SQUINTING (Strabismus). It is a condition essential to correct vision that the axes of both eyes correspond in direction, and be turned simultaneously towards the object we regard. Now to ensure the fulfilment of this condition, the orbital muscles (motores oeulorum) are so supplied with nervous influence, that we cannot will the move ment of one eye without the other being called into involuntary and harmonious action. There are some individuals, however, whose optic axes are not parallel, and whose eyes do not move iu harmony with each other; such persons are said to squint, or to be affected with strabismus. Squinting may take place either upwards, downwards, inwards, or outwards, or in the intermediate directions ; it may also be Confined to one eye; or may affect both. As the inward and outward varieties of squint are by far the moat common, we shall devote the following remarks to them alone.
Inward Squint, or Strabismus conreryens, is met with in three distinct forms : 1, single convergent strabismus ; 2, double convergent strabis mus; and 3, alternating strabismus. In the first form of the affection one eye is habitually turned more or less inwards towards the inner angle of the orbit, whilst the other maintains its natural position, and is capable of being directed to any object that the individual wishes.
On closing the sound eye, the inverted one then becomes straight, and can be turned in every direction nearly to the same extent as the other ; but as soon as it is again opened, the one affected with stra bismus revolves inwards, and there remains; or if it do move along with the good eye, yet never so as to permit the two axes to be pointed at the same object. Double convergent strabismus differs from the preceding in its affecting both eyes; the axis of each eye is inclined unnaturally inwards, as if the person were regarding some object placed close to his face. On directing his attention to distant objects, the eyes do not become parallel, but the one least affected (for one is always more so than the other) becomes straight, whilst its fellow pre serves its former position, or is turned more strongly inwards. With
regard to the relative frequency of strabismus in one or in both eyes, it is said to affect most frequently one eye only, and this the left.
Alternating Strabismus differs from the ordinary form of squint, in its affecting both eyes equally, though never both at a time. An in dividual thus affected appears to use either eye indifferently; and the change of inversion from one eye to the other is a voluntary act, independent of the opening or closing of the eyelids.
Outward Squint, or Strabismus dirergens.—Nearly all that has been said in reference to convergent strabismus, may be applied, mutatis mutandis, to divergent squint. In this form of the affection, one eye is more or loss everted, whilst the other is directed straight forwards ; the patient is likewise incapable of directing both eyes inwards simul taneously. These cases we believe to be most frequently of the alternating kind ; that ia, the individual can employ either eye singly, and bring it into the central axis, but then its fellow becomes everted. It is a more rare affection than the former one, and the deformity arising from it is seldom so obvious. Whether we regard strabismus as affecting one eye or both, it is certain that the vision of the one most distorted is nearly always imperfect, and usually in a direct ratio with the degree of distertion. Now we know that if impressions on the two retiree are dissimilar in force, the mind disregards the weaker, and takes cognizance only of the stronger ; so that a person who squints badly generally sees objects with the sound eye only. If the sight of both eyes is equal or nearly so, double vision results when ever both are employed together, because the images of objects do not fall on corresponding portions of the two retime (SIGHT, DEFECTS OF] ; and as the defect of sight is generally in a direct ratio with the degree of distortion, double vision is most frequently experienced in slight cases of squint.