should set us to try and account for it by looking for some abnormal condition of refraction, eccentric fixation or unusual shape of the eye. Clinical acknowledgment of the gamma is, I believe, the key to the successful use of ophthalmoscopic corneal images, and it is this which enforces the necessity of the patient's attention being directed to the mirror and, if possible, to its central aperture, since then, in normal eyes, the two images are symmetrical (Fig. 80). If the same eyes be allowed to wander to one or the other side, the images will, of course, appear unsymmetrical, for one will be nearer the edge of its cornea than the other, by a distance equal to twice the monocular aberration (Fig. 81). The vertical element of the angle alpha, shown by the corneal image lying generally slightly above the horizontal diameter of the cornea, seems of less clinical importance, and it is often imperceptible, though Ophthalmoscopic Cornea! Images 203 its amount is also subject to variation ; I have not devoted much attention to it, though noting many cases of very marked vertical displacement. Angle Gamma in Cataract and Iridectomy. It is very pretty to see how faithfully the corneal image occupies its correct "fixa- tion position " in cases of lamellar cataract not quite large enough .Fig. SO Fig. 81 To show the Symmetry of the images when normal eyes look straight at the mirror and the Asymmetry of the images when the same eyes look" away from the mirror, though the eyes are iiot squinting. Fig. 80 shows how and Fig. 81 how not to use corneal images. to fill the pupil, even though the reflection . lies against the most opaque portion of the cataract. The visual line, therefore, tra- verses the cataract, as, of course, it would on simple optical principles. Similarly, in cases of very peripheral iridectomy for occluded pupil, and when the iris is drawn to one side, as in old cases of prolapse, the corneal image still occupies its proper posi- tion, though against an opaque background, and demonstrates, perhaps more prettily than anything else could do, the fallacy of supposing that a nasal or temporal iridectomy predisposes to stra- bismus or alters materially the relations between convergence and accommodation. Unsymmetrical Angles Gamma. Now let us consider a dif- ficulty in the detection of strabismus by corneal images which arise very occasionally. The angle gamma may be different in the two eyes, so that the corneal images appear unsymmetrical. The asymmetry in these cases is, however, so slight that its very smallness leads us to suspect its true cause, and if we place the hand over each eye in turn, it will be found that the " fixation position " is not the same in each. Why, it may be asked, does the very smallness of the asymmetry lead us to suspect its true 204 Tests and Sttidies of the Ocular Muscles cause ? The answer is : Because minute squints are exceed- ingly rare, except when one eye is blind or its image ignored, the natural desire for single vision being too strong to allow minute squints to exist without considerable efforts being made to overcome them. In cases of alleged recent monocular blindness, the presence of a very slight squint affords presumptive evidence of the veracity of the patient, since a slight persistent squint cannot be voluntarily created. As, for instance, in the case of a young woman who stated that till a few days before she presented herself she had perfect sight in both eyes and that suddenly the sight of the left eye disappeared. No change could be detected in the fundus and the pupil reacted normally, so that the case looked like one of feigned amblyopia. Ophthalmoscopic corneal images, however, showed that there was a minute squint, and this corroborated the patient's statement. Alternation. A " monolateral' ' squint is one in which the same eye always fixes and the other always squints, in contrast to an "alternating " squint, in which latter either eye fixes indifferently. In squints of high degree it is most easy to determine whether they are alternating or monolateral, without the aid of corneal reflections, by simply covering the fixing eye for a few moments, so as to make the other one take up fixation instead ; if the latter continues to fix Fig. 82 Rather small Angle Gamma, especially in left eye, in a case of low myopia (.5 D.). when uncovered, the squint is alternating, but if fixation is at once transferred back to the originally fixing eye, the squint is mono-- lateral. With minute squints, however, it is not so easy to settle this point without the aid of corneal images, which enables us at once to see which is the fixing eye and whether, by covering this eye temporarily, fixation can be transferred to the other. Concomitancy. A still more important point to settle is that of " concomitancy," because by this alone can we tell whether or not Ophthalmoscopic Cornea/ Images 205 a squint is paralytic. In paralytic squint the degree of strabismus increases on looking in the direction of action of paralyzed muscle ; whereas, in concomitant squint, the degree remains the same in whatever direction the patient looks. The following method is one which I have found useful : Lay the palm of the left hand on the patient's head, with instructions to let the head follow the most gentle guidance of the hand without resistance. Now note the exact position of the corneal reflex in the squinting eye while the fixing eye is directed to the central aperture of the mirror, and steadily turn the head to the right and left, up and down and into intermediate positions, to notice whether the position of the reflec- tion is unchanged by these manoeuvres. If it is unchanged, the squint is concomitant ; if otherwise, the squint is paralytic, pro- vided that the movements made are not too great to bring in the fallacy of mechanical impediment from one of the corneae reaching to its motor limits. Vertical squints are just as easily detected as horizontal ones. Test for Binocular Fixation. The next use of corneal images to describe is one which I have sometimes found of value, viz. , to test for binocular fixation when its existence is doubtful. After operating for strabismus and setting a squinting eye apparently perfectly straight, we are often at a loss to be sure whether both eyes are able to work together. We have some interest in finding this out, because binocular vision is so great a preservative from any return of the strabismus, and we can give a better prognosis accordingly. By subjective tests it is often impos- sible to settle the question, the patients being so frequently either too young or unintelligent to give us any assistance. An objective test, even though difficult and requiring a rather detailed descrip- tion, is, therefore, a great help. After operation, for some weeks at least, the eye operated on remains more stationary than its fellow (Berry), so that by turning the head slowly to the right or left we make, if binocular vision is absent, the corneal image on the squinting (and operated) eye slowly and steadily move across part of the cornea. If binocular vision be present, it may be strong enough to overcome the sluggishness of the squinting eye, in which case its image remains in the ' ' fixation position ' ' throughout. But even if the desire for single vision is not strong enough to effect this, there is always, if it be present at all, a part of the field of fixation over which the 206 Tests and Studies of the Ocular Muscles "fixation position" is maintained, and at the edge of this region the corneal image suddenly moves to another point. It is the con- tinued maintenance of the fixation position during lateral movements of the head or else the sudden abandonment of the fixation posi- tion, instead of only gradually moving away from it, on which to count in making the test* To Roughly Measure a Squint. Hirschberg has shown that when the corneal reflection of a flame occupies the margin of a medium-sized pupil (3^ mm.) the amount of squint present is 15 to 20, and if it occupies the margin of the cornea about 45. This convenient mode of guessing the amount of squint, of course, neglects the aberration of the visual line, for with normal aberration the corneal reflection lies nearer the inner than the outer margin of the cornea, so that a pupillary marginal reflection means a smaller divergent squint and a greater convergent one than the mean calculation. It is easy, however, to notice what the aber- ration actually is and to allow for it. Priestley Smith's Mode of Strabismometry. This excellent procedure was published so early as 1888. A piece of tape i m. (or 60 cm. ) long, of which one end is held by the patient against his temple, while the other end is attached to a ring on the sur- geon's finger, maintains the re- quisite distance between surgeon and patient. A second piece of tape, graduated and figured, is attached by one end to the same ring and then passed between the fingers of the surgeon's free hand, at which the patient is directed to look. When the separation of the surgeon's hands reaches the measure of the squint the corneal reflection occupies the normal position of the cornea of the squinting eye. Different Points Of View. Students and onlookers some- Fig. 83 Priestley Smith's Tape Method. The rights hand figure shows a diverging squint and the left-hand figure a converging one, both of the right eye. The ophthalmo- scope is at O and the surgeon's hand at //. When the fixing eye (L) is made to look at the surgeon s hand the squint- ing eye (R) becomes straight fur the ophthalmoscope. * "Ed. Med, Journ.," loc. eii. Ophthalmoscopic Cornea/ Images 207 times forget that they do not see the corneal reflections under the same conditions as the eye behind the ophthalmoscope. Fig. 84 shows a convex mirror illuminated from a point L. An eye placed at 7 sees an image at i, an eye placed at // an image at 2, at /// at 3, at IV at 4, at V at 5, and so on ; the reflections lying in a caustic curve. Error Of Approximation. So far, I have assumed that the spot of light on the cornea marks the very point of its transit by the visual line. The assumption involves an exceedingly small error. Fig. 84 The Caustic Curve of a convex mirror whose principal focus is at F That the approximation should be trusted requires an analysis of the exact amount of error and its nature. Fig. 85 shows the principles involved. A straight line connecting the center of curvature (r) of the cornea with the center of the sight-hole in the ophthalmoscope is the line which passes through the apparent center of the corneal reflection. Another straight line connecting the anterior nodal point (A 7 ") with the center of the sight-hole is the visual line. It will be seen that they do not quite coincide, and traverse the cornea at slightly different points, the difference being exaggerated in the figure to make it evident. Without troubling the reader with calculations on so insignifi- cant a subject, I make the corneal transit of the visual line at a 208 Tests and Sttidics of the Ocular Muscles Fig. 85 To show that the Axis of Reflection (r c) is a little farther from the anterior