. Dynamic factors in education . ter. When necessary,the nerve centers enervate to their utmost power thevarious eye muscles, causing a change in the crystallinelens, stretching muscles which were too short to enablethe eyes to look in the same direction. * This resultsthen in incessant muscular strain which is a constantsource of waste. Gould, the distinguished oculist, saysthat The tremendous influence of eye strain upon dis-position, character, and vocation was borne in upon methe first year I was in practice. Almost every day sincethen the truth has become more striking and
. Dynamic factors in education . ter. When necessary,the nerve centers enervate to their utmost power thevarious eye muscles, causing a change in the crystallinelens, stretching muscles which were too short to enablethe eyes to look in the same direction. * This resultsthen in incessant muscular strain which is a constantsource of waste. Gould, the distinguished oculist, saysthat The tremendous influence of eye strain upon dis-position, character, and vocation was borne in upon methe first year I was in practice. Almost every day sincethen the truth has become more striking and fives and nunds are unconsciously and con-stantly modified, always unnaturally and morbidly,because of the fact, unconscious to them, that readingand study and writing irritates and disorders the centralnervous system, the digestional organs, , in the normal eye the lens and eyeball are so Prentice, The Eye in ReWion to Health, p. 10. In his Biographic Clinics, Vol. I, p. 28. THE EY£ IN RELATION TO NERVOUS WASTE 265. FIG. 31. Fig. 32. Fig. 33. The term refraction of the eye is used for the refracting power of the eyein repose, without any exertion of the accommodation muscle. Refraction isnormal, that is, the aiis of the eye is of normal length, when rays of lightwhich come from infinite distance are focused exactly upon the retina such a case*we say that the refraction of the eye is emmetropic (from em-metros, of the right measure, and ofs, the eye ). (Fig. 31.) Again, the axis of the eye may be too short, so that rays coming from in-finite distance are focused at a point behind the retina; this refraction istermed hypermetropic ( going beyond the measure ) or hyperopic (Fig. 32).This hyperopia must in no way be confounded with that long sight oftennoticed in old ago,-when the patient sees clearly only things at a distance, adefect caused by weakness of accommodation. Lastly, the axis of the eye may be too long, so that rays from infinite dis-tance are f
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