MYOPIA. ETIOLOGY, PA'l near vision the eye-muscles are actively innervated, and the eyeball compressed laterally between them; so that there is a constant tendency to force it to elon gate. The condition of congestion of the choroid and inflammatory softening of the sclera, that develops under excessive use of the eyes for near work, causes the sclera to give before the intra-ocular pressure; and permanent change in the shape of the eyeball results. As myopia increases, distant vision becomes less perfect, the range of clear vision more restricted, the efforts of convergence of the eyes greater and more constant; and at the same time the sclera is thinned by distension and less able to withstand the pressure of its contents. In this way the myopia tends to go from bad to worse, becomes "prog-,ressive"; and, when this progress has become so great that it can not be checked, it is said to be "malig nant myopia." The idea has sometimes been enter tained that use of the accommodation tended to increase myopia by increasing the intra-ocular tension; but this is directly disproved by both clinical and experimental studies of the subject.
Many hypotheses have been advanced re garding myopia without any sufficient basis of facts, namely: that heredity acted by determining the proportions of the cranium or the shape of the orbits, or that a special diathetic or vascular condi tion was the chief determining factor in the case. The large number of myopic eyes that also exhibit considerable astig matism make it probable that strain of the eyes from astigmatism, causing cho roidal disturbance and scleral softening, is an important factor in many cases.
Family history, etc., of 330 young myopes examined. An hereditary influ ence was certain in 216, or in 65 per cent. This hereditary form appears earlier, ad vances more rapidly, is higher in degree,