pole than the Visual Axis (v A'), c is the center of curvature of the cornea and TV the uodal point. distance from the anterior pole of the eye, which is only seven- eighths of that of the center of the corneal reflection. Hence, in emmetropia, where the corneal transit of the reflection is displaced on an average .63 mm. inwards, the displacement of the visual line is seven- eighths of this, the error of approximation being only T f T - of a millimeter. The corneal reflection, therefore, very slightly ex- aggerates the real devia- tion of the visual line, but since it does so in a uni- form proportion of 8 to 7, there is, for clinical pur- poses, no disadvantage in it. Since the fixation line proceeds from the center of motion to the object, the corneal reflection lies somewhere between the fixation and the visual lines. Photography Of Muscular Anomalies. Hitherto, for permanent records of ocular paralyses, oculists have had to confine themselves pretty much to subjective hand-made charts of diplopia. It is evident that photographic charts of the objective position of the eyes, free from all the fallacies of a subjective investigation, would be much better in some cases, and if carefully and properly utilized, the corneal reflections afford beautifully precise indices of ocular deviations of every kind except torsional. Gullstrand (1892) made a number of photographs of muscular defects, utilizing the reflection from an ordinary window ; but this source of illumination is not precise enough to afford such good results as are shown, for example, in Fig. 86, by marking out a much smaller point on each cornea through which the fixation line passes. We have seen that to get the best results from ophthalmoscopic corneal reflections, it is essential that the patient should direct his attention to the central aperture of the mirror (Figs. 80, 81) and the surgeon's eye should be behind the virtual source of light (Fig. 84). To photograph the reflections perfectly, therefore, the light should proceed from the center of the photographic lens or Ophihalmoscopic Corneal Images 209 else from an area surrounding the lens symmetrically, though indeed there would not be much error in lighting a flame or incandescent lamp just over the lens, while making the patient look at a point midway between the two. Since daylight is the best, the camera which I have designed for the purpose (shown in Fig. 87) will probably be found the Fig. 86 High Angle Gamma in left eye, with ascending convergent Sqirut iu the right. handiest kind of apparatus to use, since it is meant for work out of doors where the greater intensity of light shortens the exposure a point of great importance with so restless an organ as the eye. An elliptical mirror (m n) provided with an elliptical perforation nearer its lower than its upper end is fastened at an angle of 45 to a short cylinder of wood. This short cylinder is perforated and provided with a rapid portrait lens and pneu- matic shutter. The wooden cylinder can be revolved round its axis, so as to bring the brightest part of the sky into view. To the patient . . Author's Squint Camera. the mirror, owing to its inclination, appears perfectly circular and makes a circular reflec- tion on the cornea with a small black dot in the middle. Since the upper part of the mirror is farther from the patient than the lower, equal lengths there subtend smaller angles. For this reason the perforation of the mirror should be nearer its lower than its upper end, according to a simple calculation. The plane of the mirror should pass through the center of the photographic lens, and the major axis of the ellipse should be its minor as about 10 to 7. If d be the distance of the patient from the lens and b be the breadth of the mirror, let m represent the required length of the 2io Tests and Studies of the Ocular Muscles mirror above the center of the lens and n its length below, 6 the inclination of the mirror to the horizontal and a the angle it sub- tends at the eye. Then b cos. \ a ~ 2 sin. ( e~^-~y) b cos. \ a 2 sin. (8 + a) Therefore, m Sin. (0 -f \ a) n Sin. (0 \ a) and, since b 2 rf tan. | a a = 2 tan. 1 2 d From these formulae it is quite easy to construct a mirror for any inclination that may be most convenient, to subtend any given angle at the eye, and to appear as a perfect circle to it. In practice, the use of my camera has given me much pleasure, for it only takes a minute or two to use. To save time, it is made in the form of a wooden box with a fixed focus. The patient's distance is adjusted by a stick of the right length and the box is provided with an Eastman film roll holder. To use the camera in the consulting room, seat the patient at one corner of the window with the side of his face about a foot from the glass. Place the camera in front of him (also about a foot from the glass) and with the mirror rotate 45 to catch the sky light and reflect it into his eyes. Bid the patient look at the center of the lens, adjust the distance of the camera with the stick and give about three-seconds' Fig. 88 Slight (temporary) over-correction of congenital defect of Right Superior Rectus, by advance- ment of the Superior and tenotomy of the Interior Rectus. exposure. Lately I have used the stand of Javal's ophthalmometer for the purpose. The instrument is removed and its place taken by a wooden platform for the camera to rest on. The patient rests his chin and forehead as usual. A tiny circle of paper affixed Ophthahnoscopic Corneal Images 2 ii to the center of the lens for the patient to look at, does not impair the definition of the photograph. For those who have not a special camera, it may be well to know that a mere disk of cardboard encircling the lens of any ordinary camera suffices to give a reflection from the cornea out of Fig. 89 Chart of the Corneal Reflections of the right eye, in a case of congenital defect of the Right Superior Rectus. Being a chart of objective appearances, the patient is supposed to be behind it, so that R and L are reversed. doors, though not so excellent a one. Figs. 78, 86 and 88 were taken in this way by a skilful amateur. Since their publication I found that Gullstrand, in 1896, employed circular disks for the photographic investigation of the shape of the cornea, on the principle of Placido's disk ; so that the idea is not, in every part of it, a new one. Indoors, in the absence of a mirror, a small incandescent lamp may be found convenient if fixed just over the lens, the patient being made to fix a point mid-way between the center of the lens and the center of the lamp. An acetylene flame is very suitable. But with a mirror no artificial light is needed at all. Though a complete photographic record of an ocular paralysis would require nine photographs, yet for paralyses of single muscles 212 Tests and Studies of the Ocular Muscles three amply suffice : the first, with the head set in the favorite attitude ; the second, with the eyes brought into the area of maxi- mum diplopia ; and the third, in the area on the same level' as the last, but on the opposite side of the median plane. In each case it is only the head that is altered, since the eyes are made to fix the center of the lens always. Recording; Reflections. Fig. 89 shows a simple plan of record- ing the corneal reflections of a case of paralysis, this being, indeed, a record, before operation, of the same case as Fig. 83. In the "left superior" and "left external" areas the reflections are normal, showing that single vision exists on looking in these directions. The "left inferior" area shows slight depression and adduction of the cornea. The median areas show depression increasing on looking up, combined with abduction in the superior median area, adduction in the primary area and still greater adduc- tion in the inferior median. The right areas exhibit the same features in a more marked degree. An extremely ingenious use of the corneal reflection has been made by Lucien Howe, who has used it to determine by graphic methods the actual rate of movement of the eyeball in glancing from one object to another, the record being made on a revolving sensitized cylinder timed by a tuning fork. His photographs thus taken appear to show that in glancing forty degrees the eye takes from one-tenth to one-twentieth of a second. This experimental method might with great advantage be applied to nystagmus. CHAPTER XII Heterophoria * Latent deviations of the eyes involve the same principles as do the manifest deviations which we recognize as squints. They differ in being small enough to come within the overcoming power of the love of single vision, but are liberated from this superior influence in the dark ; or, when the vision of the two eyes is dissociated, by making single vision either impossible (prisms, etc.) or undesired (glass rod, etc.). Chief Divisions. Latent squints are, like manifest squints, grouped into "paralytic" and "concomitant" according or not as they steadily increase on looking in special directions. Dissociation of the Eyes. The demonstration of suppressed deviation depends on exclusion of one eye or on artificial diplopia of some kind, so as to "dissociate" the eyes; or else on the arrangement of two objects, so that each is only seen by one eye. By "dissociating" the eyes, we do not, of course, mean that any of the innervations are made to cease to be conjugate, but merely that the desire for single vision is removed so that the eyes fall into their "position of equilibrium." Thus, if a strong prism, with its apex upwards, be held before one eye, everything appears double, and the distance between the double images of any object as, e. g. , a candle, is so great that the cerebral centers concerned, utterly unaccustomed to so great a separation between the images of a single object, make little or no attempt to unite them. The eyes are now said to be "dissociated," and if any latent deviation exist it will express itself by a movement of one image to the right or left of an imaginary vertical line passing through the other. This device for dissociation, as introduced by v. Graefe, was for many years a favorite. It requires considerable care in the adjustment of the prism, for since the image seen through the prism is displaced in the direction of its apex, it follows that if the apex is not exactly vertical, neither will the said image be situated vertically over its fellow if no deviation exist. Thus, a lateral * Heterophoria is Stevens' name for latent deviations. (213} 214 Tests and Studies of the Ocular Muscles displacement of the image due to a badly-placed pri^m may be wrongly attributed to the eye. Since, to begin with, prisms are often incorrectly marked, precautions have to be taken accordingly and the prism be set in the trial frame so that a vertical line seen through it appears unbroken throughout. Physiological Heterophoria. Within certain limits suppressed deviations are physiological, for though the accommodating and converging centers are functionally connected in a very intimate way, they are not indivisibly one. Exclusion of one eye, while the other is engaged in distant vision, causes the excluded eye generally to deviate little, if at all, inwards or outwards from its former position. As the object fixed is brought nearer, the eyes converge less than they accommodate with each approach. In consequence of this we find that, if the excluded eye deviates out- wards in distant vision, the deviation increases more and more as vision becomes nearer.