Page:Handbook of Ophthalmology (3rd edition).djvu/46

40 myopes themselves, that straining the eyes favors the development of myopia.

The clinical history of hypermetropia shows that a long-continued tension of accommodation is of itself not enough to induce an elongation of the eye. Nevertheless it might be otherwise if the sclera possessed diminished power of resistance, and if other conditions were present, particularly strong convergence of the visual axes and a bowed position of the head. The first is associated with an increased intraocular pressure, which favors a distention of the sclera; the last causes a passive hyperæmia, which tends in the same direction. All these conditions almost of necessity exist when work is undertaken in an insufficient light. The habit which children early acquire of reading long into the twilight, and the use of ill-contrived school-furniture, are both frequent causes of myopia.

Accordingly, it is no wonder that myopia is such a wide-spread abnormity. It occurs in early childhood, and shows with the increase of years a progression both in frequency and degree of development.

Although for the majority of cases we must regard a congenital weakness of the posterior part of the sclera as the primary cause of myopia, it is certain that the same condition may be an acquired one. The circumstance that myopia often develops in children shortly after measles or scarlet fever goes to prove this. Myopia often occurs also in connection with cloudiness of the cornea; this is not because the cornea becomes more convex, for in most cases there are simultaneously present the ophthalmoscopic indications of elongation of the axis; it is more probably due to the indistinctness of the retinal images, the patient seeking to compensate by increase in size for what he loses in distinctness. In order to obtain larger retinal images, the patient brings his eye as near as possible to his work; a bowing of the head, strong tension of accommodation and a corresponding convergence of the visual axes are the consequences.

If the distention of the sclera be once established, it can be easily understood how intraocular hyperæmia and conditions of slight irritation may still further lessen its capacity for resistance and render the distention progressive. On the other hand, one often sees, in cases with great distention of the sclera and high