Page:Handbook of Ophthalmology (3rd edition).djvu/49

Rh to direct the eyes upon a point at any given distance there must be a stronger convergence of the eyes, and a greater demand upon, the action of the internal recti muscles,, than in emmetropia.

In addition to this, the abnormal shape of the myopic eye impedes its movements. While the nearly spherical emmetropic eye plays like a ball-and-socket joint in the capsule of Tenon, the myopic eye, on account of its elliptical shape, meets with considerable resistance to all lateral rotation, restricting the movement both outward and inward. But while a slight turning of the head may take the place of a rotation of the eye outward, no such compensation is possible when it is desired to turn both eyes simultaneously toward the median line. This difficulty of convergence is the reason that in high degrees of myopia binocular vision for near objects is relinquished, especially when the myopia or the acuity of vision of the two eyes does not correspond. For reading, etc., the best eye only is then used, while the axis of vision of the other eye is not directed upon the point fixed, but has a direction relatively or absolutely divergent. The inconveniences of diplopia, which one would expect under these circumstances, do not generally occur, because the attention is directed only upon the distinct retinal images in the one eye, while the retinal images of distant objects in the other eye being diffuse and indistinct, are readily neglected.

The relinquishment of binocular vision under these circumstances is to be regarded as an advantage. These myopes do exactly what we would advise them to; they avoid all straining of accommodation and strong convergence of the visual axes, and for the most part, too, the bent-over position of the head, since they, at least when reading, are accustomed to hold the book vertically and the head erect. This insufficient movement of the eyes is to be distinguished from an insufficiency of the muscles, of which we have yet to speak.

Other and very important functional disturbances are caused by the distention of the membranes in high degrees of myopia. In the first place, the distention of the retina causes a separation of the perceptive retinal elements; this, in high degrees of myopia, causes a diminution in the acuteness of vision. Nevertheless, myopes do not generally complain of this. Since they can hold objects nearer the eye, they use a larger visual angle, and can