Typical accommodative squint occurs quite independently of the will on each effort of the accommodation, and is not combined with diplopia. It is otherwise in those cases of hypermetropia of high degree in which patients voluntarily call forth convergent squint, and retain it for a short time for the purpose of distinct vision. They are then perfectly conscious of the squint, and perceive also as a rule the double images which occur at the same time; I have seen such cases in adults who could only produce the accommodation necessary for distinct vision by the aid of a too strong convergence; they, however, only now and then made use of this help. Although differing much from the typical form, these cases of voluntary accommodative squint were also included in the statistics.

In involuntary periodic (even if not accommodative) squint, the patient as a rule is not conscious of the occurrence of the false position; that exceptions to this occur Case 1 has given us an instance.

CONVERGENT SQUINT IN MYOPIA.

For the aetiology of convergent strabismus it is of interest to ascertain the age at which it is developed, and one of the first results we obtain is the exceptional position which the union of myopia with convergent strabismus takes in this category. Of the 56 cases contained in the above statistics I possess reliable information of the time of commencement in 11 cases; the squint was twice observed before the fourth year of life, once between four and ten years of age, eight times between the tenth and thirty-third years of life.

I must first state prominently with regard to the connection of myopia with convergent squint that I see no reason for holding short sight to be the cause of the squint, as v. Graefe does.

A specially severe strain of the eyes, as v. Graefe a.s.sumes, was not traceable in the cases observed by me. Excessive convergence and strain on the accommodation is often enough present in weak sight, for example, in astigmatism without the existence of squint; were short sight in general an inducement to convergent squint these cases would appear much oftener than they actually do, owing to the frequency of myopia. In my opinion the cause of their rarity lies in the fact that myopia is frequently combined with insufficiency of the interni and preponderance of the externi, but only rarely with the reverse condition of the muscles. If, however, a preponderance of the interni develops itself together with the myopia, convergent strabismus is easily produced, for without correction of the myopia by spectacles, the desire for retaining binocular single vision for everything beyond the far point is lessened by the indistinctness of the retinal images. Within the range of their field of distinct vision these squinting myopes frequently retain binocular vision, while the capacity for accepting parallel rays or retaining them for long, is lost.

Strictly speaking, the periodic squint present in these cases is of a peculiar kind, for the binocular single vision present within range of the convergence excludes the notion of squint; the latter only occurs when an object lying outside the point of convergence is fixed.

Moreover, according to the common use of language, I have only used the expression periodic convergent squint for the change between a parallel direction of the visual axes and pathological convergence.

As squint in myopia usually commences at an age when binocular fusion has already become a fixed habit, diplopia regularly takes place with it, but patients become more easily accustomed to this than in paralysis of the ocular muscles, because the retinal images are indistinct and the double images in the field of vision always keep at about the same distance, while in paralysis of the ocular muscles the distance is constantly changing.

The myopia, in these cases, is not the cause of the squint, but only a favouring circ.u.mstance. If the same preponderance of the interni is developed at the same age in emmetropia, squint is not so easily caused, as the distinct retinal images present in the whole field of vision render it easy to retain binocular single vision. Therefore we see the same form of squint arise less often in emmetropia (see Case 45) when childhood is past, than in myopia. As a rule preponderance of the interni in hypermetropia leads eventually to convergent squint even in childhood.

In emmetropia and hypermetropia convergent strabismus seldom arises after the tenth year (paresis of the abducens of course excepted), therefore in my investigations as to the time of commencement of typical squint I have only considered those patients who came under my treatment before their tenth year. We must rely for the most part on the vague statements of the parents, which lose in exactness in proportion as the origin of the squint is of distant date; moreover, I have myself seen a great many of the children before they were four years old. In this way I have collected reliable information respecting the origin of the squint in 193 cases, and of these (_a_) 88 cases occurred in children one to three years old, (_b_) 53 in children three to four years old, (_c_) 35 cases in children of over four years of age. It is thus at once seen that in the great majority of cases, convergent strabismus commences in children under four years of age, who have not yet begun to read and write, and have no inducement to use their accommodation severely, and still less continuously.

SQUINT FROM PARALYSIS OF THE ABDUCENS.

Convergent squint as a result of paralysis of the abducens is not very often seen. It is first to be observed that a convergent squint, including the whole field of vision, occurs by no means in all cases; in about half the cases binocular fusion is retained towards the healthy side, diplopia then only occurs when the weak abducens is exerted beyond its strength. In those cases in which convergent squint is present in the whole field of vision paralysis of the abducens cannot be the sole cause, but some other cause than the most apparent one must co-operate.