* By the time the object is within a foot of the eyes, the deviation has increased to 3 or 4, in the majority of people. It follows from this that in ordinary close work we habitually suppress, by our desire for single vision, a deviation to this extent. Were the same deviation to exist with both eyes uncovered, we would, of course, see double ; but the love of single vision will not allow it to exist under those circumstances. The unconscious effort required to suppress a deviation of normal amount in the interests of single vision is so slight that we experience no inconvenience from it. But if, from any cause, the deviation be a great one, the effort demanded may be sufficient to occasion headache or asthenopia, and the more so if there be any debility of the system. Indeed, the effort may at times be given up, double vision being accepted until the tired centers have had time to recuperate themselves for a fresh attempt at single vision. One form of ' ' periodic strabismus ' ' is of this nature. Direction Of Deviation. An excluded eye may deviate in any direction, upwards, downwards, inwards or outwards, or in interme- diate directions ; but in the last case we think of the horizontal element and the vertical element separately. Since the power of elevating or depressing one eye above or below the other (monocu- lar sursumduction or deorsumduction) has much smaller physio- *Syme " Fellowship Essay," 1882, and "Trans. Oph. Soc.," 1883, p. 290. Helerophoria 2 x c logical limits than horizontal powers of adjustment, it follows that vertical deviations are of so much the more importance than horizontal. Hyperphoria. Since, in concomitant vertical deviations we can- not tell which eye is at fault, the convenient name " hyperphoria " was introduced by Stevens. Instead of speaking of an upward or downward deviation of one eye, which would give us two things to remember, viz., which eye and which direction, he speaks of a right or left hyperphoria, which gives us only one thing to remem- ber. Thus, if the right eye deviate downwards on exclusion, he calls it a left hyperphoria. \Ye need, however, to always test both eyes to assure ourselves that the deviation, even if it seem concomitant at first, is of the ordinary kind, for during an extended investigation made for Mr. Berry among the patients attending his out-patient department, several cases were found in which each eye deviated upwards on exclusion. And, besides this, the use of the term hyperphoria should not make us forget that the deviation may be paretic and due to actual weakness of a muscle, causing the separation of double images to be greater in some directions, and to be greater when the paretic eye fixes (answering to the secondary deviation) than when the sound eye fixes (answering to the primary deviation). The following are the best tests for latent deviations : (i) The Objective Screen Test. Make the patient fix a very definite test object, near or distant, as may be desired. Screen one eye for fully half a minute. Suddenly withdraw the screen, watch- ing if the eye makes an instantaneous movement of recovery ("corrective movement"), and if so, in what direction. A corrective movement inwards would show that the eye had wandered outwards under the screen, so as to betray a latent divergence (exophoria*}. A similar corrective movement outwards would betray a latent convergence (esop/ioria*). Vertical corrective movements show that one eye has latent elevation (hyperphoria) or latent depression. These tests can be made a little more delicate by employing a flame for the patient to look at, or by throwing light on one eye, before covering it, from the mirror of the ophthalmoscope. On momentary unscreening of the eye the position of the corneal * Dr. Stevens' terms. 216 Tests and Studies of the Ocular Muscles reflection can be observed before the eye has had time to recover itself. Graefe recommended that both eyes be shut for a little while befoie making any test for latent deviation. (2) Subjective Screen Test. Alfred Graefe pointed out that when one eye has a manifest deviation, however small, sudden screening of the fixing eye makes a candle flame appear to the patient to move, for the deviating eye makes a corrective move- ment in order to take up fixation, and this movement betrays itself by an apparent displacement of its field of vision. The same principle has been applied to latent deviations by Duane, who has called it the "parallax test." After covering one eye for a time, he suddenly transfers the screen to the other. If the first eye wandered under the screen, the deviation is betrayed by a sudden apparent displacement of the candle in the opposite direction to that in which the eye deviated. Thus, if the right eye be the one first screened and the flame moves to the right, there is esophoria ; but if the flame moves to the left, exophoria. Duane calls them respectively "homonymous parallax" (P//) and "crossed parallax" (PX). If the flame moves down, he calls it "right parallax" (PR), because it shows that the right eye deviates upwards (right hyper- phoria) ; and if the flame moves up, "left parallax" (PL), showing left hyperphoria. This test presents the advantage of requiring no apparatus. Otherwise, its usefulness is doubtful except for a skilled patient. Parallax "can be measured in terms of the prism which causes its abolition." For near vision, a dot on a piece of paper replaces the flame. (3) Prism Tests. These were introduced by von Graefe long ago. A prism, with its base up or down, strong enough to pro- duce insuperable vertical diplopia, was held before one eye, while the patient looked either at a distant flame or at a dot on a card (with a vertical line through it, which, however, it is better without). If one image appeared to wander to the right or left of the other, the eye to which it belonged was proved to have deviated in the opposite sense. The amount of deviation was measured by the prism, base in or out, required to bring the two images again into one vertical line. Heterophoria 217 The difficulty of ensuring that a prism is strictly base up or down, and the considerable inaccuracies introduced by even slight departures from a correct position, led the writer to design a "double prism," shown in Fig. 90, like two thin prisms joined at their bases but made of one piece of glass and somewhat similar, therefore, to the bi-prism used by Fresnel to demonstrate phenomena of inter- ference. When this is held before one eye of the patient he sees with that eye two images of a flame, one above and the other below, the real image seen by the naked eye. When the two false images are vertical, the prism is correctly held and it is easy to judge whether the third and real image lies to the right or left of the imaginary line between them. Dr. Stevens, of New York, feeling the same difficulty about Graefe's prism, overcame it in a different way by his excellent and well-known " phorometer," in which prisms are mechanically Fig. yo Double Prism (square form) FIR. , so that light falling on the lens 1% cm. from its optical center is deflected 1, another 1% cm. 2, and so on. A half-meter lens will have its focus at 2 D. and the corresponding deflections will be 2 and 4. With a 5 D lens they will be 5 and 10. Prism Diopters. For those who use prism diopters the matter is simpler still, since we only divide the number of prism diopters required by the number of diopters in the lens to find the centi- meters of decentration. The formulae being C = ^ ; and A = C D. Direction of Decentering. Whatever the nature of the hetero- phoria, the following rules hold good : Displace lenses with the deviation. Displace -f- lenses against it. For example, in hyperphoria, before the highest eye Decenter -f- lenses downwards. Decenter lenses upwards. Heterophoria 233 The rationale is very easy to understand, for every student knows that lenses appear to displace objects with them, and + lenses against them. When an eye tends to deviate we displace the image in the same direction, so as to indulge the eye a little. Example. In a hypermetrope of 3D., suppose we have de- rided to relieve a right hyperphoria of 4 by i y 2 in each eye. Here P = iy 2 and D = 3, 7 P so that C = -- =- becomes 4/? X '* 875 4X3 The right lens must be displaced downwards, therefore, 8^ mm., and the left lens upwards to the same amount. Prism Diopters.* Taking prism diopters say we decide to relieve each eye by 2^ A. Then D * Operative Interference is indicated in : (1) Diplopia, however occasional, which is too high for prisms, provided not due to any passing or removable cause or to progressive disease ; and if of sufficiently long-standing to give no hope of natural cure. The correction of any error of refraction should be tried first. (2) Especially is the homonymous diplopia for distant objects experienced by some myopes (strabismus myopicus convergens) suitable for operation, since diverging effort is greater than converg- ing ; yet we take care not to over-correct if in near vision there be already exophoria, since the diminution of the distant convergence is certain to be accompanied by a corresponding increase of the near divergence. Advancements alone are suitable. (3) High esophoria, especially if it persists in near vision, is generally suitable for operation if correction of refraction fails ; but operation should only be considered if the suppressed squint cause subjective symptoms or conscious strain. (4) Exophoria, or latent divergence, is generally of very little account at all, and only very rarely calls for operation. I have *A gradient, or departure, of 1 in 100 is a convenient unit and was chosen by Charles F. Prentice, M. E., of New York, as a unit of prismatic power, being called by Swan Bur- nett a w prism-diopter." " Centime " would, perhaps, be a better name for it when used for other purposes than prisms. It is a unit which does not bear multiplication, since its higher powers are not angular multiples of its lower. 