A manual of diseases of the eye / by Charles H. May and Claud Worth.

  • May, Charles Henry, 1861-1943.
Date:
1924
Catalogue details

Licence: Public Domain Mark

Credit: A manual of diseases of the eye / by Charles H. May and Claud Worth. Source: Wellcome Collection.

    423/524 (page 367)
    the visual acuity, in a case of acquired amblyopia, falls below fr-Q the patient loses the power of ‘ fixing ’ any object steadily with this eye, even though the other eye be covered, because the macular region has ceased to be more sensitive than the paracentral region of the retina (lost fixation). Etiology of Convergent Squint.—The one essential cause of squint is a defect of the fusion faculty. So long as the fusion faculty is perfect the eyes cannot deviate from their normal relative directions without causing intolerable diplopia (e.g., in a case of paralysis of an external ocular muscle); but if the fusion faculty be absent there is no tendency to fusion, and there is no diplopia when the eyes deviate. The eyes are then merely controlled by their motor co-ordinations, and anything which upsets the equilibrium of these co-ordi¬ nations will cause a permanent squint. This disturbing influence may be : 1. Hypermetropia. — Owing to the association between accommodation and convergence (see p. 303), when the eyes accommodate in looking at a near object the visual axes converge to a proportionate degree. Hypermetropic eyes must accommodate even in distant vision, and still more in near vision. There is a tendency for an abnormal convergence to be associated with the abnormal effort of accommodation. If the control of the fusion faculty be absent this tendency will give rise to an actual deviation. 2. Anisometropia and the rare congenital amblyopia pre¬ dispose to squint by making binocular vision more difficult. 3. In the absence of the controlling influence of the fusion faculty a motor imbalance may give rise to squint. 4. Specific Fevers, especially whooping-cough, or any violent mental disturbance, may be the immediate exciting cause of the deviation in a case in which the fusion faculty is deficient. Injury to the sixth nerve during birth in some cases causes a deviation which prevents the development of binocular vision. The Method of Investigating a Case of Squint.—The presence or absence of a deviation may sometimes be determined by simple inspection, but often appearances are misleading.