Tests and studies of the ocular muscles

Front matter

F( s and Studies

OF THE

OCULAR MUSCLES

Preface

The endeavour of this little work has been to cull, from the many tests for the ocular muscles scattered throughout ophthalmic literature, a selection of the best, to be interspersed with a few original ones, and with several studies of the ocular movements.

Since, moreover, the actions of the muscles are so considerably modified bv the influence of Tenon's Fascia, a simple account of that membrane, taken chiefly from Motais, contributes to completeness.

Chapters VI I to XII can be read as an independent section by those whose time is not available for the more Dhvsiolooqcai chapters, none of which, however, are without some ultimate practical bearing.

vi PREFACE

To Dr. Percival special thanks are due tor revision of the proofs and valuable criticisms thereon ; also to Mr. Berry for many a helpful thought in years gone by.

The subject is not without its interest to physicians and neurologists, even when treated from an ophthalmic surgeon's point of view, and that the book may be of practical service to many who meet with muscle problems in their daily work, is the hearty wish of its writer.

Contents

CH AFTER I.

PAGE

The Globe and its Socket - - i

Chapter I. THE GLOBE AXD 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 eye-ball 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 common 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 forefingers 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 visual camera (Chap, xiv), which detects the movements of an eye placed in the dark.

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, observed

THE GLOBE AND ITS SOCKET.

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 eve, 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.

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 the 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 eye-ball, and the three characters of its motions as described in the commencement of this chapter, we need not wonder that astronomical language has been so freely drawn upon for their description.

Thus we speak of the globe moving in its orbit,

THE OCULAR MUSCLES.

(metaphorically like a planet^ ; and distinguish its axis, poles, meridian, and equator.

Fig. i.

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.

Fie. 2.

The equator is a circle or belt of the globe mid-way between the two poles

THE GLOBE AXD ITS SOCKET.

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 eye-ball to be spherical, though it is not strictlv so, but flattened from before backwards, so as to form a regular u oblate spheroid," interrupted by the prominence of the cornea in" front, which has a stronger curvature.

Ocular Muscles. — Each eye-ball receives the insertions 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 entrance 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 wall of the orbital outlet. Their anatomy is described on p. 68.

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 realised till

G THE OCULAR MUSCLES.

one of them is disabled from any cause, and we see double.

Normal Motions. — The ordinary movements of the eye are limited to rotations of various kinds, no appreciable translation being possible, so that the eye-ball has only three, instead of six, " degrees of freedom." We shall see that in perfect physiological conditions it loses even one of these three. 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 at least, to spasm and relaxation of the unstriped " orbital muscle " of Miiller, which spans the spheno-maxillary fissure ; in part to the vascular conditions behind the globe ; 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. 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

THE GLOBE AXD ITS SOCKET.

well described in the text-books to need repetition here.

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 centres of the orbital outlets) diverge from each other by from 240 to 302.

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 bv the incurving of its upper and outer margins. Its outer margin is considerably posterior to its inner, being about 22 mm. behind the root of the nose, 12 mm. behind the anterior ridge of the lachrymal sac, 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 pierce the centre of an average eye-ball. But great differences exist.

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 eye-ball comes nowhere in contact with it, and it constitutes something in the nature of 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 Capsule. — All the structures contained within the orbit are invested by sheaths derived from one and tJie same aponeurosis. The cornea is not of course strictly within the orbit. If it were, it would form an exception to this statement, since it is not thus invested, though all the rest of the eye-ball is encapsuled.

This "common aponeurosis of the orbit ' is in parts exquisitely differentiated to suit the requirements of the ocular motions, and that part of it which invests the sclerotic forms the outer layer of Tenon's capsule, or in other words, the external

THE GLOBE AND ITS SOCKET.

capsule of the eye. It is lined by a delicate internal capsule, which is of a different nature, being practically the serous membrane of the eye-ball.

The common aponeurosis of the orbit extends from one structure 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 hinder part of the orbit is occupied by a group of muscles which spring from the circumference of the optic foramen, and separate as they proceed so as to form a kind of cone of muscles.

Accompanying this muscular cone, there is an

Fig. 3. — Diagrammatic representation of Tenon's Fascia.

aponeurotic 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 in Fig. 3) a kind of cone of fascia within a cone of bone, with this difference between them, that while the bony cone of the orbit contracts at its brim, the fascial cone within it 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.

The fascial cone lodges the eye-ball in front, and the retro-bulbar fat behind. It is divided into two compartments — an anterior one for the eye-ball, and a posterior one for the retro-bulbar fat, by a hemispherical aponeurotic septum (P.E.C. in Fig. 4), which adapts itself to the posterior hemisphere of the eye-ball. This septum is given off from the fascial cone just opposite the equator of the globe all round, 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.

