Diseases of the eye and ophthalmoscopy : a handbook for physicians and students / by A. Eugen Fick ; authorized translation by Albert B. Hale.

Adolf Gaston Eugen Fick

Date:: 1896

Credit: Diseases of the eye and ophthalmoscopy : a handbook for physicians and students / by A. Eugen Fick ; authorized translation by Albert B. Hale. Source: Wellcome Collection.

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$(2) Elevation and depression of the visual line— (a) elevation, both rectus superior and obliquus inferior act- ing in common. {d) depression, both rectus inferior and obliquus superior act- ing in common. (3) Convergence and divergence— {a) convergence, both recti interni acting together. (^) divergence, both recti externi acting together. We make the most extensive use of these eye movements. Any- thing noticed to the side of us at once arouses our interest; invol- untarily, often unwillingly, the eye is turned thither so as to bring the object opposite the fovea centralis. The rapidity and exactness of this movement is astonishing. As a rule, the head is turned at the same time, so that, as Ritzmann has estimated, a movement of 50° toward an object is composed of jo° of eye movement and 20° of head movement. The attempt to look toward an object by moving the eyes alone can by many be accomplished only after several unsuccessful efforts. The movements of convergence and divergence need particular mention. They are inseparably connected with movements of accommodation. For example, if two normal eyes look at a point ^ m. away, an effort of accommodation is made in both eyes equivalent to ^.o D, and such a convergence of the eyes takes place that the visual lines cross on the fixation point, even if there is no need of this position, one eye being covered or useless, perhaps. Tf both eyes look straight ahead and then at a point / m. distant on the plane of the eyes and at the middle line of the body, each of the two visual lines describes an angle called by Nagel meter-angle [MA), which is chosen as the measure of that convergence of the visual lines. Expressed in degrees, a meter-angle shows a different value accord- ing as the distance of the eyes from each other is greater or smaller. The distance apart of the rotation points of the eyes is called the basal-line. A basal-line of 64 mm. gives to a Ä/A the value of 1° jo^ ; of mm. the AIA is /° J2' 43. The converging power of each eye in looking at a point is inversely proportional to the distance of this point. If the fixation point lies, for example, at ^ /«., the converging power of the eye is 2 MA ; if it lies at ]/^ m., the converging power is j MA, and so forth. The connection between accommodation and convergence, when binocular fusion is concerned, is quite elastic. If an emmetrope reads fine print in. off, each eye is capable of accommodation of D and a convergence of ^f. MA. The eyes are, however, capable of seeing at y{ m. distance with both convex and concave lenses, that is, they are able to do without accommodation$