234 Tests and Studies of the Ocular Muscles seen some very high degrees, indeed, without suggesting it. When it causes periodic squint, or diplopia, after correction of refraction, the result of operating is generally excellent, provided a sufficient change of position is obtained. Improvement of the general health, bad teeth, or digestion, and errors of refraction need look- ing to first. In America large doses of tincture of nux vomica are recommended. Orthoptic training is suitable for those who will take the trouble. The amplitude of convergence should always be tested in these cases, and if it is good, operation is more likely to succeed. The case on page 148 may be taken as one out of many to illustrate the necessity for an over-effect at first. It was that of a young school-girl with myopia = .5 ; .5, whose left eye turned outwards when tried. The tangent strabismometer showed the deviation to be one of 20 L (the negative sign meaning divergence, and L the left eye). Under chloroform, both external recti were divided and one internus advanced, producing con- siderable convergence with homonymous diplopia, which, when measured a week after, was -f- 10 in the primary position. Three weeks after the operation it became O, and seven weeks after 2, where it seems to remain. The periodical "turning out" is cured and the eyes are rapidly regaining their con- comitancy as measured by the rod test. There is no diplopia on looking in any direction. (5) Hyperphoria, without occasional diplopia, is rarely large enough to be beyond the aid of prisms. When otherwise, and it causes distress, operation is quite justifiable. When we want to produce a small effect, we have the choice of the following methods : (1) Stevens tenotomy. (2) DeWecker's capsular advancement. (3) Knapps' or Savage's tendon shortening. (4) Snellen's tenotomy with a limiting suture. (5) Ordinary advancement without tenotomy of the antagonist. These are described in all the better text-books, so that I need only mention here the convenience of using needles furnished with Hagedorn eyes and threads stained two or three different colors. For considerable advancements, where three sutures are used, the middle one should be attached strictly in the middle part of the tendon and catch in the sclerotic close to the cornea, where it is tied ; Heterophoria 235 first, the side sutures being ready to tie thereupon. The more the tendon is detached from the capsule and from its own conjunctiva, the safer the result. Graduated tenotomy is another name for careful tenotomy. Partial "buttonholing" of the middle of a tendon has no effect unless a moral one, since the tendons are peculiarly inextensible, and even a narrow strand at each margin is enough to make the effect nil. But by dividing the tendon with great delicacy, so as to leave the indirect attachments unimpaired, an extremely small effect can be produced ; and should it not be small enough, a limiting suture can be employed as well. The instruments intro- duced by Stevens for this purpose are admirably adapted for it, except that the hook and forceps are just a little too fine to do the best work, for the same reason that the finest catheter is not the easiest for strictures, being more apt to make a false passage. Marginal tenotomy has been much advocated by Savage with a view to correct at the same time any cyclophoria. After " button- holing " the tendon, as in Stevens' operation, one margin only is snipped with the scissors. The torsion of the eye is much less affected by this procedure in the case of the internal and external recti than in that of the superior and inferior, for obvious reasons. CHAPTER XIII Cyclophoria Let us now consider this subject, at which Prof. Savage has worked so much. Cyclophoria is a tendency for the principal meridians of the retina to fall out of parallelism with each other whenever the eyes are disassociated, so as no longer to be engaged in ordinary binocular vision. By far the commonest form is that in which the principal meridians diverge above (binocular extorsion). By tests made in near vision, Savage found it present in at least twenty-five per cent, of normal eyes. Cyclophoria of this kind has probably no clinical importance, unless it is very great or due to a paresis of an oblique muscle. In the latter case there will be vertical diplopia, either latent or manifest, down and to the opposite side from the paresed superior oblique. In the absence of any such tendency to vertical diplopia in the four corners of the field, as tested by the glass rod, Cyclophoria must not be attributed to the oblique muscles, but to the innervations. Should need require, the slack innervation con- cerned may be strengthened by exercise, but, in nearly all cases, non-paralytic Cyclophoria causes no symptoms and requires no treatment. Its Detection and Clinical Measurement. When a disk of mounted rods is held before one eye of a patient who is engaged in looking at a distant flame, it sometimes happens that while the rods are horizontal the streak of light created by them appears more or less sloping, and to make it vertical the rods have to be tilted from the horizontal. Such a patient has latent torsion or Cyclophoria. If the amount be considerable and the diameters of the disk be truly marked, the degree of torsion can easily be read off from an astigmatic trial frame. Its Exact Measurement. For very accurate work, the rods would be better mounted, exactly horizontal,* in a rigid stand, with a long thin wand pivoted to the wall behind the flame, or a * Optical adjustment is superior to the use of a spirit level. It can be effected by adjust- ing the rods so as to obtain the maximum definition of a thin vertical line on the wail. (236) Cyclophoria 237 cord so fastened by one end as to be adjustable to the vertical, or to any inclination from it. The tilting of the wand or slanting of the cord required to bring either parallel to the streak of light, represents the cyclophoria. Since, for clinical purposes, however, such accuracy is not necessary, simple rotation of the disk of rods in the trial frame till the streak appears parallel with a fixed vertical or horizontal line on the wall is quite sufficient. Cyclophorias are divided into " paretic " and " non-paretic. " ParetiC Cyclophoria. Whenever leaning of an image is ob- served to any marked extent, the possibility of slight paresis of one of the obliques should not be overlooked, and measures should be taken accordingly to discover whether the tilting varies on looking in certain directions ; also whether, on looking up and down, any inconcomitant hyperphoria can be demonstrated by the rods. Non-paretic Cyclophoria. Tilting which is not paretic is due to slackness of one of the conjugate innervations. It will be remembered that there are three or four of such innervations con- nected with torsion one causing parallel dextrotorsion, another parallel laevotorsion, another conjugate intorsion, and yet another (perhaps) conjugate extorsion. Explanation Of Leaning Image. It may be well to explain the relation between the tilting of the streak of light and the torsion of the eye. When a flame is fixed with both eyes without any apparatus, a vertical line inscribed on the wall, passing through the flame, throws its image on the vertical meridian of each retina, these verti- cal meridians being kept parallel to each other by torsional innerva- tion, in order to combine the two pictures into one. As soon, however, as a glass rod is placed before one eye, all active innervation exerted in the interest of single vision ceases and the eye rolls into its position of dissociation. While the glass rod is horizontal the picture formed by it upon the retina remains geometrically vertical and, therefore, now, as soon as torsion occurs, this linear picture falls on a new meridian of the retina which is not the originally-vertical one, and is pro- jected according to the rule that the false image is displaced in the opposite direction to the displacement of the eye. A dextrotorted streak, therefore, means a laevotorted eye, and vice versa. As soon as the rod is tilted so as to make the retinal picture 238 Tests and Studies of the Ocular Muscles fall on the originally-vertical meridian of the eye, the streak appears vertical.* Rule for Rod Test. This very simple rule, therefore, may be made that the torsion of the rod, required to make the streak appear vertical, represents exactly the torsion of the eye, both in sense and amount. Cyclophoria in Near Vision. Dr. Savage, of Nashville, has worked much at the subject of latent torsion in near vision by a test of his own, for which he ^ ^ ^^ ^^^ utilized the author's double prism of Fig. 91. On looking at a card ^ - ^ ^^ marked with a horizontal line, F!g - 10 through the double prism held Dext retorsion of the naked eye, causing appa- t r _ 4.U * rent laevotorsion of the middle linear image Delore One eye, the patient sees two parallel false images of the line and a third real image (Fig. 100) between them, which slopes with respect to the other two in a sense opposite to the torsion of the eye that sees it. Dr. Savage attempted to cure it by exercising the eyes in the opposite direction, using a weak cylindrical lens for the purpose, rotation of which tilts the image of a vertical object seen through it. For exercises in distant vision, Duane's suggestion is a good one to use two disks of glass rods, one for each eye, and gradually rotate them in opposite directions while endeavoring to keep the streak of light from doubling ; thus, with the bi-prism before the left eye, the appearance is generally as in Fig. 100, showing excyclo- phoria. This condition is so common as to deserve being regarded as physiological. For near vision, a steroscope such as suggested by Perry, or Javal's "stereoscope a cinque mouvements," could be used ; or, perhaps best of all, Helmholtz's rotating prisms, which enable an object to appear gradually rotated ; but the author does not think that cyclophoria which appears in near vision only, needs treatment at all. Oblique Astigmatism. When an eye with oblique astigmatism looks at a vertical line, the image of that line on the retina is twisted from the vertical towards the meridian of maximum corneal curvature. The image thrown from a horizontal line is likewise * For physiological experiments I have devised a more delicate test, in which any ten- dency to fusion is more entirely abrogated (Ophthalmic Review, June, 1894), but not be'ing a clinical test, it has no place here. Cyclophoria 239 twisted, also towards the same corneal meridian. To verify these facts let the reader look at a cross line, as in a, Fig. 100^, with the right eye, after having placed before it a strong minus cylinder with the axis down and in. The appearance is as shown in 6, Fig. ioo^, each arm of the cross being twisted toward the axis of the cylinder. A similar cylinder with its axis down and in before the left eye, the right being shut, would give the appearance of ^. It is evident that were no rotation of the globes about the lines of fixation permissible, the effect with both cylinders together a e t riff. 100% would be as in d, Fig. 100^. What really occurs, however, is that both eyes become either extorted or intorted, according as we con- fine our attention to the vertical line or the horizontal one. At e, Fig. 100^, is shown the appearance when the vertical line is attracting most attention, its double images being fused in consequence of binocular extorsion of the eyes, which allows each image to fall on the principal meridian of its retina ; by this very act the angular separation betwen the horizontal images becomes doubled. In /", Fig. 100^, the horizontal line has attracted atten- tion and its double images have been brought together by binocular intorsion, which has doubled the angular separation between the vertical images. 