When the Eye Possesses Central Fixation 117
area three or four inches in extent at twenty feet, or even less, or an area of an inch or less at the near-point, equally well at one time, while at the same time the retinoscope will demonstrate that an error of refraction has been produced. This strain, when it is habitual, leads to all sorts of abnormal conditions and is, in fact, at the bottom of most eye troubles, both functional and organic. The discomfort and pain may be absent, how ever, in the chronic condition, and it is an encouraging symptom when the patient begins to experience them.
When the eye possesses central fixation it not only possesses perfect sight, but it is perfectly at rest and can be used indefinitely without fatigue. It is open and quiet; no nervous movements are observable; and when it regards a point at the distance the visual axes are parallel. In other words, there are no muscular insuffi ciencies. This fact is not generally known. The text books state that muscular insufficiencies occur in eyes having normal sight, but I have never seen such a case. The muscles of the face and of the whole body are also at rest, and when the condition is habitual there are no wrinkles or dark circles around the eyes.
In most cases of eccentric fixation, on the contrary, the eye quickly tires, and its appearance, with that of the face, is expressive of effort or strain. The ophthalmo scope1 reveals that the eyeball moves at irregular inter vals, from side to side, vertically or in other directions. These movements are often so extensive as to be mani fest by ordinary inspection, and are sometimes suffi ciently marked to resemble nystagmus.2 Nervous move-
1A shorter movement can be noted when the observer watches the optic nerve with the ophthalmoscope than when he views merely the exterior of the eye.
2 A condition in which there is a conspicuous and more or less rhythmic movement of the eyeball from side to side.