THE GLOBE AND ITS SOCKET.

These two eye -investing membranes, when regarded as forming one capsule, are sometimes known as the outer layer of Tenon's capsule. It sends prolongations backwards in the form of sheaths for the optic nerve, and for the various

Int. C.L~<

Int.R..

L.

Ext. R.

Fig. 4. — Tenon's Fascia (Trom Motais). Int C.L. and Ext. C.L. — Internal and External Check Ligaments. Int. R. and Ext. R. — Internal and External Recti. A.E.C. and P.E.C. — Anterior and Posterior External Capsule. I. C.L. — Intra-capsular Ligament, or "Collarette." I.C. — Internal Capsule. D. — Deep Layer of Muscular Sheath.

vessels and nerves which enter the eye, and is in principle nothing more than a part of the common aponeurosis of the orbit, though highly honoured by the importance of the organ it invests, and endowed with peculiar extensibility and elasticity. The sheaths of the recti muscles, which we have hitherto looked upon as forming part of the fascial cone, might with equal propriety be regarded as being investments derived from this capsule, similar to that which it sends along the optic nerve, provided it be remembered that the sheaths of the muscles are connected with each other by more or less imperfect membranes which in turn give off septa for the lobules of orbital fat.

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 deep 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 (Z?) becomes continuous at the equator of the eye (f.C.L.) with the posterior hemisphere 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 the sheath we find the case is not so simple. As it approaches the neighbourhood 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. B.C.), and proceeds in the form of a thick band {Ext C.L.) to the orbital margin.

Cheek 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 round 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 fibres in them close to their orbital attachment.

Anteriorly they are, through the periosteum, rigidly fixed to the orbit : by their posterior extremities they are attached to —

(#,) The outer layer of the sheath of the muscle, which it will be remembered is part of the fascial cone described first of all ;

(/;,) 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

THE OCULAR MUSCLES.

investment of the globe (P.E.C.) by means of a crescentic thickening of the deep layer of the muscular sheath just where it is reflected (I.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 of 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 exerting injurious pressure on the globe during its contraction.

Uses of the Cheek Ligaments. — Besides this action, it is evident that the check ligaments, by acting on

Fig. 5. — {Motais.) The Check Ligaments during the primary position of the eye.

the posterior hemisphere of Tenon's capsule, help to draw the eye forward (like, e.g., the strings of

THE GLOBE AND ITS SOCKET.

a night-cap) against the backward traction of the recti, and in this they are aided by the entire

Fig. 6. — {Motais.) The Check Ligaments during partial contraction of the Ext. Rectus muscle, the Int. Check Ligament being in a state of maximum relaxation, and the Ext. one being somewhat stretched.

anterior portion of the fascial cone of which they are only thickenings.

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.

By their direct attachments to the recti, too, they moderate the power of backward traction, even with respect to the muscular tone in the absence of voluntarv contraction. When a muscle contracts 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 in strict proportion to their extension. (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* 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 movements 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 increases proportionately) there is a

* Motais : " Anatomie de l'appareil moteur de l'ceil." Graefe und Saemisch, Band I., p 59.

THE GLOBE AND ITS SOCKET.

constant acceleration of speed. Were such the case with the eve, its motion would be far more rapid at the end than at the beginning of each movement. Owing to the provision made for this,

A.E.C.

L XLLIXJL.

' ' J C

Fig. 8. — A horizontal section of the Globe and its Membranes (after Motais). E.C.L. and I C.L. — External and Internal Check Ligaments. A.E.C. and P.E.C. — Anterior and Posterior portions of the External Capsule. /./'., I.C.. I.C. — Reflection of Internal Capsule on the tendon. S.S. — Serous Bursa.

I think that (if there be any difference) 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 contraction almost all the force acts on the globe, through the inextensible limb of the Y, namely, the tendon, since the extensible one (the 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 continues, 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 not so much by impediments acting against the eye-ball 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 be required to produce a maximum excursion of the eye, say of 450 or 5<o=.

The excursion does not cease because the eve-ball itself is incapable 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 eye - ball * (Merkel), and the second by the fact that the contraction of the rectus required for a maximum physiological excursion of the eye is scarcely more than a quarter of its length, while it is known that striped muscles are generally in their maximum contraction shortened by half their length. Motais pointed this out.

All the muscles of the globe seem to be provided with something answering to check ligaments. The following account of them is taken chiefly from Motais.