240 Tests and Studies of the Ocular Muscles This constant alternation of the adjustment of the eyes about their fixation lines is, no doubt, what accounts for the greater fre- quency of headaches in oblique astigmatism as compared with other kinds, nor can it be corrected in any other way than by the cor- rection of the astigmatism. Too exclusive attention to the horizontal images, together with the supposition the whole picture of an object is tilted on the retina in the same direction as its horizontal lines, has led some to suppose that the correction of certain kinds of astigmatism throws strain on the superior obliques and that of other kinds on the inferior obliques. But the above simple treatment of the subject shows that the correction of oblique astigmatism relieves both pairs of obliques ; or, to put it more correctly, gives less work to the two innervations which govern binocular extorsion and intorsion. Since Savage called attention to the effect of the correction of oblique astigmatism, the subject has attracted attention. If an oblique cylinder be held before a normal eye, a square figure looks drawn out or shrunken in a direction perpendicular to the axis of the cylinder, according as the latter is convex or concave, so as to illustrate the fact that both vertical and horizontal lines are tilted, against the axis of a convex, and with the axis of a con- cave cylinder, when its axis is oblique. Were this all> every side of the square would appear double or, at least, indistinct ; but the mind prefers to see two sides clearly, even though at the expense of the other two, and this desired end is attained by either conju- gate intorsion or extorsion of the eyes, according to taste. Hori- zontal lines in near vision are preferred generally to be seen dis- tinctly at the expense of the vertical. In an astigmatic eye this state of matters is permanent, and the corrective torsion is a life-habit. When, therefore, oblique cylinders are prescribed, this life-habit has no longer any raison d' etre, and ceases. Whether in selecting the best axis for the cylinder, we do well to take account, as Savage suggests, of the altered torsional conditions, is very questionable. It should be remembered that latent torsion is much more common in near vision than in distant vision, and when it exists in both, is apt to be the greater, being in fact to a certain extent physiological, and, I believe, analogous in its own spheres to the exophoria so generally found in near vision, in the domain of the converging innervation. Cyclophoria 241 Cydophorometers. Several instruments of this name have been invented for the measurement of cyclophoria at a distance rather than in near vision. The first to be published was that of Price, who placed mounted glass rods vertically before both eyes, with, in addition, a double prism, ridge horizontal, before one eye.' Similar instruments followed, with some improvement of detail by Baxter, and Brewer, and others ; some with single prism and others with double, and all excellently planned. When a point of light is looked at through these instruments one eye sees its hori- zontal line of light inclined with respect to the line or lines seen by Fig. 101 Optomyomeler of the Geneva Optical Company the other eye when cyclophoria is present, and the measurement is effected by rotating one disk in a graduated arc till the lines are all parallel. Two more forms of apparatus for latent torsion deserve description. Oplomyometer. The first is the " optomyometer " of the Geneva Optical Company, shown in Fig. 101. It consists essen- tially of two tubes, nearly twenty inches long, one of which is capable of horizontal movement only, while the other can be elevated or depressed to any required angle. The patient is made to look, with both eyes, down these tubes, and sees a thin slit cut in a rotating disk at the far end of each. By a little adjustment of the movable tube its own slit can be made to appear vertically above the other, and then if one slants with reference to the other, the disk is rotated till the slant is corrected. If the patient have any latent torsion, it will lie found that when the slits appear to 242 Tests and Studies of the Ocular Muscles him to have the same direction, they actually are inclined to or from each other to an extent which exactly measures his latent deviation. In a variation of the experiment, one slit can be made Fig. 102 Stevens' Clinoscope to appear to the patient to lie across the other one at right angles to it ; when if a latent torsional deviation be present, the slits will be found to be really inclined to each other by a greater or less angle than 90. Clinoscope. Dr. Stevens has improved on this instrument, in his clino^cope, which consists of two tubes nearly twenty inches long, mounted on a brass platform. The attachment to the platform per- mits the tubes to be adjusted in parallelism, in convergence or in divergence, and the platform itself is attached by a movable joint to an upright standard, so that the tubes can be given any desired dip simultaneously. The tubes can be rotated about their longitu- dinal axes by thumb-screws, and this motion is recorded by an index-pointer above each tube. At the far end of each tube pro- vision is made for maintaining diagrams in position. An example of these figures is shown in Fig. 103, which represents two pins, one to be seen by each eye.* The heads of the pins blend, and by rotating one till the pins form a continuous straight line, the latent torsion is measured. 'Vo'kmann's Apparatus. A design similar to this appears, according to the language of Helniholtz, to have been that employed by Volkmann : " Instead of a whole diameter on his rotarv disks, he only traced one radius, and endeavored on binocular examination to make these radii appear in the same straight line. The head was suitably held: the rotary disks were placed in two darkened tubes which could be directed at will by the aid of s'uitable joints, so that each eye should see one disk through each of the tubes, the disk remaining always perpendicular" to the line of fixation." CyclopJioria 243 To measure the amplitude of torsion, similar disks are used, but with a complete diameter, instead of a radius (Fig. 104), on each. These diameters are fused, and by rotating both in oppo- site directions till they begin to separate the strength of the faculty of fusion is measured. Dr. Stevens finds the amplitude of extorsion for each eye to be 11 and that of intorsion to be slightly less. This holds good when both eyes are simultaneously extorted or simultaneously intorted. He finds, however, that he cannot, while maintaining the vertical position of one of the lines in the clinoscope, rotate the other to an extent double of that to which the two were rotated. With one line vertical he can only slant the other to right or left by about 14, without breaking fusion. Curiously enough, he says that horizontal lines are not held in fusion nearly so easily as vertical ones, the amplitude for each eye being only 3 inwards and 3 outwards. He says that during artificial binocular extorsion or intorsion the united line appears concave or convex, according to the will of the observer. Meissner'S Test, 1858. It is only for convenience that Meiss- ner's test is included in this chapter, since the phenomena of torsion manifested by it are not strictly those of "latent" torsion, 244 Tests and Studies of the Ocular Muscles but of physiological actual torsion during single binocular vision for near objects. This is apt to be confused with latent torsion, to which, indeed, it is closely related. It appears that in ordinary binocular vision of near objects both eyes rotate outwards about their optic axes (binocular extorsion), and the more so the nearer the object becomes. This species of torsion increases when the visual plane is elevated and lessens as it is depressed, till it at last disappears I hi Fig. 1O5 copic Figures slightly inclined (LeConte) altogether, when the fixation lines are depressed 45 below the horizontal ; if depressed more than that, intorsion of both eyes is apt to show itself. Meissner proved these points by taking a metallic thread, stretching it perpendicularly to the plane of fixaticTn and looking at it in such a way as to make the visual lines converge to a point situated a little beyond or a little behind this thread. He found the double images of the thread not parallel but relatively intorted, showing that the eyes are, to the same degree, extorted. By moving the lower end of the thread nearer the observer and the top farther away, so as to introduce an element of perspective (the top of the thread being now farther from the eyes than the bottom) the double images can be made parallel. The amount of the pre- vious intorsion of the images can easily be calculated from the amount of inclination required to be given to the thread to bring the double images to parallelism. Le Conte has carefully confirmed Meissner' s results, differing in one point only, namely, that while the latter believed that a greater inclination must be given to the thread as vision becomes nearer, Le Conte finds, with his own eyes at least, an inclination of 7 or 8 to be that required for all near distances. Cydophoria 245 Depression Of the Visual Plane. Meissner found that the more the visual plane and the thread were simultaneously depressed (the mid-point of the thread being kept at a uniform distance from the eyes all the time), the less the thread had to be displaced from the perpendicular to the visual plane, till, with a depression of 45 it needed no displacement at all. In this position of the eyes, there- fore, the torsion we are considering becomes nil. Helmholtz calls this the "primary position of the eyes for convergence," defining primary position as that of zero torsion (Nullpunkt der Raddrehun- gen), and stating that in convergence the eyes have a lower pri- mary position than when the visual axes are parallel. In his own case he found zero lie one day a little higher and another day a little lower, and even to become altered during a series of experi- ments. It is useless, therefore, to attempt too great a precision in denoting it. True Primary Position in Distant and in Near Vision. In dis- tant vision, Helmholtz defined the primary position for the parallel motions of the eyes as that in departing from which, in any cardinal direction, no false torsion was generated. During convergence he also tested the deviations from Listing's law in the secondary posi- tions of the eyes, and found them to be such as to confirm the idea of the primary position in convergence being one of depressed visual axes. Le Conte'S Confirmation. Le Conte's experiments showed that, with the point of fixation at the following distances from the root of the nose the torsion was shown in this little table : At 7 inches each eye became extorted i^ n 22" " " " " 5 t< !