eyeball (one flattened posteriorly), is almost universal. If the latent form is included, that is, hypermetropia which is made manifest only by a cycloplegic, that is, a paralyzer of the accommodation, it is present in ninety per cent of the

cases.

Generally the vision of the squinting eye is less than the other, while amblyopia, that is, defective vision which no glasses will correct, is often present. This may be due to a hazy cornea, following a former keratitis, or there may be no discoverable lesion. Diplopia is never a symptom of a wellestablished squint. The relative strength of the recti muscles may be normal, that is, the adductors three times the abductors, or there may be a decided loss of abduction.

With these facts in mind, let us consider the various theories as to cause. Donders maintained that hypermetropia was the sufficient factor in its production, the reverse condition, myopia, being the cause of divergent strabismus. It was well set forth that the excessive and constant accommodation of the hyperope exhausted the resources of the ciliary until finally binocular vision was sacrificed upon the discovery that excessive convergence augmented accommodation and relieved the overdraft on this function. The relative strength of the adductors and abductors was thought to be of little moment. Naturally the occurrence of hypermetropia in such a large proportion of the cases gives color to this view, but the other fact is that hypermetropia is the rule in children and only a small minority of children squint. Moreover, hypermetropia of high degree is not so common among strabismics as the medium amount, 2-3 D. The fact of the poor vision in so many squinting eyes was claimed to be due to a loss from disuse. "Amblyopia ex anopsia.”

But it is now held by the majority of competent observers that the amblyopia precedes the squint. It is doubtful if there exist a true amblyopia ex anopsia. There is a suppression of the retinal image received by the squinting eye, otherwise a constant diplopia would exist. Excessive adduction is not sufficient as a cause, since the relative strength of these two antagonistic forces may be normal. Defective in

It

nervation of the muscles has been assigned as a cause. has been claimed that there is a differential innervation of the different fibres of each rectus muscle. But it would seem to be more scientific to reserve this practical begging of the question as a last resort.

If

now, instead of trying to find a cause which will explain all cases, we divide the squints into four classes, it seems to the writer that a satisfactory explanation may be given for

each.

First class. Those in whom vision is normal in both eyes. Here it will be readily demonstrated that the abduction is relatively weak, so weak that the fusion power is unable to overcome the strong tendency inward. By this fusion power is meant that coördination which involuntarily turns the eyes so that the image falls on corresponding points of the two retina, which is the sine qua non of binocular vision.

Second class. Hypermetropic eyes will tend to squint if the adduction and abduction are normal (3-1) in order to relieve the excessive accommodative effort by a hyper-convergence. A difference in the refraction of the two eyes. or the occurrence of astigmatism will, of course, augment the tendency. Valk claims that all hypermetropic cases that do not squint owe their salvation to an excessive abduction.

Third class. The amblyopic eye squints because the fusion impulse is weakened or wanting. The eye has no incentive to inhibit the dynamic resultant of the muscles. Nothing short of an absolute balance of the muscles will keep this eye straight. The frequent development of squint in an eye which has become amblyopic from injury or haziness of the cornea is a sufficient demonstration of this principle.

Fourth class. Those abnormal eyes which are apparently crossed, but which show no movement with the cover test. Here we have a macula relatively misplaced. Also cases for whom binocular vision is impossible, probably from some abnormality of the distribution of the crossed and direct fibres of the chiasm. These cases may be diagnosed by the fact that with no combination of prisms can binocular vision

be effected. The double images may be made to approach, but a slight increase in the prism will throw the image to the other side. Fusion impulse is nil.

In most that has been written on this subject of etiology, it has been assumed that normal vision is present at birth. In the lower animals this is undoubtedly true. Not only do they almost immediately walk or run, but the coördination of the visual centres would seem to be at least sufficient to enable the animal to secure its food. As we advance in the animal scale toward man there is evident a lengthening of the period of infancy, the significance of which has been very generally overlooked.1 It is this lengthening of the period of plasticity that emancipates to a certain extent the young of man from the thraldom of heredity, which enables. him to mark out new paths of coördination in the jungle of his cerebral hemispheres. Preyer says: "On the whole I have found that in the newly born, one eye very often moves independently of the other, and the turnings of the head take place in a direction opposite to that in which the eyes move. The unintentional character of both movements is plainly recognizable and the combination of the two is at the beginning of life accidental."