* Graefe und Saemisch, Band I., 1874

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, greyish-white band, which leaves the external rectus 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 millimetres ; its length from the farthest back point of its adherence to the muscle is from 18 to 20 millimetres. Its greatest thickness, which varies between 3 and 6 millimetres, 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. Sappey 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 millimetres 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 millimetres. The muscle then changes its direction to incline inwards towards its sclerotic insertion. 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 mm., and a depth of 3 to 6 mm. 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.I. in Fig. 4, and I.C.L. in Fig. 8, and is broader, but thinner, than the external check ligament It has no interstices like the latter. Its colour is a yellowish gray, and near the orbital margin a pale red.

Though the prominence which forms it is much less differentiated from the neighbouring 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 millimetres. Its length from the posterior extremity of its attachment to the muscle to its bony insertion is from 15 to 18 mm. Its thickness is from 1 to ii millimetres, near to its bony insertion. 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 ligament, so that when the right internal rectus

* Panas says that between this check ligament and the corresponding palpebral ligament, and the sub-conjunctival fascia, there exists a space containing fat, and the small accessory lachrymal gland, which, he says, we constantly discover in the operation of canthoplasty.

THE OCULAR MUSCLES.

contract?, 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,

L.P.

hit. R.

Ext. R

S.R.

Inf. R.

Fig, 9. — [After Motais.) View from above, the Levator Palpebral (L.P.) being reflected forwards, to show how the fascia is reflected from its under surface, at R, on to the upper surface of the Superior Rectus (S.K.) Compare K in Fig. 10. E.S. and /.6\— External and Internal Superior Check Ligaments.

Superior Check Ligaments.— These are two. Owing to the broad tendon of the levator palpebral 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 palpebral tendon. Were there a single median check ligament, it would have to pierce this intervening tendon to reach the bone (see Fig 9).

That this affords an explanation for there being two superior check ligaments is shown by the fact, noted by Motais, that those vertebrata which possess a levator palpebral have these ligaments double, whereas those which possess no levator have a single median ligament.

He describes the internal superior check ligament (I. S.) as a fibrous cord which leaves the inner border of the superior rectus muscle, applies itself to the tendonsheath 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 (F.S.) is a more flattened band than the preceding cord, and divides into two, one 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 palpebral

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 sheath, for a length of 5 or 6 millimetres (see I.L. in Fig. 10).

It proceeds to the middle part of the inferior oblique muscle (I.O.), splitting into two so as to embrace it, as shown in the figure, establishing thus a strong connection between these two muscles.

C. P. R.

I.R

F;z to — (Arter ftlo'a'-s) Vertical longitudinal section of Eyeball and adnexa. A'. — Reflexion of fascia from the under surface of the Levator Palpebrae on to the upper surface of the Superior Rectus. C. — Part of the funnel-shaped expansion pioceecling to the margin of the orbit. P. — Expansion to contribute to the anterior part of the capsule. I.R. — Inferior Rectus. I.O. — Inferior Oblique. J.L. — Check Ligament of the Inferior Rectus, embracing the Inferior Oblique.

THE GLOBE AND ITS SOCKET.

Its appearance is whiter, and its structure more exclusively fibrous than that of the other check ligaments, so that, with a moderate thickness, it is as resistant as the external check ligament itself (Motais). It obtains no direct insertion into the margin of the

I.O.

L.O.

L.I int. R.?

Inf. R. S'R

Fig: ii. — \Motais.) View of the under surface of the Eyeball, the floor of the orbit being removed to show the Check Ligament L.O. of the Inferior Oblique muscle [I.O.) These form two limbs of a Y, the stem of which is the Check Ligament L.I.R.) of the Inferior Rectus [Inf. R. . seen to embrace the Inferior Oblique. Int. R. — Internal Rectus. S.R. — Superior Rectus.

orbit, but only through a loop of which one limb is formed by the inferior oblique muscle and the other limb by the check ligament of that same muscle. This is shown in Fig. n. For this reason it is that Fig. 10 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, 10 and n. From the point where it does so springs the structure we have next to consider.

The Check Ligament of the Inferior Oblique. — This fibrous bundle (Z. 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 millimetres from its orbital origin, and from thence courses obliquely outwards and forwards. It forms an obtuse angle of about 1200 with the check ligament of the inferior rectus muscle. With the inferior oblique it forms an angle of about no°. Its length is from 10 to 12 millimetres, and it is inserted into the lower outer angle of the orbit, 4 or 5 millimetres behind the orbital margin, about midway between the external check ligament and the origin of the inferior oblique.