/ ^ " 10 74 On looking up, i. e., with elevation of the visual plane, the extorsion increases, which Le Conte attributes, no doubt truly, to the inferior obliques ; on looking down, as already described, it becomes less. Savage's Test Compared. All this shows that in making Prof. Savage's test with the double prism, note should be taken both of the distance at which the test is made and of the inclina- tion of the head, for though his test is not the same as Meissner's, since the two eyes are thoroughly dissociated (whereas, in Meiss- ner's they are not dissociated at all), there is no doubt the results 246 Tests and Studies of the Ocular Muscles include the phenomena here treated of. His test is doubtless a good one in its place, but its value should be carefully differentiated. For example, when torsional defects are apparent in distant vision to any marked degree, it is of service to also investigate the con- ditions in near vision to see if they present any great departure from what Meissner showed to be physiological. As already confessed that in the clinical study of latent torsion, I do not attach much significance to near vision phenomena, unless coupled with distant vision defects ; but it is perhaps well not to overlook the possibility of rare cases in which Savage's and Meiss- ner' s tests might show great anomalies. Eaton's Apparatus. Perhaps the best rough clinical way to institute Meissner' s test is that suggested by Eaton of a strip of *. toe Eaton's mode of makiug Meissner's Test wood grooved at one end to rest on the root of the nose, with a white metal plate rigidly fixed to the other extremity so as to hang down therefrom at such an angle as to be perpendicular to the visual plane, when a black dot in its middle is fixed by the patient. A long hat pin, with its head downwards, is stuck by its point into the under part of the strip of wood an inch or two from the end and parallel to the metal plate.* This is shown in Fig. 106. To use the apparatus : First depress the whole till the images of the hat pin become parallel ; this discovers the amount of depres- sion which must be imparted to the visual axes to obtain zero torsion. Secondly, by making the patient look straight forward, * A graduated arc might with advantage be arranged to indicate the inclination of the pin, as shown in the figure. Cychphoria 247 the hat pin can be slanted so as to bring its head nearer the patient till parallelism of the double images is obtained the amount of slant showing the amount of extorsion. Lastly, by elevating the appara- tus the increase of the torsion on looking up can be demonstrated. Much more elaborate and accurate apparatus could be devised, but for clinical purposes they might induce us to make much of little. I have made the following simple rule, which affords a sufficiently close approximation for clinical work : Multiply the number of degrees by which Meissner's thread has to be inclined to make the images parallel, by half the interocular distance in centi- meters (generally about 3.2) and divide by the distance of the center of the thread in centimeters ; this gives the torsion of each eye in degrees. Formula for Meissner's Test. The formula I obtained, where / is the inclination of the thread in degrees, T the torsion of each eye and C the angle of convergence for each eye, is : Tan. T = Tan. / Sin. C. Putting arcs as equivalent to tangents, the formula becomes : T= I Sin. C. From which we see : ( i ) That for any fixed distance of the thread the torsion of the eye increases proportionately to the incli- nation of the thread, and (2) that for any given and constant inclination of the thread the torsion varies with the sine of the convergence. Thus, with the thread 50 cm. (about 20 inches) away, the torsion of each eye would be .064 of the inclination of the thread ; at 25 cm. (about 10 inches) it would be twice as much, namely, .128 ; at 20 cm. (about 8 inches) .16 ; at 15 cm. (about 6 inches) .21 ; at 10 cm. (about 4 inches) .32 ; and so on. It only remains to show how to arrive at the formula. How Formula Obtained. Let us, for convenience, call that meridian of each retina which is vertical while both eyes look straightforwards at distant objects, the originally vertical meridian, and that plane which passes through it, as well as through the point of fixation, the originally vertical plane. During distant straightforward vision the originally vertical planes of the two eyes are parallel, since both are vertical, and they intersect each other in a vertical line passing through the point of fixation. Were the eyes to experience no torsion during the act of convergence, this line of common section would still remain vertical for every distance of fixation. But any torsion of the eyes is, of course, accompanied by equal 248 Tests and Studies of the Ocular Muscles rotatations of their originally vertical planes about their axes of fixation, and though the originally vertical planes must still intersect one another in a straight line passing through the point of fixation, that line remains no longer vertical, but has its upper end inclined towards the observer, if the case be one of intorsion, and away from the observer if the case be one of extorsion.* Fig. 107 Fig.lOS Author's plan of solving Meissner-Torsion The greater the rotation of the originally vertical planes, during any constant distance of fixation, the greater is the tilt backwards or forwards of their line of common section. On the other hand, to produce a constant tilt of the line, greater torsion is needed for every increase of convergence. Now, it is only when Meissner's thread is held parallel to this line of common section of the originally vertical planes, that its images appear parallel to one another. Given the distance of fixation, we learn at once from the inclination which we have to give to Meissner's thread what is the rotation of the originally vertical planes, and thus the torsion of the eyes. Fig. 108 gives a horizontal plan of the problem, E P being the left axis of fixation, meeting the median plane (M P) at the point of fixation P, so that C is the angle of convergence for the left eye. The left originally vertical plane passes through the axis of fixation E P and rotates about it in strict association with the torsion of the eyeball, thus intersecting the median plane in a straight line, which ever passes through P, either perpendicularly to the plane of the paper (as when no torsion exists) or with more or less inclination from the perpendicular (according to the torsion). Select in the originally vertical plane an imaginary line running parallel to the axis of fixation, and at any unit-distance from it. So long as the originally vertical plane is vertical this line will lie immediately over the axis of fixation and, in the horizontal plan of our figure, appear to coincide with it. If the eye be extorted, however, the line will occupy some such position * The reader to whom this is not self-evident, may think of the prow of a canoe. Cyclophoria 249 as that shown in plan by S S f , meeting the median plane at 5 (which we may regard as the upper extremity of Meissner's thread, when the middle of the thread is fixed by the eyes at P}. Now, designating the angle of torsion by 7", the ordinary rule of hori- zontal projection gives us R P = Sin. T, and it is evident, from the figures, that R P _ Sin. T , . * *J r-. s~, pr- 7^. ( I ) Sin. C Sin. C But P S equals the horizontal co-ordinate of the upper half of Meiss- ner's thread, shown in side elevation in Fig. 107, as/ S. Whence Since Pp was, by construction, taken as unity, P S = Tan. t. Therefore, from (i) . Tan. T Tan. t = -=-. . Sin. C and Tan. T = Tan. i Sin. C Which means approximately that the torsion of each eye is directly propor- tional to the inclination of the thread, and also to the amount of convergence in true meter angles. True meter angles are found by measuring the dis- tance of the point of fixation from the center of the eye, and finding how many times that distance will go into a meter. Most readers will agree that this subject is a difficult one. CHAPTER XIV The Eye in Darkness The problem now before us * is to discover how an eye behaves when it is covered by the hand, or otherwise placed in total dark- ness, while its fellow is still actively engaged in near vision. We have already seen that, during deep sleep, the eyes gen- erally experience exotropia, as proved by simple inspection. But when awake in darkness it is clear that, except for more pronounced deviations than those which occur physiologically, we cannot solve this difficult problem by direct inspection of the eye or through Javal's ground glass ; neither can after-images afford us any assis- tance, since movements resulting from alterations of the conver- gence innervation are just those which after-images do not betray. Even, therefore, if we were to gaze steadily at a source of light till it became impressed on the retina before darkening the eye with the hand, the eye might then move under the hand without any apparent movement of the after-image. What is needed is an apparatus capable of placing an eye sub- jectively in absolute darkness, and yet able to take account of its movements. To solve this problem by utilizing the blind spot I devised the visual camera in 1882. It need hardly be recalled that the blind spot (or "punctum caecum," discovered by Mariotte) is an approximately circular gap in the field of vision of each eye, which was shown by Donders to be due to the fact that the entire surface of the optic disk is wholly insensible to light. There is an area, therefore, in the field of vision of each eye which is entirely devoid of visual impressions and large enough, according to Helmholtz, for eleven full moons to stand in a row in it. The center of the blind area lies about 15 to the outside of the point of fixation, and its diameter subtends an angle of about 6. The "camera" consists of a light wooden box, represented in Fig. 109, blackened inside, and of a somewhat wedge-like or pyramidal shape, its dimensions being about a foot from side to side and about nine inches from before backwards. It is one inch deep along the curved border and inclines gradually to the depth of * Trans. Oph. Soc., 1882-3, and Jour, of Anat. and Phys., vols. xx and xxi. (250) The Eye in Darkness 251 half an inch at the narrow end. The latter is provided with two visual apertures pierced through slides a a which permit of mutual approximation or the reverse, and between them is a groove for the nose. A fixed median parti- tion b extends to within two inches of the middle of the curved border and is crossed by a small transverse ' ' ob- structive " c, which is merely a little piece of wood suspen- ded through a slit in the roof, in which it can slide from side to side. The curved end of the box is built up of two arcs d d, each described about the Flg- 109 . . ... The Visual t'amer* center of motion of its corres- ponding eye and mited by a straight piece 64 mm. long, in the center of which is a tiny fixed aperture e covered by thin paper, bearing a printed letter so as to ensure accurate accom- modation. On either side of this arc are two movable aperturesyy, preferably colored red and green respectively, and pierced through brass slides ^ s, which travel so that each luminous point can be moved at pleasure along its own half of the curved end inde- pendently of the other. This is made possible by a system of long slits, so cut in the brasswork that the luminous points can be made to travel without admitting any adventitious light, since the slits mutually overlap each other. The brass slides are marked in degrees, which indicate the angular distance of each luminous point from the central aperture. To make our first experiment, adjust the two lateral apertures ff so that each shall be 15 distant from the central aperture e, and place the obstructive in the middle of its path as represented by the shading c. Now look into the camera, holding it so as to let the light fall on the three small apertures fe f but for which its interior is quite dark, and gaze at the printed letter in the central aperture e ; while doing so the two lateral apertures ff will be invisible, since they fall on the blind spot of each eye, and are lost to view. The circles of shading which are represented in the figure surrounding the apertures // correspond to the two blind spots and are at that distance about an inch in diameter. 