An insufficiency of the externi of previous existence, or an elastic preponderence of the interni may be considered. I have not been able to persuade myself of the fact that hypermetropia can play any part therein.

In by far the greater number of cases paralytic convergent squint recovers together with the paralysis of the abducens, the field of single vision transfers itself gradually from the healthy side to the side of the weak abducens, and at length governs the whole field of vision. In proportion as the muscle again fulfils its normal functions, the habit of binocular fixation regains its power, and it seldom happens that the elastic tension of the muscles has so changed during paralysis that the desire for binocular single vision does not suffice to overcome it. Case 48 furnishes an example of the fact that although the squint occurred as a consequence of paralysis of the abducens, it certainly remained in existence after healing of the paralysis on account of previously existing insufficiency of the externi.

Congenital paralysis of the abducens seems more frequently to have convergent squint as a result. If, for example, convergent squint is observed in the first year of life, and we find a complete defect of motion on the part of one abducens when the children become old enough to be examined, we may certainly a.s.sume that the case is one of congenital paralysis of this muscle, or at least that the paralysis originated soon after birth. Doubtless, however, cases appear, of congenital paralysis of the abducens without squint, and as these cases are so rare I will describe two which I observed in adults.

CASE 8. Miss H--, aet. 17, has nominally since her birth a considerable defect in the outward movement of the left eye. On looking to the left h.o.m.onymous diplopia is present, on looking to the front and the right binocular single vision and no squint; on both sides emmetropia and full acuity of vision.

CASE 9. Mr. V. W--, aet. 24, has likewise congenital paralysis of the left abducens. No squint, but as soon as the left eye is used for fixation in the left direction there occurs in the right one a strong secondary movement inwards.

HYSTERICAL SQUINT.

In the hysterical form we see rather a rare variety of convergent squint, which is conditional on contraction of the interni through restriction of movement of the externi. Hysterical symptoms may at the same time appear in the eyes or elsewhere, still this does not always happen. As these cases are rare I will relate a few of those I have observed. (These cases are not included in the above statistics.)

CASE 10. Anna R--, aet. 20, came under treatment in February, 1878, stating that on the previous day she perceived blindness of the right eye on waking; in the afternoon she felt particularly weary, and after she had slept about an hour woke with blindness in both eyes. No perception of light, good pupillary reaction, ophthalmoscopic report normal. Patient was treated with copious enemata and dismissed on the fifth day cured.

In February, 1880, she again came under treatment with blindness of both eyes, also perceived the previous day on waking. Convergent strabismus was present at the same time, of such a degree that the eyes converged to a point 10 to 20 cm. distant. The outward movement was suspended in both eyes. The attempt to turn the eye outwards is accompanied by short convulsive movements, and followed by an immediate rebound to the convergent position. She a.s.serts her inability to see the movements of a hand before her eyes, is able, however, to move about in a strange room, unsteadily certainly, but with avoidance of obstacles; she sits down on a chair indicated to her, &c. The position of the eyes proves that there was no simulation in all this; it would be impossible for any person to simulate a strong convergent squint continuously for four to five days.

Eight days after her admission the patient was dismissed with normal movement of the eyes and good vision.

CASE 11. Miss Antonie E--, aet. 15, who has been treated by her family physician for various hysterical disturbances, suffered since the middle of December, 1879, from convergent strabismus with permanent but very varying deviation, which is at times very slight, and sometimes amounted to more than 7 mm. The movement outwards is in both eyes rendered difficult, still the outer edge of the cornea is brought to the outer angle of the lids with trouble and twitching movements. h.o.m.onymous double images are present, their mutual distance is alike in the whole field of vision, but is (six or eight weeks after the commencement of the squint) signified as being slight; at the same time a difference in height is present, the image of the left eye stands lower, prism 30, base outwards, places the images just above one another. Nystagmus occasionally occurs in monocular fixation (with exclusion of the other eye). In due course a gradual improvement set in, the deviation and the distance apart of the double images became slighter, the outward movement better, and in the middle of April, 1880, four months after the trouble began, no squint and no diplopia were present, the outward movement normal, facultative divergence = 0.

The hysterical character of the visual disturbance showed itself when the vision was tested. I will first observe that repeated investigations with atropine showed emmetropia, while in the first investigation on the left side, No. 36 at 5 m. was not recognised with the naked eye, but only with weak concave gla.s.ses (with - 5 D. V. = 5/18). With the right eye No. 08 was read fluently, from 075 she a.s.serted she was unable to recognise a word, with - 2 D. V. = 5/36. It would be wrong to conclude from this myopia or spasm of the accommodation, for here, as in most cases of hysterical weak sight, it could be shown that whatever gla.s.s one chose to hold before the patient's eyes, was followed by an improvement in the statements. The same improvement in visual acuteness was repeatedly obtained in this case by a weak prism (3), held before the fixing eye during monocular examination, and in the end, V. 5/12 was obtained for the right eye, as against 5/6 with a prism of 3.