Priestly Smith says: "All eyes are highly amblyopic at birth. Those which later reach the standard of normal vision do so by a process which occupies several years." These facts bear strongly on etiology and treatment.

Any defect either in refraction or visual perception or muscle balance may start the child on wrong lines and the fusion impulse may never be realized. This early suppression may cause a true amblyopia ex anopsia, but not in the old sense, that is, it is not the loss of a faculty once possessed, but an arrested development.

Treatment.

Etiology has been entered into at some length because, without a thorough understanding of the factors which have

1 John Fiske, Essay: The Meaning of Infancy.

2 W. Preyer, The Senses and the Will, Jena, 1884.

3 British Medical Journal, July 2, 1898.

produced the condition, intelligent treatment is impossible. "Binocular vision is essentially a cerebral function," and while the fusion coördination is ordinarily involuntary, yet it is truly surprising to what extent one may control this and disassociate his convergence and accommodation by practice. The familiar example of the suppression of one image when using a monocular microscope is very suggestive of the possibilities of restoration of binocular vision by proper training.

The first and foremost indication is to determine the total refractive error. A thorough course with a cycloplegic, preferably Atrop. sulph. gr. j ad 3ij (1%) gtt. j once a day, must be instituted and continued for at least ten days, and in some cases I have used it a month. It may be necessary to repeat the instillation at intervals should there be recurrence of spasm. Glasses may be ordered of full strength, that is, equal to the total hypermetropia; but a gradual working up to the strongest is better borne by the patient. With a very young child glasses are impracticable, but with a three-yearold I do not hesitate to order them. The "golden opportunity" is while the squint is still periodic and has not passed over into the permanent form.

As previously stated, the refractive error may not be excessive, and one should not hesitate to order the correction because it seems slight, for even a slight amount may turn the scale in a case in which the fusion function is weak. Prisms, base out, may often be advantageously combined and gradually reduced if possible. It sometimes happens that as soon as the glasses are put before the eyes, an immediate straightening takes place, and the reduction of the deviation by a third or a half is not at all uncommon.

The "educative" treatment consists in first compelling. the use of the squinting eye by a bandage or pad used over the fixing eye for repeated periods of an hour or two every day. The child will rebel some against this and may at first find some difficulty in walking and estimating distances, but perseverance will usually be rewarded with great improve1 Noyes, Diseases of the Eye, New York, 1894.

If

ment in both vision and in fixing power. This method has the advantage of being applicable to the youngest child. the child has learned to read, the desire for fusion can be cultivated and strengthened by arranging a vertical obstruction in the median line about halfway between the book and the face. A lead pencil answers the purpose very well. If one eye is closed this will cut off about six letters of ordinary type, and unless one uses both eyes he will not be able to see behind the pencil.

Binocular vision exists in all degrees of perfection, and it is not necessarily secured because the eyes are properly fixed. The head, book, and pencil must not be moved, and by closing the eyes alternately it will be discovered that each can look behind the pencil about half an inch, that is, the fields of the right and left eyes overlap. The instant every letter in a line can be seen binocular vision is secured. For an older patient this is a rather pleasant occupation, and if the matter read be somewhat interesting, an extra stimulus to fusion is given.

Stereoscopic pictures have been so devised that fusion is necessary to see the whole picture. If this is impossible suitable prisms may be placed in the stereoscope to assist. These are to be replaced with weaker ones as the faculty improves. All of these educative measures require time and patience.

In the line of materia medica, Norton tells us that "the use of remedies has in the early stages of many cases relieved the tendency to permanent strabismus." He mentions particularly Cicuta vir. and Jaborandi. The writer has no personal experience in this line to offer, but suggests that it is our duty to first seek and remove the "mechanical" cause and supplement with medicine.

If after reasonable treatment all these measures prove ineffectual, recourse should be had to an operation. This is either a tenotomy of the strong muscle or a shortening of the weak one. Tenotomy is a comparatively simple operation to execute, but furnishes an opportunity for the exercise of considerable judgment. The operation for advancement