252 Tests and Studies of the Ocular Muscles Now suddenly push the obstructive to the right, into the posi- tion shown by g in the figure. The mind remains quite unconscious that the vision of the fixation aperture e by the right eye is cut off by this action, though the aperture appears a little dimmer. Now, however, the right eye is subjectively in absolute darkness, since the left luminous point f is hidden by the median partition b, the central fixation aperture e is hidden by the obstructive g and the right lumiuous point /is lost in the blind area.* The fixation aperture osition of the, 200 Corresponding points 103 Cyclophoria 236 Detection of 236 In near vision 238 Measurement of 236 Non-paretic Paretic 237 D Darkness, The eye in 250 Deorsumductors 167 257 Tests and Studies of the Ocular Muscles Dextroduction 49, 56 Dextroductors 167 Dextrotorsion 42 Motion of 45 Deviation secondary 135 Consecutive 149 Direction of 214 Latent 97 Measurement of 143 Reason of 136 Deviometer, Worth's . . 145 De Wecker's capsular advancement 234 Diplopia 104, 157 Breadth of 108 Crossed 108 Distal 104 Homonymous 108 Monocular 158 Peripheral 96 Physiological 104 Post-operative 125 Proximal 104 Suppressed 122 Direction of objects 91 Displacement, horizontal 74 Dissociation of the eye 213 Graefe's device for 213 Distance of objects 91 Donder's Law 36, 40 Plane of reference 41 Dynamics of the eye 70 Dynamometer, Landolt's 154 Eaton's apparatus 216 Elevation angle of fixation 49 Equator of the eye 16 Error of approximation 207 Esophoria 129,215 Exophoria 75, 96, 215 In near, oblique, vision 95 Physiological 227 Speed of 254 Exophthalmos 17 Paralytic 75 Extorsion 56 Faculties, extension of partially pre- served 133 Recovery of lost 131 Field of projection 103 Fixation 44, 101 Axis of 100 Binocular .... 101 Central 100 Central imperfect 123 Defective diagnosis of 124 Fixation Eccentric 124 Field of 124 Line of 44, 100 Lost 123 Persistent 99 Point of 44, 98 Reflex 99 Fovea centralis 200 Anatomical 100 Foveal differentiations 99 Foveal projections 103 Fusion 106 Breadth of 108, 227 Diluted 255 Faculty, deficiency 121 False 265 Horizontal breadth .... 109, 229 Negative 109 Positive 109 Sense, training the 147 Total amplitude of 109 Tubes 131 Vertical 109 Q Giddiness 157 Globe of the eye 15 Translations of the 30 Graefe's view 79 H Helmholtz, analysis of ocular motions . 49 Law ...... 40 Plane of reference 41 Hering's drop test 114 Heterophoria 97, 108, 215 Correction of 228 Decentering rules for 231 Diagnosis of 230 Direction of 232' IB near vision 226 Physiological 214 Simple rules for 230 Treatment of 231 Hirschberg's method . . . 139 Hooke's law . . 22 Hyperophoria, congenital 84 Hysteria 81 Image, leaning, suppression of the false . 122 Tilting of 39 Trembling of the 124 True 107 Images, suppression of 104 Impulse 94 And work . . 95 Index 259 Initial position of fixation plane .... 49 Inuervatious .81 Conjugate 81, 86 Convergence 81, 87, 91 Dextrodueting 86 Horizontal conjugate 86 Laevoducting 86 Parallel 90 Variations 81 Voluntary 81 Intorsion . . 56 Isogonai line 88 K Kataphoria 101 Kinetic energy 37 Kuapp's tendon shortening 234 Laevoduction .......... 49, 56 Laevoductors ............. 137 Laevotorsion .......... 42 Landolt's figure for field of fixation . . 101 I>e Conte's experiment ......... 38 lecture coutrollee ........... 113 Levator palpebne ...... ..... 29 Ligaments, check ........... 20 Extensibility .......... 23 External ..... ........ 27 External superior ........ Inferior ............. 31 Inferior oblique ........ " In teuotomy .......... 26 Internal ............ 28 Lines of force ............. 58 Listing's Law ............. 3e Proof of ............ 35 Reasons for ........... 37 Listing's plane ............ 40 Vertical diameter of ....... 46 Love of single vision ......... 106 Maddoxrod .............. 220 Manufacture of ........ 220 Mode of use ..... ..... 221 Test for .......... 220,226 Mai-projection ....... 502, 106, 156 Mathematical perspective ....... 110 Mauthner's hypothesis ........ 75 Meissner's test ............. 243 Formula for . . ....... 247 Meissner's torsion, plan of solving . . . 248 Meridians of the eye ......... 16 Mnemonics ........... 161, 185 Mobility, comparative ......... I 53 Conjugate ............ 153 Motions, normal ........... 17 Testing horizontal ........ 160 Motorfield ...... ......... 101 Homony mous restriction of ... 187 Medial depression ........... 58 Elevation ............ 58 Muscular axes ............ 58 Functions ............ 53 Muscular anomalies, photography of . . 208 Coue ............... 19 Planes .............. 60 Planes, properties of ....... 60 Muscles ...... ..... 17, 52, 68 Associate ........... 68 Cardinal groups of ........ 160 Medial origins of ........ 57 Oblique ............ 53 Recti .............. 52 Single spasm of ......... 80 Motion, monocular ........... 87 Movements of the eye ....... 15, 48 Of the parallel .......... 37 N Nystagmus ............. 192 Curious case of ......... 196 Etiology of ........... 194 Examination of ......... 194 Excursions in ...... ... .195 Motions ............. 193 Rotation ...... ..... 82 Treatment of ......... 196 o Ocular movements ........... Object of ........ .... Precision of the ........ Silence of the ....... Swiftness of the ......... Ocular motions ............ Laws of ........... Ocular muscles . ........ Secondary effect of ........ Occlusion . . ...... Ophthalmotropes ........... Anderson Stuart's ........ Landolt's ........ .... Optic foramen ............ Optomyometer ............ Orbit of the eye ............ Axisot ............. Capacity of .......... Outlet of ............ Shape of ............. Orthophoria ........... Orthoptic training ........ 137, 63 66 63 18 241 18 18 230 Parallax Crossed 260 Tests and Studies of the Ocular Muscles Parallax Homonymous 216 Left 216 Right 216 Test of Duane 216 Paralysis 160 Isolated 74 Left external rectus 170 Left inferior oblique 179 Left inferior rectus .... 176 Left internal rectus 172 Left superior oblique 177 Left superior rectus 174 Of third nerve 180 Operative treatment in 80 Right external rectus 171 Right inferior oblique 178 Right inferior rectus 175 Right superior rectus 173 Right superior oblique 176 Single . 61 Paralysis conjugate 88 Precise tests for 189 Rough tests for 188 Paralysis ocular 151 Examination of 153 Measurement of 182 Symplons of 151 Paralytic equilibrium 149 Semi-orbits 77 Pathetic nerve . . 181 Percival's experiment 89 Phorometer 217 Prince's 217 Risley's 217 Stevens' 218 Perception of distance . 102 Binocular 110 Monocular ... 109 Of relief Ill Pole, anterior . . 16 Posterior 16 Primary position 35, 39 Priestley Smith's tape method 144 Prism diopters 232 In near vision 227 Perimeter method 139 Perimeter method, Scluveigger's . 155 Prism test ... 216 Deviating angle of 107 Prism, deorsumducent 228 Sursumducent 228 Prisms, power of overcoming 107 Prismatic displacement, apparent ... 107 Projection 102 Field of 102 Illustrative errors of 105 Line of direction 102 Origin of . . 105 True test for . . . 105 Pupillary light-reflex 121 Pupillary reflection 32 Pulley of superior oblique 54 Recorded reflections 212 Recti muscles 52 Description of 52 Insertion of 53 Relative strength of 53 Refraction surmisable 202 Rente's experiment 37 Retinal horizon 41 Retractor muscle 17 Rod-test, rule for 238 Rotation 34 Amount of 70 Axis of 60, 70 Center of 34 Composition of 69 Paralytic 78 Resolution of 7.S Sense of . .70 s Savage's tendon, shortening 23 Test 245 Schurrman's figures 101 Semiluuar membrane 29 Sphenoidal fissure IS Snellen's test, Worth's modification of . 145 Squint 133 Apparent 137 Camera 20!) Convergent, over correction of . . 128 Convergent, under correction of . 138 Divergent, over correction of ... 128 Divergent, under correction of . . 128 Evidences of 13;i Exclusion test for . . 133 Fixed convergent 130 Infantile 146 Latent 134 Manifest 134 Squinting eyes, deficient adduction sec- tion of the right eye 72 Spasm lateral 881 Idiopathic 80 Steadiness of the eyeball 15 Stereoscope, Berry's .' 113 Magnetic 132 Wheatstone's 131 Stereoscopic bar reader and squint . . 129 Stereoscopic vision 109 Stereoscopic experiments . 112 Screen test, subjective 135 Strabismometer, Lawrence's 138 Index 261 Strabismometry, Priestley Smith's mode of 206 Linear 103 Screen test, subjective 216 Screen test, objective 216 Subjective 148 Strabismus 115 Accommodative 118 After treatment for 128 Alternating 115 Anisometropia 115 Comitant 115 Convergens concomitans . . . 116 Couvergeus myopicus 125 Convergent without hypermetro- pia 119 Definition of 115 Deviation of 115 Divergent 127 Horizontal 115 Incomitaut 115 In myopia 127 Non-paralytic 115 *aralytic 115 Periodic 214 Unilateral 115 Vertical 115 Tangent scales 223 Advantages of 224 Hirschberg's 224 Landolt's 224 Tenon's capsule 18, 149 Fascia 18 Tenon's space 18 Tenotoiny . . 80 Double prismatic 229 For convergent squint ..'.... 147 Graduated 235 Marginal 235 Stevens' 234 M'cllen's 234 i'rial frame 228 Torsion .... 35 Calculator 42 False 42 Geometry of 46 Index of 42 Paretic 46 Secondary 36 Touching point 56 Trcipnmeter. Stevens' 190 Tunica ad veiititia ocua . . 25 Vasa uorticosa is Vision direct 98 Indirect 98 Visual camera . . 16, 251 As a test tor oiojection 254 Visual piane 41 w White spot 89 Worth's amblyoscope M7 Four-dot test ... 145 Tubes . ...... 