Finally, on May 1st, full visual acuteness was present on both sides.

Field of vision and sense of colour normal.

CASE 12. Mrs. B--, aet. 30, previously treated for various hysterical disturbances, has complained for about eight days of disordered vision, the binocular nature of which was proved as patient had herself observed that on closing one eye she could at once see clearly. Near objects to 15 cm. are seen distinctly. With all this, at the first examination it was impossible to produce diplopia, either with the aid of a red gla.s.s or prisms, &c., the images of first one eye, then the other were always seen by turns. A few days later, on repeating the examination, double images were perceived, they were h.o.m.onymous with slight difference in height (image of the right eye lower), the lateral displacement is corrected by a prism of 28. Micropsia of one image was also perceived.

On both sides the outward movement is rather difficult. Full visual acuity on both sides--in the first examination slight myopia - 75 D. is specified, afterwards emmetropia. The visual disturbance was removed by goggles with faintly ground gla.s.s on the right side--preparations of iron, bromide salts, shampooing with cold water and electricity were used. In six weeks' time binocular single vision was again restored; the facultative divergence = 0. With red gla.s.s and vertically deviating prisms h.o.m.onymous diplopia corrected by prism 3. Field of vision and sense of colour remained normal throughout.

DIVERGENT SQUINT.

If we want to draw a comparison between convergent and divergent squint, we must consider only absolute divergent strabismus, for convergent strabismus does not offer a parallel to relative divergent squint. In absolute divergent squint the direction of the visual axes is such that they would meet behind the patient's head; in the relative divergent squint the axes of vision are parallel or slightly convergent, but they do not cross at the point fixed by the one eye, but at a greater distance off.

If we then only compare that which admits of comparison, we first find out that divergent squint is rarer than the convergent form, and the cause contained in the ocular muscles is here brought to light still more clearly than there.

We must next distinguish between permanent and periodic squint, and we see the latter so frequently continue as such, that we must not consider the transition from this variety to the permanent one to be the rule.

In 183 cases of absolute divergent strabismus which appeared in my private practice in the same s.p.a.ce of time as the cases of convergent squint above discussed I have been able to obtain exact determinations of the refraction and visual acuteness. The weakness of the fixing eye was the test for cla.s.sing them among the statistics, and in patients who had been long under observation, the first certain determination of refraction, which was necessary, as several children are included who came under treatment with divergent strabismus and emmetropia whilst myopia developed itself later.

A. Divergent squint with hypermetropia.

(_a_) Permanent 4 cases. Visual acuteness of the squinting eye more than 1/7 1 case, V. less than 1/36 1 case, 2 excluded, one on account of complication with detachment of retina, the other on account of impossibility of testing vision.

(_b_) Periodic squint 5 cases. Among them 3 with double hypermetropia, 2 with emmetropia in one, and hypermetropia in the other eye. Visual acuteness of more than 1/7 in 3 cases; V. = 1/9 1 case; V. = 1/36 1 case.

B. Divergent squint in emmetropia.

(_a_) Permanent 32 cases. Among them 10 with alternating strabismus and anisometropia of at least 2 D. And in 9 cases emmetropia in one, myopia in the other eye; once simple hypermetropic astigmatism in one, with myopic astigmatism in the other eye. Visual acuteness of both eyes in these 10 cases more than 1/7. In the 22 cases of monocular squint the visual acuteness of the squinting eye amounted 8 times to more than 1/7 -, 10 times 1/12 to 1/36 (in 1 case V. = 1/36 with nystagmus of the squinting eye when put into fixation). V. less than 1/36 in 3 cases; 6 cases excluded on account of complications.

(_b_) Periodic squint 28 cases. Among them 5 with anisometropia of at least 2 D. (emmetropia in one, myopia in the other eye). Visual acuteness of the squinting eye more than 1/7 in 27 cases, less than 1/7 to V. = 1/12 in 1 case.

C. Divergent squint in myopia to M. = 2 D.

(_a_) Permanent 24 cases (among them 6 with anisometropia of at least 2 D.). Visual acuteness of the squinting eye more than 1/7 in 15 cases. V.

less than 1/7 to V. = 1/12 2; V. less than 1/12 to V. = 1/36 3; V. less than 1/36 2 cases; 2 cases excluded on account of complications (one on account of atrophy of the optic nerve, the other on account of posterior polar cataract).