BLINDNESS may arise from any cause intercepting the rays of light on their way to the optic nerve, or from disease of the optic nerve, or of that part of the brain connected with it. B. may vary in degree; it may exist from birth, or be the result of extreme old age. It may only be present during the day or the night, or a few weeks of the year, or it may be permanent.
Congenital B. is generally from some deficient development of the nervous apparatus, and is detected by the child being indifferent to light, and throwing its head from side to side. Occasionally, but very rarely, the power of vision is subsequently developed. Ainaurosis has been already described.
Opacity of the vitreous humor, or of the crystalline lens—the latter is generally- known as cataract—causes B. which comes on gradually. The patient with cataract can see best in the evening. or when the pupil is dilated, as then some rays of light are able to miter by the side of the opacity. The B. from cataract is seldom so complete as to pre vent the person from distinguishing day from night, or from being aware of opaque bodies passing between him and the light (see CATARACT). Opacities of the cornea, if extensive, or in the axis of vision, produce some degree of B., whether they are on or in its substance. In general, these are irremediable; but if there is a spot, an artificial pupil may be made. Some years ago, Mr. Bowman, of London, met with a case in which the opacity consisted of a layer of phosphate and carbonate of lime: he removed It, and restored the vision, which had been totally lost for several years.
Night B. is a rare condition, in which a person finds, towards evening, that objects are becoming less and less distinct, and at last that he is totally blind. This may occur without previous warning, and cause great alarm, but next morning he finds that his sight is restored. This is repeated every night, but at last the eves become weak during
the day also, and may finally- become amaurotic. This strange affection may be epidemic; it has attacked bodies of troops exposed to great fatigue and the glare of the sun's rays. If there are no symptoms of disease within the brain, recovery generally results from protecting the eyes from the light, entire repose, such remedies as correct any constitu tional defect in the individual attacked, and repeated blistering.
Day B. is characterized by inability to see in a bright light; the subjects of it see more than usually well at night, but during the day have to be led about. Captives who have been long immured in dark cells are often affected with it, as a galley-slave men tioned by Larrey,who had for 33 years been shut up in a subterraneous dungeon, and when liberated could only e3e by night.
The structural causes of 13. will be better understood when the eye (q.v.) is described, when it will be seen that advances in our knowledge of its anatomy have enabled sur geons to restore sight in cases which, some years ago, would have been considered hope but it can never be too strongly impressed, opecially on the young, that overwork wears out the eyes, whatever be the pursuit, and that, without being wholly dark, a degree of blindness may be induced, such as to render the eyes useless for practical pur poses. This condition, astlienopia or weak sight, is frequently met with iu young lads with sedentary occupations, students, dressmakers; and, says Dr. Mackenzie of Glasgow, "what may be called the hothouse education of modern times is a fruitful source of it." The only cure is avoiding the evident causes.