137 Clinics In Optometry CLINICS IN OPTOMETRY BY c. H. BROWN, M. D. Graduate University of Pennsylvania; Professor of Optics and Refraction; formerly Physician in Philadelphia Hospital; Member of Philadelphia County, Pennsylvania State and American Medical Societies "Clinics in Optometry" is a unique work in the field of practical refraction and fills a want that has been seriously felt both by oculists and optometrists. The book is a compilation of optometric clinics, each clinic being complete in itself. Together they cover all manner of refractive eye defects, from the simplest to the most complicated, givingin minutest detail the proper procedure to fol- low in the diagnosis, treatment and correction of all such defects. No case can come before you that you cannot find a similar case thoroughly explained in all its phases in this useful volume, making mistakes or oversights impossible and assuring correct and successful treatment. The author's experience in teaching the science of refraction to thousands of pupils peculiarly equipped him for compiling these clinics, all of which are actual cases of refractive error that came before him in his practice as an oculist. A copious index makes reference to any particular case, test or method, the work of a moment. Sent postpaid on receipt of $2.50 Published by THE KEYSTONE PUBLISHING CO. PHILADELPHIA, U. S. A. THE OPTICIAN'S MANUAL VOL. I. By C. H. Brown, M. D. Graduate University of Pennsylvania: Professor of Optics and Refraction; formerly Physician in Philadelphia Hospital; Member of Philadelphia County, Pennsylvania State and American Medical Societies. The Optician's Manual, Vol. I, was the most popular and useful work on practical refraction ever written, and has been the entire optical education of many hundred successful refractionists. The knowledge it contains was more ef- fective in building up the optical profes- sion than any other educational factor. It is, in fact, the foundation structure of all optical knowledge as the titles of its ten chapters show: Chapter I. Introductory Remarks. Chapter II. The Eye Anatomically. Chapter III. The Eye Optically; or, The Physiology of Vision. Chapter IV. Optics. Chapter V. Lenses. Chapter VI. Numbering of Lenses. Chapter VII. The Use and Value of Glasses. Chapter VIII. Outfit Required. Chapter IX. Method of Examination. Chapter X. Presbyopia. The Optician's Manual, Vol. I, was the first important treatise published on eye refraction and spectacle fitting. It is the recognized standard text-book on practical refraction, being used as such in all schools of Optics. A study of it is essential to an intelligent appreciation of its companion treatise, The Optician's Manual, Vol. II, described on the opposite page. A comprehensive index adds much to its usefulness to both student and practitioner. Bound in Cloth 422 pages colored plates and illustrations. Sent postpaid on receipt of $2.50 Published by THE KEYSTONE PUBLISHING CO. PHILADELPHIA, U. S. A. THE OPTICIAN'S MANUAL VOL. II. By C. H. Brown, M. D. Graduate University of Pennsylvania; Professor of Optics and Refraction; formerly Physician in Philadelphia Hospital; Member of Philadelphia County, Pennsylvania State and American Medical Societies. OPTICIAN 5 ' MANUAL The Optician's Manual, Vol. II., is a direct continuation of The Optician's Manual, Vol. I., being a much more advanced and comprehensive treatise. It covers in minutest detail the four great subdivisions of practical eye re- fraction, viz: Myopia. Hypermetropia. Astigmatism. Muscular Anomalies. It contains the most authoritative and complete re- searches up to date on these subjects, treated by the master hand of an eminent oculist and optical teacher. It is thor- oughly practical, explicit in statement and accurate as to fact. All refractive errors and complications are clearly explained, and the methods of correction thoroughly elucidated. This book fills the last great want in higher refractive optics, and the knowledge contained in it marks the standard of professionalism. Bound in Cloth 408 pages with illustrations. Sent postpaid on receipt of $2.50 Published by THE KEYSTONE PUBLISHING CO. PHILADELPHIA, U. S. A. THE PRINCIPLES OF REFRACTION in the Human Eye, Based on the Laws of Conjugate Foci BY SWAN M. BURNETT, M. D., PH. D. Professor of Ophthalmology and Otology in the Georgetown University Medical School ; Director of the Eye and Ear Clinic, Central Dispensary and Emergency Hospital ; Ophthalmologist to the Children's Hospital and to Providence Hospital, etc., Washington, L>. C. In this treatise the student is given a condensed but thor- ough grounding in the principles of refraction according to a method which is both easy and fundamental. The few laws governing the conjugate foci lie at the basis of whatever pertains to the relations of the object and its image. To bring all the phenomena manifest in the refraction of the human eye consecutively under a common explanation by these simple laws is, we believe, here undertaken for the first time. The comprehension of much which has hitherto seemed difficult to the average student has thus been rendered much easier. This is especially true of the theory of Skiascopy, which is here eluci- dated in a manner much more simple and direct than by any method hitherto offered. The authorship is sufficient assurance of the thoroughness of the work. Dr. Burnett is recognized as one of the greatest authorities on eye refraction, and this treatise may be described as the crystallization of his life-work in this field. The text is elucidated by 24 original diagrams, which were executed by Chas. F. Prentice, M. E. , whose pre-eminence in mathematical optics is recognized by all ophthalmologists. Bound in Silk Cloth. Sent postpaid to any part of the world on receipt of price, $1.50 Published by THE KEYSTONE PUBLISHING CO. PHILADELPHIA, U. S. A. PHYSIOLOGIC OPTICS Ocular Dioptrics Functions of the Retina Ocular Movements and Binocular Vision By Dr. M. Tscherning Director of the Laboratory of Ophthalmology at the Sorbonne, Paris AUTHORIZED TRANSLATION By Carl Weiland, M. D. Former Chief of Clinic in ths Eye Department of the Jefferson College Hospital. Philadelphia, Pa. This book is recognized in the scientific and medical world as the one complete and authoritative treatise on physiologic optics. Its distinguished author is admittedly the greatest authority on this subject, and his book embodies not only his own researches, but those of the several hundred investigators who, in the past hundred years, made the eye their specialty and life study. Tscherning has sifted the gold of all optical research from the dross, and his book, as now published in English, with many additions, is the most valuable mine of reliable optical knowledge within reach of ophthalmologists. It contains 380 pages and 212 illustrations, and its reference list comprises the entire galaxy of scientists who have made the century famous in the world of optics. The chapters on Ophthalmometry, Ophthalmoscopy, Ac- commodation, Astigmatism, Aberration and Entoptic Phenom- ena, etc. in fact, the entire book contains so much that is new, practical and necessary that no refractionist can afford to be without it. Bound in Cloth. 380 Pages, 212 Illustrations. Price $3.00 Published by THE KEYSTONE PUBLISHING CO. PHILADELPHIA, U. S. A. OPHTHALMIC LENSES Dioptric Formulae for Combined Cylindrical Lenses, The Prism-Dioptry and Other Original Papers By Charles F. Prentice, M.E. A new and revised edition of all the original papers of this noted author, combined in one volume. In this revised form, with the ad- dition of recent research, these standard papers are of increased value. Combined in one volume, they are the greatest compilation on the subject of lenses extant. This book of over 200 pages contains the following papers: Ophthalmic Lenses. Dioptric Formula? for Combined Cylindrical Lenses. The Prism-Dioptry. A Metric System of Numbering and Measuring; Prisms. The Relation of the Prism-Dioptry to the Meter Angle. The Relation of the Prism-Dioptry to the Lens-Dioptry. The Perfected Prismometer. The Prismometric Scale. On the Practical Execution of Ophthalmic Prescriptions in- volving" Prisms. A Problem in Cemented Bi-Focal Lenses, Solved by the Prism- Dioptry. Why Strong Contra-Generic Lenses of Equal Power Fail to Neutralize Each Other. The Advantages of the Sphero-Toric Lens. The Iris, as Diaphragm and Photostat. The Typoscope. The Correction of Depleted Dynamic Refraction (Presbyopia). PRESS NOTICES ON THE ORIGINAL EDITION: Ophthalmic Lenses. "The work stands alone, in its present form, a compendium of the various laws of physics relative to this subject that are so difficult of access in scattered treat- ises." Neie England Medical Gazette. "It is the most complete and best il- lustrated book on this special subject ever published." Horological Review, New York. "Of all the simple treatises on the properties of lenses that we have seen, this is incomparably the best. . . . The teacher of the average medical stu- dent will hail this little work as a great boon." Archives of Ophthalmology, ed- itnl by H. Knapp, J/.Z). Dioptric Formula? for Combined Cylindrical Lenses. "This little brochure solves the prob- lem of combined cylinders in all its as- pects, -and in a manner simple enough for the comprehension of the average student of ophthalmology. The author is to be congratulated upon the success that has crowned his labors, for nowhere is there to be found so simple and yet so complete an explanation as is contained in these pages." Archives of Ophthalmology, ed- ited ly H. Knapp, M.D. "This exhaustive work of Mr. Prentice is a solution of one of the most difficult problems in ophthalmological optics. Thanks are due to Mr. Prentice for the excellent manner in which he has eluci- dated i subject which has not hitherto been satisfactorily explained." The Oph- thalmic Review, London. The book contains 1 10 Original Diagrams. Bound in cloth. Price $2.00 Published by THE KEYSTONE PUBLISHING CO. PHILADELPHIA, U. S. A. OPTOMETRIC RECORD BOOK A record-book, wherein to record optometric examina- tions, is an indispensable adjunct of an optician's outfit. The Keystone Optometric Record-book was specially pre- pared for this purpose. It excels all others in being not only a record-book, but an invaluable guide in examination. The book contains two hundred record forms with printed headings, suggesting, in the proper order, the course of ex- amination that should be pursued to obtain most accurate re- sults. Each book has an index, which enables the optician to refer instantly to the case of any particular patient. The Keystone Record-book diminishes the time and labor required for examinations, obviates possible oversights from carelessness, and assures a systematic and thorough ex- amination of the eye, as well as furnishes a permanent record of all examinations. Sent postpaid on receipt of $2.00 Published by THE KEYSTONE PUBLISHING CO. PHILADELPHIA, U. S. A. New and Enlarged Edition of STATE BOARD EXAMINATIONS QUESTIONS and ANSWERS Compiled by C. Henry Brown, M. D. Author of "Clinics in Optometry," Etc. COMPILED especially for the student who wishes to quiz himself before taking the State Board Examination or the practicing Optometrist who desires to "brush up" on the subjects of his profession and find his "weak spots." Twice the size of the old edition. This new book contains 1000 questions asked in State Board Examinations, together with concise answers. Eleven chapters covering practi- cally every question asked by State Boards. Bound in Silk Cloth Price $3.00 Place your order at once Published by THE KEYSTONE PUBLISHING CO. PHILADELPHIA, U. S. A. UNIVERSITY OF CALIFORNIA LIBRARY Los Angeles is DUE on the last date stamped below. AUG4 BKMS>:jULl7'73 ~: LIB. AUG4 BbW * 72 REC'D Form L9-Series 4939 A 000414559 5