=WEAKNESS OF THE EYE MUSCLES.=--There are six muscles attached to the outside of the eyeball which pull it in various directions, and so enable each eye to be directed upon a common point, otherwise objects will appear double. Weakness of these muscles or insufficiency, especially of those required to direct the eyes inward for near work, may lead to symptoms of eye-strain. When reading, for example, the muscles which pull the eye inward soon grow tired and relax, allowing the opposing muscles to pull the eye outward so that the eyes are no longer directed toward a common point, and two images may be perceived or, more frequently, they become fused together producing a general blurring on the page. Then by a new effort of will the internal muscles pull the eyes into line again, only to have the performance repeated, all of which entails a great strain upon the nervous system, and may lead to permanent squint, as has been pointed out. In addition to these symptoms caused by weakness of the eye muscles--seeing double, blurred vision, and want of endurance for close work--there are others which are common to eye-strain in general, as headache, nausea, etc., described in the following paragraph. =Symptoms of Eye-strain.=--Headache is the most frequent symptom. It may be about the eyes, but there is no special characteristic which will positively enable one to know an eye headache from that arising from other sources, although eye-strain is probably the most common cause of headache. The headache resulting from eye-strain may then be in the forehead, temples, top or the back of the head, or limited to one side. It frequently takes the form of "sick headache" (p. 113). It is perhaps more apt to appear after any unusual use of the eyes in reading, writing, sewing, riding, shopping, or sight-seeing, and going to theaters and picture galleries, but this is not by any means invariably the case, as eye headache may appear without apparent cause. Nausea and vomiting, with or without headache, nervousness, sleeplessness, and dizziness often accompany eye-strain. Sometimes there is weakness of the eyes, i. e., lack of endurance for eye work, twitching of the eyelids, weeping, styes, and inflammation of the lids. In view of the extreme frequency of eye-disorders which lead to eye-strain, it behooves people, in the words of an eminent medical writer, to recognize that "the subtle influence of eye-strain upon character is of enormous importance" inasmuch as "the disposition may be warped, injured, and wrecked," especially in the young. Some of the more serious nervous diseases, as nervous exhaustion, convulsions, hysteria, and St. Vitus's dance may be caused by the reflex irritation of the central nervous system following eye-strain.