iarly marked in certain cases of esophoria. In such cases there has been, during the history of the patient, little need of performing a positive act of adduction. Habitually the external muscles have by severe tension, maintained the parallelism of the visual lines, and when it is required to converge these lines the act is performed, not altogether by the muscular contraction of the interni, but largely by their natural elasticity, acting when the opposing tension of the externi is removed.

TREATMENT OF EXOPHORIA.

In the condition of insufficient abducting power, such as may be found with moderate exophoria, with no especial deviating tendency, or with esophoria, the adduction may be greatly improved by gymnastic exercises of the interni conducted by the aid of prisms.

In these exercises the eyes are required to unite images in overcoming gradually increasing obstacles. A prism of a few degrees, perhaps 10°, is placed, base out, before one of the eyes, while gazing at a lighted candle placed at twenty feet distance, when an effort is at once made to prevent diplopia. As soon as the images are blended, another prism, of perhaps less degree, is placed in the same manner; the images being united, a stronger prism takes the place of one of those already in place, or one is added to those already in position. Thus, little by little, the eyes are required to overcome prisms until the images can no longer be united. Then all the glasses are removed and the process is repeated; with each repetition something may be gained. The exercise should

not be continued at a single sitting more than five or six minutes, and only a single sitting daily is desirable.

By this means the adducting power can, in most cases, be raised after a few exercises to the desired point. It is an interesting fact that in most cases of moderate exophoria, or of no especial heterophoria, the exercise is attended with much more speedy results than in a certain proportion of cases of esophoria.

The effect of such exercises upon the eyes is very often extremely salutary. With greater freedom of muscular action comes a sense of relief from nervous strain, which is often of a most gratifying character. Such an exercise is in no way related to the practice sometimes adopted, and which should be condemned, of requiring the patient to gaze for a length of time at a near object. In this latter case the act of accommodation is associated with the convergence of the optic axis, and there is simply an exaggeration of the accustomed strain. In the exercise with prisms the accommodation is at rest, and the action of the recti muscles is almost completely dissociated from that of the ciliary muscles. The exercise then selects the muscles to be acted upon.

In exophoria of a moderate degree, prisms with the base in may be found useful in reading. It is in this condition, of all the forms of heterophoria, that prisms are most likely to prove of any permanent use. In general, even for moderate exophoria, a radical relief is to be preferred to the perpetual employment of glasses.

The method for performing tenotomy, as described

already, should be followed closely in this as in other conditions of heterophoria. The advice given by Graefe, and followed up to the present time in the text-books, to sever the externus completely and to induce homonymous diplopia, is not to be followed. Graefe performed his operations, for the most part, in cases of extreme myopia. In these cases, the vision of his patients being defective at a distance, an insufficiency of the externi did not appear to be a matter of serious consequence. It may well be supposed also that, for the most part, his cases were not of the class which has here been described as exophoria, but cases of positive but slight strabismus.

The result of a tenotomy for exophoria should not be homonymous diplopia, nor even esophoria to exceed 1° or 2°, and the abducting power should immediately after the operation not be less than sufficient to overcome a prism of 6°. Should exophoria again manifest itself, the operation may be made upon the opposite eye; and it is better to make repeated operations than in any instance to obtain diplopia or considerable esophoria.

The study of muscular anomalies, in which gross deviations are found, strabismus concomitans, or strabismus from paralysis, constitutes a subject of much interest to the student of the effects of ocular anomalies upon the nervous system. Cases of this class are much less frequent than those already discussed, and, to a considerable extent, the principles which have been laid down respecting the more ordinary class of anomalies will apply to non-paralytic strabismus.

It is the purpose of this supplement only to introduce the practitioner into a field not usually investigated, except by specialists, and to assist him in his efforts to find and to remove a class of causes largely instrumental in inducing an important class of disorders.

To add largely to what has been said would be outside the design of this work. The author, therefore, reserves the discussion of his personal views upon strabismus for a future work upon the ocular muscles.

THE END.