CHAPTER III.

EXTERNAL MUSCLES OF THE EYEBALL.

Strabismus (squint).-"Strabismus is a deviation in direction of the axes of the two eyeballs, in consequence of which the two yellow spots receive images. from different objects. In convergent strabismus the two visual lines do not cross each other at the point it is desired to observe; only one of the two, that of the undeviating eye, reaches it. Under this deviation not only does the expression of the face suffer from want of symmetry in its most eloquent parts, but the power of vision, at least in one of the eyes, is usually disturbed, and the squinter always looses the advantage of binocular vision." (Donders).

Strabismus must not be looked upon as a special form of disease; it is in by far the greater number of cases associated with some anomaly of refraction of which it is only a symptom; other conditions which may induce strabismus will be subsequently considered.

I.

Two forms of squint are commonly met with. 1. Convergent. 2. Divergent. Other rare forms are superior and inferior strabismus; these will receive no further notice.

Convergent strabismus is the most common of all, and is almost always the result of hypermetropia.

Divergent strabismus is frequently the result of myopia.

Convergent strabismus, as just stated, is nearly always the result of hypermetropia. It may be congenital but most commonly makes its first appearance

soon after birth or within the first two or three years of life. Squint may disappear spontaneously, consequently the parents are often told that the child will grow out of it," no greater mistake can be made as it is much more likely to grow into it. It is attributed to a variety of causes, such as teething, injury, worms, convulsions, habit, trick, nervousness, copying, etc., none of which, however, can produce it unless hypermetropia is present. It is due to the association between the action of accommodation and convergence (see p. 49).

The hypermetropic individual must always accommodate when looking at even a distant object; and as the object is brought nearer, the tension of accommodation must be correspondingly increased. Now, the greater the degree of convergence the more strongly is the accommodation brought into play; consequently there is an ever increasing tendency on the part of the hypermetropic individual to converge too much, in order to bring his accommodation into the highest possible state of tension. If the visual lines converge to a point nearer the eyes than the object looked at, convergent strabismus at once results, and no doubt double images appear which the patients are too young to notice; very soon, however, one eye comes more into use than the other, and its visual line is habitually directed to the object looked at, whilst that of its fellow is directed to a point nearer the eyes "habitual squint." Or the visual lines may be alternately directed to the object "alternating squint." We speak of the eye, the visual line of which is properly directed as the "fixing eye," of that of which the visual line is improperly directed as the "deviating eye." In some cases the deviation is not always present, but only occasional, when it is called "periodic squint."

The question next arises, Why do not all hypermetro

pic individuals squint? The reason is that if both eyes are of the same refraction, and have equal acuteness of sight, there is always such a desire to maintain binocular vision that the visual lines will remain directed to the same point, even though the eyes are not accurately accommodated for that point, the individual being content with ill-defined retinal images rather than sacrifice binocular vision by increasing his convergence.

But if vision of one eye is less acute than that of the other, or if there is a difference of refraction between the two, the desire for binocular vision is lost, or its value very much lessened, and the necessity for a welldefined image on one retina is immediately felt. The accommodation is put fully on the stretch, and with it the degree of convergence becomes excessive.

Ordinary convergent or "concomitant" squint has to be distinguished from squint the result of paralysis, "paralytic" squint. This can be done by noticing the relation of the convergence of the eye which is observed to be squinting-"primary" squint-to the deviation"secondary" ́squint-which occurs in the properly directed eye when it is covered and an attempt made to fix an object with the squinting eye. In concomitant squint the primary and "secondary" deviations are equal; in paralytic squint the "secondary" deviation is greater than the "primary."

Treatment.-Slight cases of convergent strabismus, especially if the deviation is not constantly present, but only occasional (periodic squint), may be cured by the constant use of glasses which accurately neutralise the existing hypermetropia.

In more severe cases division of the internal rectus tendon in one or both eyes is necessary; but in no case should tenotomy be performed before the age of seven years. Glasses should be ordered as soon as the child

is old enough to wear them, and should be worn constantly. We often meet with cases in which the visual lines remain properly directed as long as the glasses are worn, but squint immediately occurs when they are removed. In such the necessity for wearing glasses for distance may be obviated by tenotomy, and if the patient wishes to go about without them the operation should be performed; if, on the contrary, he is content to wear his glasses constantly, no operation is requisite. It is often difficult to decide whether only one or both eyes should be operated on.

If it is found than one eye squints habitually and to no great extent, the other being always used for fixing an object, division of the internal rectus of that eye which habitually deviates alone is necessary.

If each eye deviates alternately (alternating strabismus), and to no great extent, division of one internal rectus may be sufficient; but if three weeks or a month after the operation the squint still continues, tenotomy of the internal rectus of the other eye should be performed.

If one eye squints considerably and habitually, or if the deviation, though alternating, is excessive, the internal rectus in both eyes must be divided. In any case, if there be a doubt as to whether one or both eyes should be operated on, it is well to be on the safe side, and do only one at a time.

Operations for convergent strabismus.-There are two principal methods of operating for convergent strabismus.

1. The operator should stand on the right side of the patient, placed in the usual position (fig. 32, p. 204), and the eyelids being kept well open with a wire speculum, should seize the conjunctiva and subconjunctival fascia with the toothed forceps (fig. 38) at a point about mid

way between the margin of the cornea and semilunar fold, and just below the inferior margin of the tendon of the internal rectus muscle. An incision should then be made with strabismus scissors (fig. 40) through the conjunctiva and subconjunctival fascia, well down to the sclerotic, and the strabismus hook (fig. 39) passed

FIG. 39.-Strabismus
hook.

FIG. 38.-Toothed and fixing forceps. a, their points shown in side view; b, front view.

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FIG. 40.-Strabismus scissors.

through the opening and inserted between the tendon and the eyeball. If properly introduced the hook will. be brought up short at the insertion of the tendon into the sclerotic when it is pulled forwards; it should be held firmly in position, the scissors passed through the