have ground to believe that from the senile changes of the lens its cortical part becomes harder, so that its refracting power more nearly approaches that of the nucleus; an increase of focal distance is the result. Moreover, in extreme old age the lens appears to become actually flatter, a further cause for the diminution of refraction. In this way hypermetropia may develop in eyes originally emmetropic.
The diminution of the range of accommodation by a withdrawal of the near point from the eye is, then, to be called presbyopia only when it is a co-symptom of senile changes in the eye. These changes are numerous. All the refracting media of the eye become less transparent; this is noticeable when one compares ophthalmoscopically a senile eye with a youthful one; the anterior chamber becomes shallower; the iris loses its motility; the pupil becomes smaller, and often does not dilate normally upon the use of atropine; the choroid and membrane of Descemet (the last, fortunately, as a rule, only near the corneal margin) become thickened and uneven; phosphate of lime, as Donders has shown,[1] is deposited in the sclera; the muscle of accommodation atrophies; the retina in its vessels and in the structure of its peripheral parts also shows changes which must be regarded as senile. Diminished clearness of vision follows all these phenomena. We thus find presbyopia to be a co-symptom of a considerable series of changes, and must, therefore, regard the withdrawal of the near point as a normal phenomenon only when it stands in due proportion to the age. Emmetropes, on an average, first experience the inconveniences of presbyopia between the forty-fifth and fiftieth years of life. Only very sharp-sighted eyes can, at a greater age, be used by the hour in reading or writing without experiencing fatigue.
Premature presbyopia occurs in connection with premature marasmus, after prostrating sicknesses, with incipient cataract and with the development of glaucoma.
The diagnosis of presbyopia is easy. It is first to be ascertained that the vision for distance is good, then that the indistinctness of near objects is corrected immediately by weak convex lenses, and, finally, that the diminution of accommodation stands
- ↑ Arch. f. Ophth., ix. 2, pag. 217.
